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6 Learning Outcomes

After reading this chapter, you will be able to:

6.1 Describe protein, its basic structure and shape, and the classification of amino acids.

6.2 Identify the key steps in digesting proteins and absorbing amino acids.

6.3 Explain the metabolism of amino acids and the role of the amino acid pool.

6.4 Describe the functions of protein in the body.

6.5 Calculate the daily amount of protein recommended based on the Dietary Reference Intakes.

6.6 Describe the best food sources of protein and the methods available to determine protein quality.

6.7 Explain the health consequences of consuming too much or too little protein.

6.8 Describe the benefits and risks of a vegetarian diet.

True or False?

1. Proteins are chemically different from carbohydrates or lipids because they contain nitrogen. T/F

2. Proteins are made up of 20 essential amino acids. T/F

3. The first step in the chemical digestion of protein occurs in the mouth with the enzyme pepsin. T/F

4. Hydrochloric acid denatures protein in the stomach. T/F

5. The body can use protein as a source of glucose. T/F

6. The primary function of protein is to provide energy to the cells. T/F

7. Growing children are in a state of negative nitrogen balance. T/F

8. Animal products are a good source of incomplete protein. T/F

9. Eating too much protein is associated with high blood cholesterol levels. T/F

10. Consuming a diet inadequate in protein may lead to a disease called kwashiorkor. T/F See page 244 for the answers.

Proteins

206 Chapter 6 | Proteins

What Are Proteins?

LO 6.1 Describe protein, its basic structure and shape, and the classification of amino acids.

Chemically, the structure of proteins is similar to that of carbohydrates and lipids in that all three nutrients contain atoms of carbon (C), hydrogen (H), and oxygen (O). Protein is unique, however, because 16 percent of each protein molecule is nitrogen (N). In fact, protein is the only food component that provides the nitrogen the body needs for impor- tant processes, such as the synthesis of neurotransmitters. Some proteins also contain the mineral sulfur (S), which is not found in either carbohydrates or lipids.

The Building Blocks of Proteins Are Amino Acids

In Chapters 4 and 5, you learned that dietary carbohydrates are chains of glucose units and that most dietary lipids contain fatty acid chains (Figure 6.1). Proteins are also made up of chains, but the units (or building blocks) of these chains are amino acids.

proteins Large molecules, made up of chains

of amino acids, found in all living cells.

amino acids Fundamental units of proteins;

composed of carbon, hydrogen, oxygen, and

nitrogen.

P roteins are the predominant structural and functional materials in every cell of the body. In fact,

protein alone makes up 50 percent of your body’s dry weight. Proteins carry out most of the work

of body cells. Your protein-rich muscles enable you to swim, jog, walk, and hold your head up so you

can read this chapter. Without adequate protein, your body couldn’t replace the skin cells that slough

off when you shower or produce sufficient antibodies to fight off infections. Your hair and fingernails

wouldn’t grow and you wouldn’t be able to digest food. Many enzymes and some hormones, which

control essential metabolic processes, are made of proteins. They direct how fast the body burns

kilocalories, how quickly the heart beats, and possibly your attraction to another person.1 In short,

proteins are involved in all of the body’s functions, and without them, you wouldn’t survive.2

In this chapter we discuss the structure and roles of proteins and how they are digested, absorbed,

and synthesized. We also cover the health risks associated with consuming too much or too little protein

and the pros and cons of different eating patterns, including vegetarian diets.

Protein-rich muscles enable you to perform

daily activities.

▶ Figure 6.1 Macronutrient Structure

Carbohydrates, some lipids, and proteins

are similar in their chainlike structures. The

main difference is that proteins contain

nitrogen and carbohydrates and lipids do

not. Carbohydrates are composed of glucose

chains; triglycerides and phospholipids

contain fatty acids; and proteins are made of

chains of amino acids.

Macronutrients Composed of Example

Carbohydrates

Lipids

Proteins

Monosaccharides Polysaccharide

Fatty acids Triglyceride

Amino acids Peptide

Fatty acids

Glucose units

Amino acids

What Are Proteins? 207

All proteins in the body consist of a unique combination of up to 20 different amino acids and are classified according to the number of amino acids in the chain. Proteins typically contain between 100 and 10,000 amino acids in a sequence. For instance, the protein that forms the hemoglobin in red blood cells consists of 574 amino acids. In con- trast, collagen, a protein found in connective tissue, contains approximately 1,000 amino acids.

Amino acids are like numeric digits: Their specific sequence determines a specific function of the protein. Consider that telephone numbers, Social Security numbers, and bank PIN numbers are all made up of the same digits (0–9) arranged in different sequences of varying lengths. Each of these numbers has a specific purpose. Similarly, amino acids can be linked together to make unique sequences of varying lengths, giving each protein a specific function.

As illustrated in Figure 6.2, each amino acid contains a central carbon (C) surrounded by a carboxylic acid group (COOH), which is why it is called an amino “acid”; an amine group (NH2) that contains the nitrogen; a hydrogen atom; and a distinctive side chain also referred to as the “R” group. All amino acids contain the same five parts, and the side chain makes each amino acid unique.

Side chains can be as simple as a single hydrogen atom, as in the amino acid glycine, or they can be as complex as the ring structure in phenylalanine (see Figure 6.2b). Though each side chain is distinct, some have similar properties. For example, some side chains cause their amino acids to be basic, such as those found in arginine and histidine, whereas others, such as the side chain in aspartic acid, cause their amino acid to be acidic. The side chains of two amino acids, methionine and cysteine, contain sulfur. Some side chains are branched, as in the case of leucine, isoleucine, and valine.

Side chains influence the function of each amino acid, whether the body can make the amino acid, and the metabolic pathway the amino acid follows after absorption. Side chains also influence the shape of the protein because of the way they bond with the side chains of other amino acids. Certain side chains are attracted to certain others; some are neutral; and some repel each other, causing the protein to be either linear or globular in shape. Because the shape of the protein determines its function in the body, anything that alters the attractions between the side chains will alter the protein’s shape and thus its function.

Any amino acid chain that contains fewer than 50 amino acids is called a peptide. If a chain consists of two joined amino acids it is called a dipeptide; three joined amino acids form a tripeptide; more than 10 amino acids joined together is called a polypeptide. A polypeptide chain that contains 50 or more amino acids is called a protein.

Peptide Bonds Peptide bonds link amino acids into unique chains. They form when the carbon from the acid group (COOH) of one amino acid bonds with the nitrogen atom from the amine group (NH2) of another amino acid by a condensation reaction (Figure 6.3), releasing a molecule of water. In contrast, peptide bonds are broken by hydrolysis, which is particu- larly important during digestion. In this process, a molecule of water is used to split the bond, adding the hydroxyl (OH) group to one amino acid and hydrogen to the other.

Essential, Nonessential, and Conditional Amino Acids Nine of the 20 amino acids that the body uses to make protein are classified as essential amino acids. It is essential that the diet provide them because these amino acids either cannot be made by the body or cannot be made in sufficient quantities to sustain the body’s needs.

The remaining 11 amino acids are considered nonessential amino acids because the body can make them. It is not essential to consume them. Table 6.1 lists the 20 known, nutritionally important amino acids by their classification.

amine group Nitrogen-containing compound

(NH2) connected to the central carbon of an amino acid.

side chain Part of an amino acid that

provides its unique qualities; also referred to as

the R group.

peptide Chain of amino acids.

dipeptide Chain of two amino acids joined

together by a peptide bond.

tripeptide Chain of three amino acids joined

together by peptide bonds.

polypeptide Chain consisting of 10 or more

amino acids joined together by peptide bonds.

peptide bonds Bonds that connect amino

acids; created when the acid group of one

amino acid is joined with the amine group of

another through condensation.

essential amino acids Nine amino acids

that the body cannot synthesize; they must be

obtained through dietary sources.

nonessential amino acids Eleven amino

acids the body can synthesize and that therefore

do not need to be consumed in the diet.

▲ Figure 6.2 The Anatomy of an Amino

Acid

NH2

OHC

Side

chain

Amine

group

Carboxyl

(acid)

group

Glycine (Gly)

Phenylalanine (Phe)

Aspartic acid (Asp) NH2

NH2

OHC

COOH

C HH

Amino acid structure. All amino acids

contain a central carbon surrounded by a

side chain, carboxylic acid (COOH), a

hydrogen, and an amine group (NH 2 ).

Different amino acids with their

unique side chains. A unique side chain

(shown in yellow) distinguishes the

various amino acids.

208 Chapter 6 | Proteins

Some nonessential amino acids may become conditionally essential amino acids if the body cannot make them because of illness or because the body lacks the necessary precursors or enzymes. In such situations, the amino acid involved is considered essential and must be consumed. An example of this is when premature infants are not able to make enough of the enzymes needed to synthesize arginine; they need to get this amino acid in their diet.

The Organization and Shape of Proteins Affect

Their Function

Every protein has four different levels of structure: primary, secondary, tertiary, and quaternary; each level must be correct in order for the protein to function (Figure 6.4).

conditionally essential amino

acids Nonessential amino acids that become

essential (and must be consumed in the diet)

when the body cannot make them.

▶ Figure 6.3 Condensation and

Hydrolytic Reactions

A peptide bond forms by condensation when the acid

group (COOH) and amine group of two different amino acids

join and release a molecule of water.

When peptide bonds are broken by hydrolysis, the

hydroxyl group (OH) and hydrogen (H) from water are added.

H2O

NH C

Amino acid Amino acid

Dipeptide

Condensation

Peptide

bond

Hydrolysis

+ OHC

NH C

O CH H

H H

NH C

OHC

N C

O CH H

H H

NH C

OHC

N C

O CH H

OHC

NH C

OHC

NH C

O CH H

Essential Amino Acids

Nonessential Amino Acids (Conditionally

Essentialb Amino Acids in Italics)

Histidine (His)a

Isoleucine (Ile)

Leucine (Leu)

Lysine (Lys)

Methionine (Met)

Phenylalanine (Phe)

Threonine (Thr)

Tryptophan (Trp)

Valine (Val)

Alanine (Ala)

Arginine (Arg) Asparagine (Asn)

Aspartic acid (Asp)

Cysteine (Cys) Glutamic acid (Glu)

Glutamine (Gln) Glycine (Gly) Proline (Pro) Serine (Ser)

Tyrosine (Tyr) aHistidine was once thought to be essential only for infants. It is now known that small amounts are also needed for adults.

bThese amino acids can be “conditionally essential” if there are either inadequate precursors or inadequate enzymes

available to create these in the body.

TABLE 6.1 The Mighty Twenty

What Are Proteins? 209

The primary structure is the order in which the amino acids are assembled and the total length—up to thousands of amino acids—of the chain. The amino acids are held together by peptide bonds in a sequence that is unique to that protein. The gene that codes for that protein determines the sequence. A change in just one amino acid in the sequence results in a dramatic change in the eventual shape of the protein and, therefore, its function, in the same way that a telephone number with digits out of order or missing will no longer work.

Once the sequence is formed, the amino acids either attract and form bonds with one another or repel each other. The formation of hydrogen bonds between the carboxyl and amine groups of amino acids in the chain creates the secondary structure. The hydrogen bonding causes the straight chain to fold, twist, and coil.

Side chains—especially sulfur-containing side chains—can be attracted to (hydro- philic) or repelled by (hydrophobic) water in the cells. This property affects how they interact

primary structure First stage of protein

synthesis after transcription when the amino

acids have been linked together with peptide

bonds to form a simple linear chain.

secondary structure Shape of a protein in

which hydrogen bonding between carboxyl

and amine groups has caused the straight

chain to fold and twist.

◀ Figure 6.4 The Organization and

Shape of Proteins

The primary structure of a

protein consists of the length

and the sequence of amino

acids in the chain.

The hydrogen bonds

between the carboxyl and

amine groups cause the

straight chain to fold and

twist, forming the secondary

structure of a protein.

The tertiary structure

results when the side chains

of amino acids, especially

the sulfur-containing side

groups, bond together,

resulting in more bends and

folding.

When two or more

polypeptide chains join, they

form the quaternary structure

of a protein. In the molecule

of hemoglobin shown here,

heme is also included.

Glycine (Gly)

NH 2

OHC

Aspartic acid (Asp)

NH 2

OHC

COOH

C HH

NH 2

Phenylalanine (Phe)

OHC

C HH

Gly Asp Phe

A polypeptide

Peptide bond

Heme group

210 Chapter 6 | Proteins

with their environment. Hydrophobic side chains cluster together on the inside of the protein, combining to form a globular shape called the tertiary structure. Hydrophilic side chains assemble on the outside of the protein and interact with the watery portion of blood and other body fluids.

Finally, the quaternary structure of a protein forms when two or more polypep- tide chains bond together with a hydrogen bond, or when there is a reaction between the sulfur-containing side chains of the amino acids methionine and cysteine. A good example of a quaternary structure of a protein is hemoglobin (Figure 6.4d). Notice that the iron-containing heme groups shown in red in the illustration assist in binding oxygen and are not part of the normal quaternary structure of proteins.

Denaturation of Proteins Changes Their Shape

Heat, acids, bases, salts, or mechanical agitation can unfold, or denature, proteins (Figure 6.5). Denaturation doesn’t alter the primary structure of the protein (the amino acids stay in the same sequence), but does change the shape.

You can see denaturation in action when you cook an egg. When you apply heat to a raw egg, such as by frying it, the heat denatures the protein in both the yolk and the egg white. Heat disrupts the bonds between the amino acid side chains, causing the protein in the egg to uncoil. New bonds then form between the side chains, changing the shape and structure of the protein and the appearance and texture of the egg.

Similarly, mechanical agitation, such as beating egg whites when you prepare a meringue, can denature protein. Beating an egg white uncoils the protein, allowing the hydrophilic side chains to react with the water in the egg white, while the hydrophobic portions of the side chains form new bonds, trapping the air from the beating. The stiffer the peaks of egg white, the more denatured the protein. Whether you cook an egg, or whip egg whites, the change in the protein’s shape and structure is permanent because new bonds between the amino acids have been formed.

Salts and acids can also denature proteins. For example, when you marinate a chicken breast or a steak before cooking, you might use salt (such as in soy sauce) or acid (such as wine or vinegar), which denatures its protein. The end result is juicier, more tender meat. During digestion, acidic stomach juices help denature and untangle proteins to reveal the peptide bonds. This allows digestive enzymes to break the bonds apart.

tertiary structure Protein structure that

occurs when the side chains of the amino

acids, most often containing sulfur, form bonds

resulting in loops, bends, and folds in the

molecule.

quaternary structure Rod-like or globular

structure of a protein formed when two or

more polypeptide chains cluster together.

denature To alter a protein’s secondary,

tertiary, or quaternary structure, thereby

disabling its function; the amino acids of the

primary structure remain linked together by

peptide bonds.

▲ Figure 6.5 Denaturing a Protein

A protein can be denatured, or unfolded,

by exposure to heat, acids, or salts or by

mechanical agitation. Any change in a pro-

tein’s shape will alter its function.

Heat, acids, salts,

and mechanical agitation

Normal protein

Denatured protein

LO 6.1: THE TAKE-HOME MESSAGE Proteins are chains of amino acids linked together with peptide bonds. Amino acids, which contain carbon, hydrogen,

oxygen, nitrogen, and, in some cases, sulfur, are composed of a central carbon

with a carboxyl group, a hydrogen, a nitrogen-containing amine group, and a

unique side chain. There are 20 different side chains and therefore 20 unique

amino acids. Eleven are classified as nonessential and nine are classified

as essential. Under certain circumstances, some nonessential amino acids

become conditionally essential. Interactions between the side chains cause the

protein to fold into a precise three-dimensional shape that determines its func-

tion. Heat, mechanical agitation, acids, bases, and salts can denature a protein

and alter its shape and function.

Whipping egg whites denatures the protein.

What Are the Key Steps in Digesting and Absorbing Protein? 211

What Are the Key Steps in Digesting and Absorbing Protein?

LO 6.2 Identify the key steps in digesting proteins and absorbing amino acids.

When you eat a peanut butter sandwich, what happens to the protein in the peanut butter and the whole-wheat bread after it has been chewed and swallowed? How do proteins in food become body proteins?

Protein Digestion Begins in the Stomach

The peanut butter sandwich is prepared for digestion in the mouth, where teeth tear and shred the food, breaking the sandwich into smaller pieces while mixing it with saliva (Focus Figure 6.6). This mechanical digestion helps make the food easy to swallow and is the only digestion of protein that takes place in the mouth. No chemical or enzymatic digestion of proteins occurs in the mouth.

After you eat a meal, the hormone gastrin directs the release of hydrochloric acid (HCl) from the parietal cells in the stomach wall. At the same time, gastrin directs the release of pepsinogen, an inactive protein enzyme, from the chief cells. Once the bolus enters the stomach, HCl begins to denature the protein strands. HCl also converts the pepsinogen to the active enzyme pepsin, which begins breaking the polypeptides into shorter chains via hydrolysis. As part of chyme, they are then propelled into the small intestine. Table 6.2 provides a complete list of enzymes that participate in protein digestion.

Protein Digestion Continues in the Small Intestine

When the protein-rich chyme reaches the small intestine, the intestinal cells release the hormone cholecystokinin into the blood. This hormone stimulates the pancreas to secrete proteases such as trypsin, chymotrypsin, and carboxypeptidase, through the pancreatic duct into the small intestine (see Table 6.2). Trypsin and chymotrypsin con- tinue to break apart the peptide bonds in the center of the polypeptide chain, resulting in smaller and smaller peptide chains. The enzyme carboxypeptidase breaks apart the first peptide bond closest to the carboxylic end of the chain. What started out in the peanut butter as a very large protein molecule has now been reduced to tripeptides and dipeptides. Dipeptidases and tripeptidases help break these small peptide chains into single amino acids.

Amino Acids Are Absorbed in the Small Intestine

The single amino acids are absorbed into and pool inside the enterocytes, where they can be used for energy or to synthesize new compounds. Most enter the bloodstream and are transported via the portal vein to the liver. After reaching the liver, amino acids can be used to synthesize new proteins or can be converted to adenosine triphosphate (ATP), glucose, or fat. When other cells need them, the liver releases amino acids into the blood- stream and they are transported throughout the body.

Almost all dietary proteins are digested, absorbed, and transported via the portal vein as single amino acids. However, newborns have a limited ability to absorb whole proteins—for example, the antibodies in breast milk—intact.

212 Chapter 6 | Proteins

Head to Mastering Nutrition and watch a narrated video tour of this figure by author Joan Salge Blake.

Figure 6.6 Protein Digestion and AbsorptionFOCUS

Produces proteases that are

released into the small intestine via

the pancreatic duct.

Uses some amino acids to make

new proteins or converts them to

glucose. Most amino acids pass

through the liver and return to

the blood to be picked up and

used by body cells.

PANCREAS

LIVER

MOUTH

STOMACH

SMALL INTESTINE

Protein digestion begins in the stomach with the aid of hydrochloric acid (HCl) and the enzyme pepsin. Proteases continue the digestion in the small intestine, releasing single amino acids to be absorbed into the portal vein for delivery to the liver.

ORGANS OF THE GI TRACT ACCESSORY ORGANS

Mechanical digestion of protein begins with

chewing, tearing, and mixing food with salivary

juices to form a bolus.

Hydrochloric acid denatures protein

and activates pepsinogen to form

pepsin.

Pepsin breaks the polypeptide chain

into smaller polypeptides.

Proteases continue to cleave peptide

bonds, resulting in dipeptides,

tripeptides, and single amino acids.

Tripeptidases and dipeptidases on the

surface of the enterocytes finish the

digestion to yield single amino acids,

which can then be absorbed into the

bloodstream and travel through the

portal vein to the liver.

Amino acids

Capillary

Enterocytes

How Are Amino Acids Metabolized? 213

How Are Amino Acids Metabolized?

LO 6.3 Explain the metabolism of amino acids and the role of the amino acid pool.

How the liver metabolizes newly absorbed amino acids depends on the needs of the body. For example, amino acids might be used to replace old proteins or synthesize new ones or, if necessary, they may be used as an energy source. If an individual is not eating suf- ficient carbohydrates, amino acids can be converted to glucose through gluconeogenesis, as discussed in Chapter 4. However, most amino acids travel back out to the blood to be picked up and used by cells.

Amino Acid Pools Allow Protein Synthesis on Demand

Proteins don’t last indefinitely. The daily wear and tear on the body causes the breakdown of hundreds of grams of proteins each day. For example, the protein-rich cells in the skin are constantly sloughed off, and proteins help create a new layer of outer skin every 25 to 45 days.3 Because red blood cells have a short lifespan—only about 120 days—new red blood cells must be continually regenerated. The cells that line the inner surfaces of the organs, such as the lungs and intestines, are recycled and replaced every 3 to 5 days, thanks to protein synthesis.

In addition to regular maintenance, extra protein is sometimes needed for emergency repairs. Protein is essential in healing, and a person with extensive wounds, such as severe burns, may have dietary protein needs that are more than triple his or her normal needs.

Newly absorbed amino acids collect in limited amounts in amino acid pools found in the blood and inside cells. When cellular proteins are degraded or broken down into

amino acid pools Limited supplies of amino

acids that accumulate in the blood and cells;

amino acids are pulled from the pools and

used to build new proteins.

Digestive Enzyme Where Released Purpose

Pepsinogen From chief cells in the stomach

(activated to pepsin by HCl)

Breaks apart polypeptides into shorter polypeptide chains

Trypsin From pancreas into small intestine Breaks apart peptide bonds

Chymotrypsin From pancreas into small intestine Breaks apart peptide bonds

Carboxypeptidase From pancreas into small intestine Breaks free one amino acid at a time from the carboxyl end of a

peptide chain

Aminopeptidase Brush border of the small intestine Breaks free the end amino acids from tri- and dipeptides into single

amino acids

Tripeptidase Brush border of the small intestine Breaks tripeptides into single amino acids

Dipeptidase Brush border of the small intestine Breaks dipeptides into single amino acids

TABLE 6.2 Enzymes Involved in Protein Digestion

LO 6.2: THE TAKE-HOME MESSAGE Chemical digestion of protein begins in the stomach. Gastrin stimulates the release of HCl from the parietal cells and the

inactive enzyme pepsinogen from the chief cells. HCl denatures the protein

and converts pepsinogen to pepsin, which breaks polypeptides into shorter

chains. Cholecystokinin from the duodenum stimulates release of trypsinogen,

carboxypeptidase, and chymotrypsinogen from the pancreas. These prote-

ases hydrolyze the shorter chains into tripeptides and dipeptides. Dipeptidases

and tripeptidases hydrolyze the tripeptides and dipeptides into single amino

acids that are absorbed through the enterocytes via the portal vein to the liver.

Absorbed amino acids are used to synthesize new proteins or are converted to

nonessential amino acids, ATP, glucose, or fat.

214 Chapter 6 | Proteins

▲ Figure 6.7 Metabolic Fate of Amino Acids

Once in the amino acid pool, most amino acids are used for protein synthesis. Under certain conditions, amino acids can be used for

gluconeogenesis or energy production, or converted to fatty acids and stored in fat cells.

ATP

Amino acid pool

Protein turnover:

Proteins are constantly made

and broken down, releasing their

amino acids into the amino acid

pool or using the amino acids for

protein synthesis and other

important compounds such as

niacin and serotonin.

Gluconeogenesis:

Amino acids can be used

to make glucose when

glucose is limited.

Energy production:

Amino acids can be used

for energy when the diet is

deficient in kilocalories.

Fat cells:

Amino acids can be converted

to fatty acids and stored as a

triglyceride in the adipose tissue

when kilocalorie intake is sufficient.

their component parts, the resulting amino acids also enter the amino acid pools. The body can then use the amino acids in the pool to create proteins on demand. This process of degrading and synthesizing protein is called protein turnover (Figure 6.7). More than 200 grams of protein are turned over daily. The proteins in the intestines and liver— two tissue types with rapid degradation and resynthesis rates—account for as much as 50 percent of this turnover. Some of the amino acids in the pools are also used to syn- thesize nonproteins, including thyroid hormones, which contain iodine, and melanin, a complex pigment that gives color to dark skin and hair.

Protein Synthesis Is Regulated by Genes

When the cell is ready to synthesize new or repair old proteins, it uses the instructions for assembly coded in the genes that you inherited from your parents. Unless you have an identical twin, your specific genotype or genetic makeup is unique to you.

Genes are segments of deoxyribonucleic acid (DNA), which is stored in the nucleus of most body cells. DNA is a two-stranded compound. Each strand is made of combinations of four bases (nucleotides): adenine (A), thymine (T), guanine (G), and cytosine (C). Bases on opposite strands of DNA pair specifically: an A always pairs with a T and a C always pairs with a G. Within a gene, each group of three nucleotide bases codes for a single amino acid. For example, A-T-G codes for the amino acid methionine. In this way, the order of the As, Ts, Cs, and Gs that make up a gene code for the precise protein to be synthesized.

The body is made up of over 100,000 different proteins. When the body requires a protein, the cells receive a signal to begin the process of protein synthesis. This signal is communicated to a cell receptor by hormones, cell-to-cell contact, or neurotransmitters. Cells can “turn on” and “turn off “ protein synthesis as needed.

Because the DNA cannot leave the nucleus of the cell and protein is synthesized out- side the nucleus in the cytoplasm, a gene’s code must be copied and delivered to the cyto- plasm. There, cellular organelles called ribosomes translate the message and assemble the exact sequence of amino acids described in the code. Let’s walk through the process of protein synthesis (Focus Figure 6.8).

protein turnover Continual process of

degrading and synthesizing protein.

genes A segment of DNA that codes for a

protein; genes are inherited from our parents

and determine a variety of characteristics.

ribosomes Organelles found in the cytoplasm

that read the mRNA and build the protein in

the proper sequence during elongation.

How Are Amino Acids Metabolized? 215

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Figure 6.8 Protein SynthesisFOCUS

Protein synthesis is the process by which the DNA code within a cell’s nucleus is transcribed and translated to produce specific proteins.

mRNA

Completed

Nucleus

TRANSCRIPTION

TRANSLATION

ELONGATION

Nucleus

Cell

Cytoplasm

Ribosome

DNA unwinds

1 In the nucleus, DNA unwinds to allow a copy of the code, called messenger RNA (mRNA) to be made. This process is called transcription.

2 The mRNA leaves the nucleus and travels to the cytoplasm. 2

3 Once the mRNA reaches the cytoplasm, it binds to a ribosome. 3

4 Translation is initiated as the ribosome moves along the mRNA, reading the code. Transfer RNA (tRNA) brings specific amino acids to the ribosome based on the code.

5 The process of elongation occurs as translation continues. The ribosome builds a chain of amino acids (the protein) in the proper sequence, based on the code in the mRNA.

6 Translation and elongation are terminated when all the appropriate amino acids are added and the protein is complete. The protein is released from the ribosome.

Amino acids

tRNA

216 Chapter 6 | Proteins

1. Transcription. The first step in making a new polypeptide chain is to transcribe (or copy) the DNA into ribonucleic acid (RNA). This is somewhat similar to transcribing Chinese characters into English words. The information transcribed is the nucleotide sequence, which as noted earlier determines the amino acids in the protein chain. The bonds between the two strands of DNA break, the strands unwind, and the nucleotide code is transcribed into an RNA molecule called messenger RNA (mRNA).

2. Translation. Once the code has been transcribed from DNA to mRNA, the new mRNA detaches from the DNA, leaves the nucleus, enters the cytoplasm, and attaches to a ribosome. The ribosome moves along the mRNA, reading the nucleo- tide instructions recorded in the mRNA. The ribosome then sends a second type of RNA, called transfer RNA (tRNA), to collect the corresponding amino acid from the cytoplasm and transport it to the ribosome. Note that 20 unique tRNAs exist, each of which can bind to one and only one of the 20 different amino acids. This gathering and building step is called elongation and continues until the full sequence of amino acids has been completed and a new protein is released.

When the sequence of amino acids is incorrect, abnormalities occur and medical conditions can result. One such condition is sickle cell anemia. The most common inherited blood disorder in the United States, sickle cell anemia is caused by an abnormal variant of the gene that codes for the assembly of the protein hemoglobin. The displace- ment of just one amino acid, glutamine, with another amino acid, valine, in polypeptide chains of hemoglobin makes the chains likely to stick to one another and form crescent- shaped blood cells. Whereas red blood cells with normal hemoglobin are smooth and round, those with this mutation are stiff and form a sickle shape under certain conditions, such as after vigorous exercise, when oxygen levels in the blood are low. These abnormal sickle cells are recognized and destroyed by the immune system, causing anemia; more- over, because of their shape, they can build up in blood vessels, causing painful blockages and damage to tissues and organs. Approximately one in 12 African Americans and one in 100 Hispanic Americans are carriers of the mutated gene that causes the disease.4

Deamination Removes the Amine Group from Amino Acids

What happens if amino acids in the pool are not used for protein synthesis? As the amino acid pool reaches capacity, amino acids that are not used to build proteins are broken down into their component parts. These component parts are used for other purposes, such as energy production, or stored as triglycerides.

Before amino acids can be used for energy production or converted to other com- pounds, the amine group must be removed and converted to ammonia (NH3) in a process called deamination (Figure 6.9a). Because ammonia in high amounts can be toxic to cells, the ammonia is sent through the bloodstream to the liver, where it is quickly con- verted to urea, CH4N2O, a waste product that is released into the blood, filtered out by the kidneys, and eventually excreted in urine. Once the nitrogen has been removed, the carbon-containing remnants of the amino acids are eventually converted to glucose, used as energy, or stored as fat, depending on the needs of the body.

Nonessential Amino Acids Are Synthesized through

Transamination

As you learned earlier in this chapter, nonessential amino acids can be made in the body when needed or not present in your diet. These amino acids can be made from the nitro- gen provided by the amine group of an amino acid or by ammonia, and another com- pound referred to as a keto acid (Figure 6.9b). In this process, called transamination, the

translation Second phase of protein

synthesis; the process of converting the

information in mRNA to an amino acid

sequence in the ribosomes.

transcription First stage in protein synthesis,

in which the DNA sequence is copied from the

gene and transferred to messenger RNA.

transfer RNA (tRNA) Type of RNA that

transfers a specific amino acid to a growing

polypeptide chain in the ribosomes during

protein synthesis.

messenger RNA (mRNA) Type of RNA

that copies the genetic information from the

DNA and carries it from the nucleus to the

ribosomes in the cell.

elongation Phase of protein synthesis in

which the polypeptide chain grows longer by

adding amino acids.

sickle cell anemia Blood disorder caused by

a genetic defect that results in the synthesis

of hemoglobin S, which makes the red blood

cells likely to distort into a sickle shape.

deamination Removal of the amine group

from an amino acid.

urea Nitrogen-containing waste product of

protein metabolism that is mainly excreted

through the urine via the kidneys.

transamination Transfer of an amino group

from one amino acid to a keto acid to form a

new nonessential amino acid.

Red blood cells with normal hemoglobin

are smooth and round, like the three red

doughnut-shaped cells. A person with sickle

cell anemia has red blood cells that are stiff

and form a sickle (half-moon) shape, like

the orange cell, when blood oxygen levels

are low.

What Are the Functions of Protein in the Body? 217

liver transfers the amine group to the keto acid, creating a nonessential amino acid and a different keto acid. This reaction requires vitamin B6 and will be discussed in greater detail in Chapter 8.

Excess Protein Is Converted to Body Fat

If you add too much water to a swimming pool, the excess overflows. The same is true of an amino acid pool. When the diet contains sufficient carbohydrates, and protein intake exceeds requirements, the amino acid pool becomes saturated. The “overflow” amino acids are deaminated, and the remaining carbon remnants are converted to fatty acids and stored as triglycerides in adipose tissue.

◀ Figure 6.9 Deamination

and Transamination

NH 3 (Ammonia)

Urea

Deamination: In the liver, the amine group is removed,

producing ammonia and a keto acid. The ammonia is used to

form urea, which is excreted in the urine.

Transamination: An amine group from an essential amino

acid is transferred to a keto acid, producing a nonessential

amino acid and a new keto acid.

OHC

H H

NH C

OHH C

Amino acid

OHC

NH C

Amino acid A

OHC

NH C

Amino acid B

Keto acid

OHHO C C

O O

Oxaloacetate

OHHO C C

H H

Alpha-ketoglutarate

+ +

Side

chain

Side

chain

Side

chain A

Side

chain BH

LO 6.3: THE TAKE-HOME MESSAGE During digestion, proteins are broken down into amino acids with the help of gastric juices, enzymes in the stomach and

small intestine, and enzymes from the pancreas and small intestinal lining.

A limited supply of amino acids exists in the amino acid pools, which act as

a reservoir for protein synthesis. Surplus amino acids are deaminated, with

the carbon-containing remnants used for glucose or energy or stored as fat,

depending on the body’s needs. The nitrogen in the amine groups is eventually

converted to the waste product urea and excreted in urine. Nonessential amino

acids are synthesized through transamination.

What Are the Functions of Protein in the Body?

LO 6.4 Describe the functions of protein in the body.

Proteins play many important roles in the body, from providing structural and mechani- cal support and maintaining body tissues to functioning as enzymes and hormones and helping maintain acid–base, and fluid balance. They also transport nutrients, assist the

218 Chapter 6 | Proteins

albumin Protein produced in the liver and

found in the blood that helps maintain fluid

balance.

immune system, and, when necessary, are a source of energy. Let’s examine each of these vital functions in more depth.

Proteins Provide Structural Support and Enable

Movement

Proteins provide much of the structural and mechanical support that keeps the body upright, moving, and flexible. Collagen, the most abundant pro- tein in the body, is found in all connective tissues, including bones, tendons, and ligaments, which support and connect joints and other body parts. This fibrous protein is also responsible for the skin’s elasticity and forms the scar tissue necessary to repair injuries.

The proteins actin and myosin enable movement by contracting mus- cle fibers. They are also involved in nonmuscle movement, such as when cells divide during mitosis or when chemicals are transported along actin filaments in the cell cytoplasm.

Proteins Act as Catalysts

Recall that enzymes are biological catalysts that speed up reactions. Most enzymes are proteins, although to be activated, some may also need a coenzyme, such as a vitamin. Without enzymes, reactions would occur so slowly that you couldn’t survive.

Each of the thousands of enzymes in the body catalyzes a specific reaction. Some enzymes, such as digestive enzymes, are catabolic enzymes: They break compounds apart. The enzyme lactase is needed to break down the milk sugar lactose (see Chapter 4). Other enzymes are anabolic, and build substances. For example, the anabolic enzyme glycogen synthetase converts excess glucose units one by one into a chain of glycogen that is then stored. Enzymes aren’t changed, damaged, or used up in the process of speeding up a particular reaction (see Figure 3.10 on page 87). Thus, an enzyme is available to catalyze additional reactions.

Proteins Act as Chemical Messengers

Recall that hormones are chemical messengers with regulatory functions. There are over 70 trillion cells in the body, and all of these cells interact with at least one of over 50 known hormones.5 Although some hormones are made from cholesterol, many are proteins or peptides (amino acid based). Familiar examples include insulin and glucagon, which regu- late blood glucose levels, and leptin and ghrelin, which regulate appetite. Antidiuretic hor- mone (ADH) contributes to blood pressure regulation, whereas growth hormone (GH) supports childhood development and many other body functions.

Proteins Help Regulate Fluid Balance

The body is made up predominantly of water, which is distributed both outside (extra- cellular) and inside (intracellular) the cells. Fluid can generally flow easily in and out of cells. However, proteins are too large to move across the cell membranes and thus stay either within the cells or outside in the extracellular fluid. Normally, blood pressure forces nutrient- and oxygen-rich fluids out of capillaries (the smallest blood vessels of the body) and into the spaces between the cells (called interstitial spaces). But proteins, including an important blood protein called albumin, remain in the blood. As fluid is forced out of the blood with each heartbeat, the concentration of albumin increases, drawing the fluid from the interstitial spaces back into the blood by osmosis. (Osmosis is described in detail in Chapter 11.) Hence, albumin and other blood proteins play an important role in keeping body fluids dispersed evenly inside and outside of cells, helping to maintain a state of fluid

catabolic Energy-releasing process that

breaks larger molecules into smaller parts.

anabolic Energy-requiring process in which

smaller molecules are combined to form larger

molecules.

Proteins play an important role in keeping skin healthy and

nails strong.

What Are the Functions of Protein in the Body? 219

balance. (Note: Sodium and other minerals, discussed in Chapter 12, also play a major role in fluid balance.)

When fewer proteins are available to draw the fluid from between the cells back into the bloodstream, as during severe malnutrition, a fluid imbalance results. The interstitial spaces between the cells become bloated and the body tissue swells, a condition known as edema (Figure 6.10).

Proteins Help Regulate Acid–Base Balance

Proteins can alter the pH of the body fluids. Normally, the blood has a pH of about 7.4. Even a small change in the pH of the blood in either direction can be harmful or even fatal. A blood pH below 7.35, a condition called acidosis, can result in a coma. A blood pH above 7.45, known as alkalosis, can cause convulsions.

Proteins act as buffers and minimize the changes in acid–base levels by picking up hydrogen ions in the blood or donating hydrogen ions to the blood. Should the blood become too basic (contain too few hydrogen ions), the carboxyl groups of amino acids lower the pH of the blood by donating hydrogen ions; if blood becomes too acidic (too many hydrogen ions), the amine groups bind the excess hydrogen ions and restore the pH to an optimal level. This dual buffering role helps maintain the acid–base balance in the cells and the blood.

Proteins Transport Substances throughout the Body

Transport proteins shuttle oxygen, waste products, lipids, some vitamins, sodium and potassium, and other substances through the blood and across cell membranes. For example, hemoglobin is a transport protein that carries oxygen to cells from the lungs; hemoglobin also picks up carbon dioxide for delivery to the lungs to be exhaled. Lipoproteins are another example; they transport fat-soluble nutrients through the bloodstream (see Chapter 5).

Some nutrients, such as vitamin A, are fat soluble and need assistance to move through the water-based blood. Vitamin A attaches to the blood protein albumin for transport to the liver and other cells. Essential minerals—for example, iron and zinc—have special- ized transport proteins whose sole function is to escort them across the enterocytes.

Channel proteins in cell membranes form passageways that allow ions such as sodium and potassium to pass in and out of cells (Figure 6.11). In contrast, carrier proteins change their shape to allow the entry of substances such as glucose into cells. Without channel and carrier proteins, cells would be unable to maintain an optimal concentration of such nutrients or remove waste from the cell. If your diet is deficient in essential amino acids,

edema Accumulation of excess water in the

spaces surrounding the cells, which causes

swelling of the body tissue.

acidosis Condition in which the blood pH is

too low, generally due to excessive hydrogen

ions.

alkalosis Condition in which the blood

pH is too low due to a low concentration of

hydrogen ions.

buffers Substances that help maintain

the proper pH in a solution by accepting or

donating hydrogen ions.

transport proteins Proteins that carry other

substances, mainly nutrients, through the

blood to various organs and tissues. Proteins

can also act as channels through which some

substances enter your cells.

▲ Figure 6.10 Edema

Inadequate protein in the blood can cause

edema.

◀ Figure 6.11 Proteins as Ion Channels

Channel proteins form tunnels through which

ions such as sodium and potassium can

move from one side of the cell membrane to

the other.

Outside cell

Channel

protein

Inside cell

Potassium

Sodium

Sodium binds to

channel protein

Channel protein

releases sodium

outside of cell

Potassium binds

to channel protein

Channel protein

releases potassium

inside the cell

220 Chapter 6 | Proteins

fewer such proteins are produced, causing an unhealthy balance of nutrients inside and outside of the cell membrane.

Proteins Contribute to a Healthy Immune System

The immune system protects the body from pathogens. Once pathogens, including bac- teria and viruses, enter the cells, they can multiply rapidly, eventually causing illness. An army of specialized protein “soldiers,” called antibodies, quickly recognize and assist in the destruction of pathogens before they have a chance to multiply.

Once the body knows how to create antibodies against a specific foreign substance, such as a particular virus, it stores that information, giving the body immunity to that pathogen. The next time the invader enters the body, the body can respond very quickly (producing up to 2,000 precise antibodies per second!) to fight it.

Sometimes, the body incorrectly perceives a nonthreatening substance, typically a protein, as harmful and attacks it. A substance of this type is called an allergen. Proteins in certain foods, such as peanuts, wheat, and eggs, commonly act as allergens.Food aller- gies are discussed in detail in Chapter 17.

Proteins Can Provide Energy

Because proteins provide 4 kilocalories per gram, they can be used as an energy source. After amino acids are deaminated, the remaining carbon remnants can enter the energy cycle to produce ATP. (We cover this process in depth in Chapter 8.)

When an individual eats too few kilocalories or carbohydrates, the stores of glycogen in the liver and muscle become depleted and blood glucose levels drop. To raise blood glucose levels, the body turns to specific amino acids called glucogenic amino acids. These amino acids are converted to glucose through gluconeogenesis. (Remember that the red blood cells and brain need glucose to function properly.)

However, the last thing you want to do is use this valuable nutrient, which plays so many important roles in the body, as a regular source of fuel because carbohydrates and fats are far better suited to providing energy. When the diet contains adequate amounts of kilocalories from carbohydrates and fat, proteins are spared and used for their more important roles. For optimal health, individuals need to eat enough protein daily to meet the body’s needs and enough carbohydrates and fats to prevent protein from being used as energy.

Table 6.3 summarizes the many structural and functional roles proteins play in the body.

Certain white blood cells of the immune sys-

tem (see upper right) produce antibodies,

proteins that defend against harmful agents

such as the Staphylococcus bacteria shown

(in yellow) in this photo. (The red cells are red

blood cells.)

Role of Protein How It Works

Structural and mechanical support

and maintenance

Proteins are the body’s building materials, providing strength and flexibility to tissues, tendons,

ligaments, muscles, organs, bones, nails, hair, and skin. Proteins are also needed for the ongoing

maintenance of the body.

Enzymes and hormones Proteins are needed to make most enzymes that speed up reactions in the body and many hor-

mones that direct specific activities, such as regulating blood glucose levels.

Fluid balance Proteins play a major role in ensuring that body fluids are evenly dispersed in the blood and

inside and outside cells.

Acid–base balance Proteins act as buffers to help keep the pH of body fluids within a tight range. A drop in pH will

cause body fluids to become too acidic, whereas a rise in pH can make them too basic.

Transport Proteins shuttle substances such as oxygen, waste products, and nutrients (such as sodium and

potassium) through the blood and into and out of cells.

Antibodies and the immune response Proteins create specialized antibodies that attack pathogens that may cause illness.

Energy Because proteins provide 4 kilocalories per gram, they can be used as fuel or energy.

Satiety Protein increases satiety, which can help control appetite and weight.

TABLE 6.3 The Many Roles of Proteins

How Much Protein Do You Need Daily? 221

Protein Improves Satiety and Appetite Control

In addition to its structural and functional roles, protein also improves satiety after a meal more than either carbohydrate or fat.6 Eating a meal that contains a good source of protein will leave you more satisfied than will a high-carbohydrate meal with the same number of kilocalories. The satiety following a high-protein meal may be due to dietary protein suppressing the release of ghrelin.7 Recall that ghrelin, which is produced in the stomach, stimulates the hypothalamus to sense hunger. Including protein in each meal helps to control appetite, which in turn can help maintain a healthy weight.

antibodies Proteins that identify and

participate in the destruction of pathogens as

part of the body’s immune response.

immunity State of having built up memory

immune cells that target a particular pathogen

so that any subsequent encounter with that

pathogen prompts rapid production of specific

antibodies.

allergen Substance, such as wheat protein,

that causes an allergic reaction.

glucogenic amino acids Amino acids

that can be used to form glucose through

gluconeogenesis.

LO 6.4: THE TAKE-HOME MESSAGE Proteins play many important roles in the body: (1) synthesizing, repairing, and maintaining structural tissues; (2) helping

facilitate muscular contraction; (3) catalyzing reactions as enzymes; (4) acting as

hormones; (5) maintaining fluid balance; (6) maintaining acid–base balance; (7)

transporting nutrients throughout the body; (8) providing antibodies for a strong

immune system; (9) providing energy when kilocalorie intake doesn’t meet daily

energy needs; and (10) promoting satiety and appetite control.

How Much Protein Do You Need Daily?

LO 6.5 Calculate the daily amount of protein recommended based on the Dietary Reference Intakes.

Healthy, nonpregnant adults should consume enough dietary protein to replace the amount they use each day. However, pregnant women, people recovering from surgery or an injury, and growing children need more protein to supply the necessary amino acids and nitrogen to build new tissue. Nitrogen balance studies are often used to determine how much protein individuals need to replace or build new tissue.

Healthy Adults Should Be in Nitrogen Balance

A person’s protein requirement can be estimated by using what we know about the struc- ture of an amino acid. We know that 16 percent of every dietary protein molecule is nitrogen. And we know that the body retains this nitrogen during protein synthesis. It follows that we can assess a person’s protein status by checking their nitrogen balance— measuring the amount of nitrogen they consume and subtracting the amount of nitrogen they excrete, mostly as urea. The goal is to achieve nitrogen balance. See the Calculation Corner for an example of the calculation done to assess nitrogen balance.

nitrogen balance Difference between

nitrogen intake and nitrogen excretion.

Calculation Corner

Nitrogen Balance

(a) Using the fact that protein is 16 percent nitrogen, a common factor of 6.25 is used to calculate

how much nitrogen is in a given amount of food (100/16 = 6.25). We analyzed the food and beverages from a meal and found that it contained 73 grams (g) of

protein. Divide this by 6.25 to determine the nitrogen content of the meal:

Nitrogen = 73 g protein

6.25 g nitrogen = 11.6 g nitrogen

222 Chapter 6 | Proteins

Once we know the amount of nitrogen consumed, we can compare that to the amount of nitrogen excreted to determine if an individual is in nitrogen balance (Figure 6.12). If the nitrogen intake from dietary protein is equivalent to the amount of nitrogen excreted as urea in the urine, then a person is in nitrogen balance. Healthy, nonpregnant adults are typically in nitrogen balance.

A body that retains more nitrogen than it excretes is in positive nitrogen balance. Rapidly growing babies, children, and adolescents are all in positive nitrogen balance. They excrete less nitrogen than they take in because their bodies incorporate nitrogen

(b) Now calculate the amount of nitrogen lost from the body. First, nitrogen is lost in the

urine as urea nitrogen, and in other nitrogen sources that are not part of the urea mol-

ecule. Because it’s difficult to account for the nonurea sources directly, a factor of

0.2 grams * urinary urea nitrogen (UUN) is used to determine these losses. In this example, let’s assume we analyzed the urine and found 8 grams of UUN. The total nitrogen lost in the urine would

be calculated as follows:

8 g UUN + (0.2 * 8 g UUN) = 9.6 g nitrogen lost

approximately 2 grams per day. Add this to the equation.

8 g UUN + (0.2 * 8 g UUN) + 2 g = 11.6 g nitrogen lost

(d) Now let’s put it all together with the equation:

Nitrogen balance = nitrogen in - nitrogen out Nitrogen balance = (73 g protein/6.25) - (8 g UUN + [0.2 * 8 g UUN] + 2 g)

= 0 g of nitrogen

The calculation for nitrogen balance can be

useful to dietitians as well as researchers to

determine protein requirements.

▶Figure 6.12 Nitrogen Balance

and Imbalance

Positive

nitrogen

balance

Nitrogen intake

Nitrogen

excretion

Equilibrium

Negative

nitrogen

balance

Pregnant women,

growing children

and adolescents,

and some athletes

tend to be in

positive nitrogen

balance.

A healthy adult is

typically in nitrogen

equilibrium.

An individual who

is experiencing a

medical trauma or

not eating a

healthy diet is

often in negative

nitrogen balance.

Nitrogen

excretion

Nitrogen

excretion

Go to Mastering Nutrition and complete a Math Video activity similar to the problem in this Calculation Corner.

How Much Protein Do You Need Daily? 223

into new tissues as they grow, build muscles, and expand their supply of red blood cells. When a woman is pregnant, she, too, is in positive nitrogen balance because her body is building a robust baby.

An individual is in negative nitrogen balance when nitrogen losses are greater than nitrogen intake. This occurs immediately following surgery, when fighting an infection, or when experiencing severe emotional trauma. These situations all increase the body’s need for both kilocalories and protein. If the kilocalories and protein in the diet are inad- equate to cover these increased demands, then proteins from tissues are broken down to meet the body’s needs.

You Can Determine Your Own Protein Needs

There are two ways to determine whether or not your protein intake falls within recom- mended levels. The RDA measures grams of protein per kilogram of body weight, and the AMDR measures protein intake as a percentage of total kilocalories.

The RDA for Protein The RDA for protein has been established to provide adequate amounts of essential amino acids and nitrogen. This ensures the body will have the materials necessary to make the nonessential amino acids and body proteins necessary to meet daily needs. The RDAs for the essential amino acids are illustrated in Figure 6.13. As you can see, the RDA per day for each of the nine essential amino acids is based on grams per kilogram of body weight. This is also true for the RDA for protein. Adults older than 19 years of age should consume 0.8 grams of protein for each kilogram of body weight. For individuals under the age of 19, the RDA is somewhat higher (see the tables in the front cover of this textbook).

In the United States, men age 20 and older consume, on average, more than 100 grams of protein daily, and women on average consume more than 70 grams. In general, Americans are meeting, and even exceeding, their dietary protein needs. Follow the steps in the Calculation Corner to calculate your RDA for protein.

▲ Figure 6.13 RDAs for Essential Amino Acids

Recommended Dietary Allowances for the nine essential amino acids

based on body weight.

Source: Data from Institute of Medicine. 2005. Dietary Reference Intakes for Energy, Carbohydrate, Fiber, Fat, Fatty Acids, Cholesterol, Protein, and Amino Acids (Washington, DC: National Academies Press).

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Is ol eu

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M et

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224 Chapter 6 | Proteins

The AMDR for Protein Total protein intake is also measured as a percentage of total kilocalories. The latest AMDR for protein, based on data from numerous nitrogen balance studies, is 10–35 percent of total daily kilocalories. Currently, even though many adults in the United States are consuming more grams of protein than they need, they consume about 15 percent of their daily kilocalories from protein, which falls within the AMDR. This is because Americans are overconsuming kilocalories from carbohydrates and fats, which lowers the percentage of the total kilocalories coming from protein.

An overweight individual’s protein needs are not much greater than those of a nor- mal-weight person of similar height because the RDA for dietary protein is based on a person’s need to maintain protein-dependent tissues, like lean muscle and organs, and to perform protein-dependent body functions. Because people who are overweight carry more of their weight as fat, they do not need to consume significantly more protein than normal-weight people.

The American College of Sports Medicine, the Academy of Nutrition and Dietetics, and other experts have advocated an increase of 50–100 percent more protein for competi- tive athletes participating in endurance exercise (marathon runners) or resistance exercise (weight lifters).8, 9 However, athletes typically maintain a higher intake of food and thus already consume higher amounts of both kilocalories and protein.

LO 6.5: THE TAKE-HOME MESSAGE When protein intake equals the amount of nitrogen excreted, a healthy body is in nitrogen balance. Nitrogen balance stud-

ies suggest adults should consume 0.8 gram of protein for each kilogram of

body weight, and 10–35 percent of their daily energy intake as protein. In the

United States, men typically consume more than 100 grams of protein daily

and women more than 70 grams—in both cases, far more than is needed—

but their AMDR is about 15 percent, which is within the recommended range.

Athletes have higher protein needs than nonathletes, but their higher food

intake means that they typically meet or exceed these higher needs.

Calculation Corner

Protein Requirements

(a) To calculate protein requirements, the first step is to convert body weight in pounds to

kilograms. The conversion factor is 2.2. For example, an adult who weighs 176 pounds (lb) would

weigh 80 kilograms (kg):

176 lb , 2.2 = 80 kg

(b) Next, multiply weight in kilograms * 0.8 grams of protein. In this example, a healthy adult who weighs 80 kg should consume 64 grams (g) of protein per day.

80 kg * 0.8 g = 64 g protein

Source: Data from Institute of Medicine, National Academy of Science. 2002. Dietary Reference Intakes for Energy, Carbohydrate, Fiber, Fat, Fatty Acids, Cholesterol, Protein, and Amino Acids (Washington, DC: National Academies Press).

Go to Mastering Nutrition and complete a Math Video activity similar to the problem in this Calculation Corner.

What Are the Best Food Sources of Protein? 225

What Are the Best Food Sources of Protein?

LO 6.6 Describe the best food sources of protein and the methods available to determine protein quality.

The protein content of foods varies greatly. Although fruits are an excellent dietary choice, most contain one gram or less of protein per serving. Other foods, especially animal products, can contribute substantial amounts of protein to the diet.

Not All Protein Is Created Equal

A high-quality protein is digestible, contains all the essential amino acids, and provides a sufficient quantity of amino acids to be used to synthesize the nonessential amino acids and support the body’s requirements for growth and maintenance. If a single essential amino acid is in low supply in the diet and thus in the amino acid pool, the body’s ability to synthesize proteins will be limited. Thus, while it is important to eat enough protein, the quality of protein also matters.

Which protein is best? Several methods have been used to yield chemical scores to answer this question. These methods include the amino acid score, the protein digestibility corrected amino acid score, and the biological value.

Amino Acid Score The amino acid score is used to determine if a protein is complete. To calculate an amino acid score, the amount of an essential amino acid per gram of food protein (mg/g) being tested is divided by the standard amount for that same amino acid in a gram of the refer- ence protein. Egg white protein is typically used as the reference because it is known to have a balance of all of the essential amino acids needed to support growth. Amino acid scores range from 0 to 1, with 1 representing an optimal score. The Calculation Corner provides an example of how to calculate this score for peanut butter using the information shown in Table 6.4.

amino acid score Composition of essential

amino acids in a protein compared with a

standard, usually egg protein.

Essential Amino Acid

Peanut Butter

(mg/g)

Egg Protein

(mg/g)

Amino Acid

Score

Histidine 30 22 1.36

Isoleucine 40 54 0.74

Leucine 77 86 0.90

Lysine 39 70 0.56

Methionine plus cysteine 24 57 0.42

Phenylalanine plus tyrosine 108 93 1.16

Threonine 30 47 0.64

Tryptophan 12 17 0.71

Valine 46 66 0.71

Source: Data from Institute of Medicine. 2002. Dietary Reference Intakes for Energy, Carbohydrate, Fiber, Fat, Fatty Acids, Cholesterol, Protein, and Amino Acids (Washington, DC: National Academies Press).

TABLE 6.4 Amino Acid Scores for Peanut Butter

226 Chapter 6 | Proteins

The essential amino acid that has the lowest score is called the limiting amino acid. In the case of peanut butter, the limiting amino acid is methionine, with a score of 0.42. This score means that peanut butter contains 42 percent of the methionine found in egg protein.

The amino acid score is one method for comparing food proteins and their essential amino acid composition. Though it is a useful method of comparison, it doesn’t take into consideration how protein is digested.

Protein Digestibility Corrected Amino Acid Score (PDCAAS) The protein digestibility corrected amino acid score (PDCAAS) combines the chemical score with the digestibility of a food protein to give a more accurate indication of quality. This is important because only amino acids that are digested and absorbed can contribute to the amino acid pool and be used to build and maintain body proteins. An example of the PDCAAS calculation for peanut butter is presented in the Calculation Corner.

limiting amino acid Essential amino acid that

is in the shortest supply, relative to the body’s

needs, in an incomplete protein

protein digestibility corrected amino

acid score (PDCAAS) Score measured as

a percentage that takes into account both

digestibility and amino acid score and provides

a good indication of the quality of a protein.

Calculation Corner

Amino Acid Score The formula used to calculate the amino acid score for any food is:

Amino acid score = essential amino acid for protein (mg/g)

essential amino acid for standard (mg/g)

We can use peanut butter to illustrate this calculation. Table 6.4 shows the essential amino acid

content for peanut butter in column two. The third column shows the essential amino acid content

for egg protein, which is most often used as the standard for this calculation. As you read across

the table to column four, you can see the amino acid score. This means that peanut butter contains

136 percent of the histidine that egg protein does. This is how you calculate the amino acid score.

Begin with histidine and divide the amount of histidine in peanut butter by the amount found in egg

protein:

30 mg/g , 22 mg/g = 1.36

Continue the calculation for each of the amino acids. Each of the nine essential amino acid

scores is calculated individually and presented in column four in Table 6.4. Now add together all the

amino acid scores found in peanut butter and compare with egg protein using the total mg/g for

each as follows:

406 mg/g peanut butter , 674 mg/g egg protein = 0.60 or 60,

This amino acid score means that overall, peanut butter contains 60 percent of the essential

amino acids that egg protein does.

Egg protein is the highest quality protein

in the diet.

Calculation Corner

PDCAAS The digestibility of a food has a major impact on the protein quality. This calculation, called the

Protein Digestibility Corrected Amino Acid Score (PDCAAS), compares the amino acid content of a

food with the amino acid requirement for humans and then corrects for digestibility. This is how the

calculation works:

(1) First you need to know the amino acid score of the food. For this calculation you use the low-

est amino acid score. From Table 6.4 you see that the lowest amino acid score for peanut butter is

methionine plus cysteine, or 0.42.

What Are the Best Food Sources of Protein? 227

In general, animal proteins are more digestible than plant proteins. Some plant pro- teins, especially when consumed raw, are protected by the plant’s cell walls and cannot be broken down by the enzymes in the intestinal tract, whereas 90–99 percent of the proteins from animal sources (cheese and other dairy foods, meat, poultry, and eggs) are digestible. Plant proteins, such as in oatmeal (86 percent digestible) and soybeans (78 percent digestible), are generally only 70–90 percent digestible.10

Milk protein, which is easily digested and provides all essential amino acids, has a PDCAAS of 1.00. In comparison, kidney beans garner a PDCAAS of 0.68, and wheat has a score of only 0.40. However, when wheat is combined with another protein source, such as peanut butter, the protein quality of the meal is improved.

The PDCAAS is used by the Food and Drug Administration to calculate the % Daily Value of protein used on food labels. Manufacturers use 50 grams of protein as the standard to calculate the %DV for a serving of a food for adults and children 4 or more years of age. Recall from Chapter 2 that the %DV for protein is only required on the Nutrition Facts panel if the manufacturer has made a nutrient claim for the protein in the food.

Complementary and Complete Proteins Protein from animal products is considered a high-quality, complete protein that provides all nine of the essential amino acids, along with some of the 11 nonessential amino acids. Plant proteins are considered to be incomplete protein because plants are deficient in one or more essential amino acids. Some exceptions to this generaliza- tion are gelatin, quinoa, and soy. Gelatin, an animal protein, is not a complete protein because it is missing the amino acid tryptophan. Quinoa and soy, plant proteins, have amino acid profiles that resemble those of animal proteins, and are considered complete proteins. Examining the Evidence: Does Soy Reduce the Risk of Disease? discusses a variety of soy products to incorporate into a vegetarian diet to improve the quality of your meals.

complete protein Protein that provides all

the essential amino acids, along with some

nonessential amino acids. Soy protein and

protein from animal sources are complete

proteins.

incomplete protein Protein that is low in one

or more of the essential amino acids. Proteins

from plant sources tend to be incomplete.

(2) Next, to determine the PDCAAS for peanut butter, multiply the score for its lowest limiting

amino acid (0.42) by the protein digestibility of peanut butter (95%):

PDCAAS for peanut butter = 0.42 * 0.95 = 0.40

In other words, because the protein in peanut butter is not completely digested, the amino acid

score for methionine plus cysteine has been corrected from 0.42 to 0.40.

These calculations are used by the Food and Drug Administration to determine the %DV repre-

sented on food labels. For example, one serving (2 tbsp) of peanut butter contains 8 grams (g) of

protein. To calculate the %DV, this number is multiplied by the PDCAAS:

8 g * 0.40 = 3.2

Next, divide this by 50 grams, which is the recommended intake of protein for adults used on

the label:

3.2 , 50 g = 0.064 or 6.4,

In this example, a serving of peanut butter would represent 6.4 percent of the %DV for protein.

Peanut butter on whole-wheat bread is a

protein-rich snack.

228 Chapter 6 | Proteins

E X A M I N I N G T H E

EVIDENCE

S oy consumption in the United

States, in foods ranging from

soy milk to soy burgers, has

been increasing in recent decades at a

rate of 5 percent annually.1 According

to a survey conducted by the United

Soybean Board, 81 percent of U.S.

consumers perceive soy foods as being

healthy.2 The popularity of soy foods

has increased 14 percent among many

age groups and ethnic groups, including

baby boomers; Asian populations in the

United States looking for traditional soy-

based foods; and young adults with an

increasing interest in vegetarian diets.3

Soy is a complete, high-quality pro-

tein source that is low in saturated fat

and that contains isoflavones, which

are naturally occurring phytoestrogens

(phyto = plant). These plant estrogens have a chemical structure similar to

human estrogen, a reproductive hor-

mone present in lower levels in males

and higher levels in females. However,

they are considered weak estrogens,

as they have less than a thousandth

the potential activity of human estro-

gen. Though isoflavones can also be

found in other plant foods, such as

grains, vegetables, and other types of

legumes, soybeans contain the largest

amount found in food.

Epidemiological studies have sug-

gested that eating soy protein as part

of a heart-healthy diet may reduce the

risk of heart disease by lowering cho-

lesterol levels. Research suggests that

soy protein can lower LDL cholesterol

and raise HDL cholesterol levels.4 Soy

protein may also help lower blood pres-

sure, a risk factor for heart disease.5

Observational studies suggest

that isoflavones may help relieve

menopausal symptoms, although a

well-designed, double-blind study

reported no benefit.6 At the same time,

because isoflavones act as weak estro-

gens in the body, some concern exists

that they may have a harmful impact

on certain estrogen-sensitive cancers

(tumors that use estrogen for growth),

including an estrogen-sensitive form

of breast cancer. Recently, however,

several new reports have found no link

between breast cancer prognosis and

soy isoflavones.7 In fact, some stud-

ies have suggested that isoflavones

may actually reduce the risk of breast cancer, possibly by competing with the

hormone estrogen for its binding site

on tumor cells.8

Timing may be an important part

of the preventative role that soy plays

in breast cancer. A study of Chinese

women revealed that those who ate the

most soy during their adolescent years

had a reduced risk of breast cancer in

adulthood.9 Early exposure to soy foods

may be protective by stimulating the

growth of cells in the breast, enhancing

the rate at which the glands mature, and

altering the tissues in beneficial ways.

Research supports the safety of

soy isoflavones when consumed in soy

foods and beverages such as soymilk.10

According to the American Cancer

Society, soy foods and beverages are

healthy and safe, and women with

breast cancer may consume them in

moderate amounts. They should, how-

ever, avoid soy-containing pills, pow-

ders, and supplements with high levels

of isoflavones.11

Soy can be an inexpensive, heart-

healthy protein source that may also

help lower your LDL cholesterol and

blood pressure, raise HDL choles-

terol, and reduce your risk of certain

cancers.

References

1. United Soybean Board. 2014. Bite. The Data is Delicious. Available at www.soyconnection. com/sites/default/files/Consumer%20Atti- tudes_Med_062714.pdf. Accessed February 2017.

2. United Soybean Board. 2016. Consumer Attitudes About Nutrition, Health, and Soy

Foods. Available at www.soyconnection.com. Accessed February 2017.

3. United Soybean Board. 2016. 4. Wofford, M. R., C. M. Rebholz, et al. 2012.

Effect of Soy and Milk Protein Supplementa- tion on Serum Lipid Levels: A Randomized Controlled Trial. European Journal of Clinical Nutrition 66:419–425.

5. He, J., M. Wofford, K. Reynolds, et al. 2011. Effect of Dietary Protein Supplementation on Blood Pressure: A Randomized Con- trolled Trial. Circulation 124:589–595.

6. Levis, S., N. Strickman-Stein, et al. 2011. Soy Isoflavones in the Prevention of Menopausal Bone Loss and Menopausal Symptoms: A Randomized Double-Blind Trial. Archives of Internal Medicine 171(15):1363–1369.

7. Messina, M. 2016. Soy and Health Update: Evaluation of the Clinical and Epidemiologic Literature. Nutrients 8(12):754. doi:10.3390/ nu8120754.

8. Fritz, H., D. Seely, et al. 2013. Soy, Red Clo- ver, and Isof lavones and Breast Cancer: A Systematic Review. PLoS One 8(11):e81968. doi: 10.1371/journal.pone.0081968.

9. Maskarinec, G., D. Ju, et al. 2017. Soy Food Intake and Biomarkers of Breast Cancer Risk: Possible Difference in Asian Women? Nutrition and Cancer 69(1):146–153. doi: 10.1080/01635581.2017.1250924.

10. American Cancer Society. 2015. How Your Diet May Affect Your Risk of Breast Can- cer. Available at www.cancer.org. Accessed March 2017.

11. Ibid.

isoflavones Naturally occurring

phytoestrogens, or weak plant estrogens, that

function in a similar fashion to the hormone

estrogen in the human body.

Does Soy Reduce the Risk of Heart Disease and Cancer?

Soy meat analogs, such as hot dogs,

sausages, burgers, and cold cuts, are

made using soy.

What Are the Best Food Sources of Protein? 229

Does this mean that most plant proteins are of less value in the diet? Absolutely not. When incomplete proteins are eaten with modest amounts of animal proteins or soy, or combined with other plant proteins that are rich in the incomplete protein’s limit- ing amino acids, the incomplete protein is complemented. In other words, the amino acid profile of the meal is complete. For example, when rice, which is low in lysine but high in methionine, is combined with beans, which provide lysine, they complement each other and provide all nine essential amino acids. In addition, adding a small amount of cheese or meat to a plant protein, such as in macaroni and cheese or a shrimp stir-fry, provides the amino acid that is limited in the plant food.

Complementary proteins do not need to be consumed at the same meal to improve the quality of the protein source. As long as the foods are consumed in the same day, all the essential amino acids will be available to meet your biological needs. Vegetarian diets can contain a sufficient quality as well as quantity of protein in carefully planned meals. Read more on this topic in the Health Connection later in this chapter.

Many Healthy Foods Provide Significant Protein

The best sources of protein are low-fat dairy foods, lean meats, fish, poultry, and meat alternatives such as dried beans, peanut butter, nuts, quinoa, and soy (Figure 6.14). A 3-ounce serving of lean meat, poultry, or fish (about the size of a deck of cards) provides 21–25 grams of protein, or about 7 grams per ounce. Grains and vegetables are less robust protein sources, providing about 3–4 grams per serving, but as part of a varied, balanced diet, they can contribute significantly to daily needs.

Chickpeas are short of the limiting amino

acid methionine. The addition of sesame

seed paste, which has an abundance of

methionine, completes the protein. Add garlic

and lemon as seasonings for a completely delicious hummus.

▲ Figure 6.14 Food Sources of Protein

Meat, poultry, fish, eggs, soy and other legumes, nuts and seeds, and dairy products are the most abundant sources of dietary

protein. Grains and vegetables provide less protein per serving, but as part of a varied, balanced diet can contribute to daily

needs.

Source: Data from USDA National Nutrient Database for Standard Reference Release 28. Revised 2016. Available at www.nal.usda.gov/fnic. Accessed March 2017.

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230 Chapter 6 | Proteins

Eating a wide variety of foods is the best approach to meeting protein needs. A diet that consists of the recommended servings from the five food groups based on 1,600 kilocalories (which is far less than most adults consume daily) will supply an ade- quate amount of protein for adult women and most adult men. In fact, many people have met their daily protein needs before they even sit down to dinner.

Most People Don’t Need Protein Supplements

Some of the most popular supplements in the United States are protein and amino acids, often marketed especially to athletes (see Spotlight: Protein Supplements). Physically active people may take protein supplements as an ergogenic aid to increase muscle size and strength and endurance performance. However, the DRIs for protein are based on healthy food choices and you do not need additional protein in the form of supplements. (We discuss protein and sport performance in greater detail in Chapter 16.)

LO 6.6: THE TAKE-HOME MESSAGE Protein quality is determined by the digest- ibility of protein and the amino acid profile, which includes the types and

amounts of amino acids it contains. Protein from animal foods is more digest-

ible than plant proteins. Complete protein, found in animal foods, quinoa, and

soy, provides a complete set of essential amino acids and some nonessential

amino acids. Incomplete plant proteins can be complemented with protein

from other plant sources or animal food sources to improve their protein qual-

ity. Low-fat dairy, eggs, lean meat, fish, poultry, and meat alternatives such as

dried beans, peanut butter, nuts, and soy are healthy foods rich in protein.

Protein Supplements 231

or combinations not found naturally in

foods. High amounts of single amino

acids can compete with other amino

acids for absorption, possibly caus-

ing a deficiency of other amino acids.

Furthermore, over-consuming specific

amino acids can lead to side effects

such as nausea, lightheadedness, vom-

iting, and drowsiness.

Protein Bars and

Energy Bars

Protein bars and energy bars are com-

monly marketed as a portable snack or

a quick meal, but if convenience is the

main attraction, consider a peanut but-

ter sandwich. It can be made in a snap,

and since it doesn’t have to be refriger-

ated, it can travel anywhere. From a

price standpoint, a peanut butter sand-

wich is a bargain compared with pro-

tein bars and energy bars, which can

cost 5–10 times as much. Whereas the

kilocalories and protein content of the

sandwich and most bars are similar, the

sandwich is lower in saturated fat and

has no added sugars. In contrast, some

bars contain up to 7 teaspoons (about

28 grams) of added sugar. Many bars

are also low in fiber. Thus, the peanut

butter sandwich is the healthier choice.

health consequences, including organ

damage, anemia, and osteoporosis.

Protein intake enhances muscle

synthesis,4 but athletes consume enough

protein for muscle growth and repair in

an average mixed diet. The same whey

protein used in protein supplements is

abundant in milk and dairy products,

including Greek yogurt.5 Any protein not

used for protein synthesis is either burned

for energy or stored as fat.

The key to increasing muscle mass

is a well-designed strength-training

program combined with additional

kilocalories from all three macronutrients.

These kilocalories allow dietary protein

to be used for muscle synthesis instead

of energy. Another important key is tim-

ing. Research suggests that ingestion

of protein with carbohydrate before and

immediately after a workout improves

muscle synthesis.6 A glass of low-fat

chocolate milk, rather than a protein

supplement, before and after a workout

will provide both key amino acids and

carbohydrate.

Protein shakes and powders are also

marketed as meal replacers. Dieters

might lose weight using a high-protein

meal replacer, but the same results can

be obtained with a kilocalorie-controlled

meal of whole foods, without the health

risks and the added expense.

There are instances where pro-

tein shakes and supplements may be

advised. Older adults, who may have

limited appetites and be less likely to

consume adequate nutrients in foods,

may benefit from drinking a protein shake

every day. However, these products

should be used to supplement meals, not replace them.

Amino Acid Supplements

Amino acid supplements, including

those containing individual amino acids

such as tryptophan and lysine, are

marketed as remedies for a range of

health issues, including chronic pain,

depression, insomnia, and certain infec-

tions. Typically, these supplements

contain single amino acids in amounts

T he sale of protein supplements has

skyrocketed over the last decade,

fueling an industry that now gener-

ates almost $2 billion annually.1 These

products are marketed with promises

to give you an energy boost, help shed

those unwanted pounds, build muscle,

fight aging, and cure a host of health

problems. Their manufacturers use

phrases such as “scientifically proven,”

but dietary supplements do not undergo

rigorous testing for quality, efficacy, or

safety. So how do you know if these

supplements contain what they say they

contain, do what they say they do, and

are safe? Is more protein always better?

Protein Shakes and Powder

Most protein shakes and powders

use whey, soy, or rice protein as a key

ingredient. The amount of protein their

label claims they contain ranges from

10 to 40 grams per serving, along with

added vitamins and minerals. Muscle

Milk® lists 25 grams of protein per serv-

ing and suggests 3 servings per day for

a total of 75 grams of protein, almost

100 percent of the total RDA for protein

for a male weighing 176 pounds.

Do these products actually contain

what is listed on the label? Recently,

a consumer research lab found that a

popular brand of protein powder provided

16 fewer grams of protein per scoop than

stated on the label. Instead, it contained

an extra 16 grams of carbohydrates, in-

cluding 3 grams of sugar, not accounted

for on the label.2 Other labs have found

arsenic, cadmium, lead, and mercury in

15 tested samples.3 Chronic ingestion

of these toxic metals can cause severe

Protein SupplementsSPOTLIGHT

232 Chapter 6 | Proteins

References

1. Euromonitor International. 2014. The Rise of Protein in the Global Health and Wellness and Supplement Arenas: Examining the Global Protein Surge. Available at http://globalfoodforums. com/wp-content/uploads/2014/04/Chris- Schmidt-Euromonitor- 2014-Protein-Trends- Technologies.pdf. Accessed February 2017.

2. ConsumerLab.com. 2015. 31% of Protein Powders and Drinks Flunk Test of

Quality. Available at www.consumerlab. com/reviews/Protein_Powders_Shakes_ Drinks_Sports_%20Meal_Diet/Nutrition- Drinks/. Accessed February 2017.

3. Consumer Reports Magazine. 2010. Alert! You Don’t Need the Extra Protein or the Heavy Metals Our Tests Found. Consumer Reports 75:24.

4. R. J. Maughan and S. M. Shirreffs. 2012. “Nutrition for Sports Performance: Issues and Opportunities.” Proceedings of the Nutrition Society 71:112–119.

5. Oosthuyse, T., M. Carstens, and A. M. Millen. 2015. Whey or Casein Hydrolysate with Carbohydrate for Metabolism and Performance in Cycling. International Journal of Sports Medicine 36(8):636–646. doi: 10.1055/s-0034-1398647.

6. Naclerio, F., et al. 2016. Effects of Protein- carbohydrate Supplementation on Immunity and Resistance Training Outcomes: A Dou- ble-blind, Randomized, Controlled Clinical Trial. European Journal of Applied Physiolog y. doi: 10.1007/s00421-016-3520-x.

What Happens If You Eat Too Much or Too Little Protein?

LO 6.7 Explain the health consequences of consuming too much or too little protein.

Most people in industrialized nations consume more than enough protein, while people from less developed countries may struggle to meet even the minimum requirements. Let’s look at what happens to the human body when it gets too much or too little protein.

Eating Too Much Protein May Contribute

to Chronic Disease

Eating too great a percentage of the diet as protein could cause the displacement of other nutrient-dense foods such as whole grains, fruits, and vegetables, all of which also provide disease-fighting phytochemicals and dietary fiber. Moreover, a diet with excessive protein has long been thought to increase the risk of heart disease, kidney stones, osteoporosis, and some types of cancer. However, recent research provides some reassurance that eating too much protein (0.9–2.0 g/kg per day) may not be as bad as once thought. 11

Heart Disease Recent research reports that the type of protein is more important in reducing the risk of heart disease than the quantity.12 A diet low in red meat that contains nuts, low-fat dairy, poultry, or fish lessens the risk for heart disease compared with a diet high in red meat and high-fat dairy. A high-red-meat diet may mean overloading on heart-unhealthy saturated fat (see Figure 6.15). Even lean meats and skinless poultry, which contain less saturated fat than some other cuts of meat, are not free of saturated fat. A diet high in saturated fat can raise LDL cholesterol levels in the blood, whereas lowering the saturated fat may lower the risk of heart disease. While the overall effect of high protein intake on heart disease is still not clear, it is clear that eating a variety of plant proteins low in saturated fat is the best heart-healthy approach.

Kidney Stones A high-protein diet may increase the risk of kidney stones. Eating a diet high in animal protein and low in carbohydrate lowers the pH of the urine, which raises the risk of devel- oping kidney stones, especially in people who are more susceptible to the condition.13 This change in pH may be due to higher levels of oxalates in the urine from oxalic acid;

What Happens If You Eat Too Much or Too Little Protein? 233

oxalic acid combines with other compounds, including calcium, to form kidney stones. The impact of protein on the change in pH may be the type of protein, not the amount of protein, consumed. A diet that contains plenty of fluid, a lower protein intake (not greater than 0.8 g/kg), a proper calcium intake, and a balance of fruits and vegetables may be beneficial, especially for people who already have kidney disease.

Osteoporosis Though still controversial, past studies have shown that bones lose calcium when a per- son’s diet is too high in protein. The loss seems to occur because calcium is removed from bone to neutralize the acid generated when specific amino acids are broken down. In fact, in a study of individuals on a low-carbohydrate, high-protein diet, researchers observed a 50 percent loss of calcium in the subjects’ urine.14 The calcium loss was not observed when these individuals were on a lower-protein diet, so the researchers concluded that it was due to the buffering effect.

More recent research suggests a more positive effect of high protein intake on bone health as long as dietary calcium is sufficient.15, 16 Foods such as low-fat milk, yogurt, and cheese can add both protein and calcium to the diet.17 Unfortunately, many American adults are falling short of their recommended calcium intake, and if their diets are also high in protein, the combination may not be healthy for their bones.

Eating too little protein can also lead to loss of bone mass. A study of more than 900 elderly men and women showed that higher protein consumption was associated with denser bone.18 Similar results were also reported in younger adults. When it comes to our bones, too much protein coupled with low calcium intake, or too little protein intake, can both be unhealthy.19

▲ Figure 6.15 Where’s the Protein and Saturated Fat in Foods?

Though many foods, in particular dairy foods and meats, can provide a hefty amount of protein, they can also provide a large amount of

saturated fat. Choose nonfat and low-fat dairy foods, lean cuts of meat, and skinless poultry to avoid overloading on saturated fat.

Source: USDA National Nutrient Database for Standard Reference (www.nal.usda.gov). 2016. Accessed March 2017.

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234 Chapter 6 | Proteins

Cancer The relationship between high-protein diets and cancer is also less than clear. Although large amounts of meat, especially red and processed meats, may increase the risk for colon cancer, research doesn’t support a connection between high amounts of total protein and increased colon cancer risk.20 Processed meats have also been associated with an increased risk for bladder cancer.21

The Dietary Guidelines for Americans recommend avoiding processed meats that have been preserved by smoking, curing, salting, or adding chemical preservatives. If you are in the habit of eating bacon, sausage, ham, deli meats, and other processed meats, try replac- ing them with other high-protein foods, including peanut butter, low-fat dairy foods, eggs, soy-based meat alternatives, or low-fat yogurt with berries and nuts.

Eating Too Little Protein Can Lead to Protein-Energy

Malnutrition

Whereas many individuals have the luxury of worrying about consuming too much pro- tein, others are desperately trying to meet their daily needs. Every day, 795 million people, or one in nine (many of them children), around the world don’t have access to enough food.22 These children’s diets are inadequate in either protein or energy or both, a condi- tion known as protein-energy malnutrition (PEM). When kilocalories and protein are inadequate, dietary protein is used for energy rather than for its other roles in the body. Moreover, other important nutrients, such as vitamins and minerals, also tend to be in short supply, which further compounds PEM.

Many factors can lead to PEM, including poverty, poor food quality, insufficient food intake, unsanitary living conditions (causing diarrhea and infection), ignorance regarding the proper feeding of children, and the cessation of breastfeeding in the first few months of age.23 Because infants and children are growing, they have higher nutri- tional needs for their size than adults. They are also dependent on others to provide them with food. For these reasons, PEM is more frequently seen in infants and children than in adults.

Because protein is needed for so many body functions, it isn’t surprising that chronic protein deficiency can lead to numerous health problems. Without adequate dietary pro- tein, cells lining the gastrointestinal tract aren’t sufficiently replaced as they’re sloughed off. The inability to regenerate these cells inhibits digestive function. Absorption of the little amount of food that may be available is reduced, and bacteria that normally stay in the intestines can contaminate the blood, causing septicemia. Malnourished individuals frequently have a compromised immune system, which can make fighting infection, such as a respiratory infection or diarrhea, impossible. Malnourished children have a much greater risk of death after exposure to measles or after bouts of diarrhea.24, 25

Though deficiencies of kilocalories and protein often occur simultaneously, some- times one may be more prevalent than the other. A severe deficiency of protein is called kwashiorkor, whereas a severe deficiency of kilocalories is called marasmus. A con- dition that is caused by a chronic deficiency of both kilocalories and protein is called marasmic kwashiorkor.

Kwashiorkor Kwashiorkor was first observed in the 1930s in tribes in Ghana (a republic of West Africa) when firstborn toddlers became malnourished following the birth of a sibling. Typically, when the newborn began receiving the mother’s nutritionally balanced breast milk, the firstborn child was relegated to an inadequate and unbalanced diet high in

protein-energy malnutrition (PEM) Lack of

sufficient dietary protein and/or kilocalories.

kwashiorkor State of PEM in which there is a

severe deficiency of dietary protein.

marasmus State of PEM in which there

is a severe deficiency of kilocalories, which

perpetuates wasting; also called starvation.

What Happens If You Eat Too Much or Too Little Protein? 235

carbohydrate-rich grains but severely deficient in protein. This breast milk displacement set the stage for a serious decline in the child’s health.

A classic symptom of severe kwashiorkor is edema in the legs, feet, and stomach (Figure 6.16 ). Because protein plays an important role in maintaining fluid balance in the blood and around the cells, a protein deficiency can cause fluid to accumulate in the spaces surrounding the cells, causing swelling. In addition, as muscle proteins are broken down to generate the amino acids needed to synthesize other body proteins, muscle tone and strength diminish. Those with kwashiorkor may also have skin that is dry and peeling. Rashes or lesions can also develop. Their hair is often brittle and can be easily pulled out. Children with kwashiorkor often appear pale, sad, and apathetic, and cry easily. They are prone to infections, rapid heartbeat, excess fluid in the lungs, pneumonia, septicemia, and fluid and electrolyte imbalances—all of which can be deadly.

Marasmus The bloating seen in kwashiorkor is the opposite of the frail, emaciated appearance of marasmus (Figure 6.17). Because they are not consuming enough kilocalories, marasmic individuals are literally starving. They are often not even at 60 percent of their desirable body weight. Marasmic children’s bodies use all available kilocalories to stay alive; thus, growth is interrupted. Such children are weakened and appear apathetic. Many can’t stand without support. They look old beyond their years, as the loss of fat in their face—one of the last places that the body loses fat during starvation—diminishes their childlike appearance. Their hair is thin and dry and lacks the sheen found in healthy children. Their body temperature and blood pressure are both low, and they are prone to dehydration, infections, and unnecessary blood clotting.

Individuals with marasmic kwashiorkor have the worst of both conditions. They often have edema in their legs and arms, yet have a “skin and bones” appearance in other parts of the body. When these individuals are provided with medical and nutritional treat- ment, such as adequate protein, the edema subsides and their clinical symptoms more closely resemble that of a person with marasmus.

Appropriate medical care and treatment can dramatically reduce the 22–40 percent mortality rate seen among children with severe PEM worldwide.26 The treatment for PEM should be carefully and slowly implemented using a three-step approach. The first step addresses the life-threatening factors, such as severe dehydration and fluid imbal- ances, using electrolyte solutions. The second step is to restore the individual’s depleted tissues by gradually providing nutritionally dense kilocalories and high-quality protein. The third step involves transitioning the person to solid foods and introducing physical activity.

▲ Figure 6.16 Kwashiorkor

The edema in this child’s belly is a classic sign

of kwashiorkor.

▲ Figure 6.17 Marasmus

The emaciated appearance of this child is a

sign of marasmus.

LO 6.7: THE TAKE-HOME MESSAGE Too many protein-rich foods can displace whole grains, fruits, and vegetables, which have been shown to help reduce

many chronic diseases. A high-protein diet may increase the risk of heart dis-

ease, kidney problems, and calcium loss from bone. Consuming too much

protein from animal sources can increase the amount of saturated fat in the

diet. A low-protein diet has also been shown to lead to loss of bone mass. PEM

is caused by an inadequate amount of protein, kilocalories, or both in the diet.

A severe deficiency of protein results in kwashiorkor; a severe deficiency of

kilocalories causes marasmus.

236 Chapter 6 | Proteins

What Is a Vegetarian Diet? LO 6.8 Describe the benefits and

risks of a vegetarian diet.

Whereas many people choose to become a vegetarian for ethical, religious, or environmental reasons, others follow a vegetarian diet for health reasons.27 An estimated 3.3 percent of Americans con- sider themselves vegetarians.28 There are several types of vegetarians and associated ranges of acceptable foods. See Table 6.5 for a description of different vegetarian diets and the foods associated with each.

Because vegetarians avoid meat, poul- try, and fish, which are high in complete protein, they need to be sure to get ade- quate protein from other food sources. Vegetarians can meet their daily protein needs by consuming a varied plant-based diet that contains meat alternatives such as soy, dried beans and other legumes, and nuts. Vegetarians who consume some animal products, such as milk, eggs, and/ or fish, can use these foods to help meet their protein needs.

In the United States, the vegetarian food market continues to grow as manufacturers accommodate increased consumer demand with an array of new vegetarian products each year.29 Many sit-down restaurants offer vegetarian entrées on their menus, and even some fast-food restaurants now offer veggie burgers. University food services are increasingly making vegetarian options available to meet growing student demand.

Balanced Vegetarian

Diets Confer Health

Benefits

A plant-based vegetarian diet can be rich in high-fiber whole grains, legumes and other vegetables, fruits, and nuts; naturally

HEALTHCONNECTION

vegetarian Person who avoids eating animal

foods. Some vegetarians only avoid meat, fish,

and poultry, while others (vegans) avoid all

animal products, including eggs and dairy.

Type Eats Avoids

Lacto-vegetarian Grains, vegetables, fruits,

legumes, seeds, nuts, dairy foods

Meat, fish, poultry, and eggs

Lacto-ovo-vegetarian Grains, vegetables, fruits,

legumes, seeds, nuts, dairy foods,

eggs

Meat, fish, and poultry

Ovo-vegetarian Grains, vegetables, fruits,

legumes, seeds, nuts, eggs

Meat, fish, poultry, dairy

foods

Vegan Grains, vegetables, fruits,

legumes, seeds, nuts

Any animal foods (meat, fish,

poultry, dairy foods, eggs)

Pescetarian Grains, vegetables, fruits,

legumes, seeds, nuts, dairy foods,

eggs, and fish

Meat and poultry

Semivegetarian A vegetarian diet that occasionally

includes meat, fish, and poultry

Meat, fish, and poultry on

occasion

TABLE 6.5 The Many Types of Vegetarians

Beans add protein, antioxidants, and a variety of minerals to a healthful plant-based diet.

lower in saturated fat; and cholesterol free. These qualities are fundamental for reducing the risk of heart disease, high blood pressure, diabetes, cancer, stroke, and obesity, assuming the diet is not lim- ited or unbalanced in nutrients.

Vegetarian food staples such as soy, nuts, and soluble fiber–rich foods includ- ing beans and oats have all been shown to reduce blood cholesterol levels. Research collected from numerous studies has shown that deaths from heart disease are about 29 percent lower among vegetarians than among nonvegetarians.30

Vegetarians also tend to have lower blood pressure than those who consume meat. The incidence of high blood pres- sure has been shown to be over two times higher in nonvegetarians.31 High blood pressure is a risk factor not only for heart disease but also for stroke.

A plant-based diet can help reduce the risk of type 2 diabetes, and vegetar- ians as a population group tend to have a lower incidence of diabetes. Among people who already have diabetes, the predominance of foods rich in fiber and low in saturated fat and cholesterol in

HEALTHCONNECTION (CONTINUED)

What Is a Vegetarian Diet? 237

vegetarian diet means that you lose out on this benefit and are more likely to develop a zinc deficiency. Phytates found in grains and rice also bind zinc, making it less bio- available. A vegan’s zinc needs may be as much as 50 percent higher than a nonveg- etarian’s. A diet rich in soybeans, fortified soy burgers, legumes, nuts, and seeds will increase the zinc content of the diet.

Calcium is abundant in low-fat dairy foods such as nonfat or low-fat milk, yogurt, and cheese. If you follow a lacto- vegetarian diet, these foods would meet your calcium needs; however, if you follow a vegan diet, you may have more difficulty meeting your calcium needs. Calcium-forti- fied soymilk and orange juice as well as tofu can provide about the same amount of cal- cium per serving as is found in dairy foods. Adding calcium-rich vegetables including bok choy, broccoli, kale, collard greens, and okra will enhance calcium intake.

Vitamin A and vitamin D may be low in a vegetarian diet. Preformed vitamin A is found only in animal foods. However, veg- etarians can meet their needs by consum- ing the vitamin A precursor beta-carotene found in vegetables such as carrots and spinach. Some vegetarians will need to consume vitamin D–fortified milk, yogurt, or soymilk to maintain adequate levels of

a vegetarian diet can help manage the disease.32

Vegetarians also have lower cancer rates compared with the general population. The latest World Cancer Research Fund Report advocates a plant-based diet that is high in nutrients and dietary fiber yet low in energy-dense foods (highly processed foods with added sugars and saturated fat, as well as sugary beverages) to reduce the risk for cancer.33

Also, a plant-based diet that contains mostly fiber-rich whole grains and low-kilo- calorie, nutrient-dense vegetables and fruits tends to be one that “fills you up before it fills you out,” which means that you are likely to eat fewer overall kilocalories. Con- sequently, eating the plant-based foods of a vegetarian diet can be a healthy and satisfy- ing strategy for fighting the battle against obesity. Figure 6.18 can help vegetarians easily incorporate these foods into their diet.

A Healthy Vegetarian

Diet Requires Planning

The biggest risk of a vegetarian diet is underconsuming certain nutrients, such as protein, or certain vitamins and miner- als. Vegetarian foods contain protein, but

▲ Figure 6.18 My Vegan Plate

Source: The Vegetarian Resource Group, www.vrg.org.

A vegetarian diet combines a variety of plant foods with fortified foods or supplements rich

in vitamin B12.

the amount per serving is lower than that found in animal sources. For example, 100 grams, or a serving, of kidney beans contains 7 grams of protein, whereas 100 grams of cooked chicken breast, or the size of a medium boneless chicken breast, contains 17 grams. Most plant foods also do not contain complete protein. A vegetarian’s protein needs can be met by consuming a variety of plant foods. A combination of protein-rich soy foods,

legumes, nuts, or seeds should be eaten daily.

The form of iron in plants is not as easily absorbed as the form of iron in meat, fish, and poultry. Also,

phytate in grains and rice, and polyphenols

in tea and coffee, can inhibit iron absorption. The iron needs of vegetarians are about 1.5 times higher

than those of nonvegetar- ians. Consuming iron-fortified

cereals, enriched grains, pasta, wheat germ, and nuts and seeds can

improve iron intake in vegetarians. Consuming animal protein enhances

the absorption of zinc. Following a

HEALTHCONNECTION (CONTINUED)

238 Chapter 6 | Proteins

vitamin D. Egg yolk and ready-to-eat cere- als, in addition to a vitamin supplement, will add sufficient vitamin D to the diet.

Vitamin B12 is also a concern in veg- etarian diets because it is only found in animal-based foods.34 If the vegetarian diet contains eggs and dairy products, the diet may contain some vitamin B12. If the diet plan is strictly vegan, fortified foods or sup- plements should be included. Plant foods that are fermented or sprouted may contain a little vitamin B12 but the amount is not consistent or dependable. Foods fortified with vitamin B12 such as cereals, fortified soy products, nutritional yeast, and yeast extracts should be incorporated into the menu. Vitamin B12 supplements can also be used or vitamin B12 intramuscular injections from a physician will prevent vitamin B12 deficiencies.

If your vegetarian diet doesn’t include fish, you may not be consuming enough of the essential omega-3 fatty acid alpha- linolenic acid, which is a precursor to eicosanoids. Moreover, fatty fish such as salmon and sardines are direct sources of both EPA and DHA. Walnuts, flax- seed and flaxseed oil, and soybean and canola oil are other good vegetarian food sources to increase the omega-3 content of the diet.

To avoid nutrient deficiencies, vegetar- ians must consume adequate amounts of a wide variety of foods. Monitor those nutrients found in abundance in animal- based foods, including protein, iron, zinc, calcium, vitamin D, vitamin B12, vitamin A, and omega-3 fatty acids. A vitamin and mineral supplement may be necessary. The tips in Table 6.6 can help vegetar- ians increase these nutrients in their diet. Analyze the dietary habits of Megan, a col- lege sophomore who has recently become a vegan, in the Nutrition in Practice on page 239.

TABLE 6.6 Suggested Servings for a Healthy Vegetarian Diet

Food Group

Number of

Servings Serving Size

Calcium-Rich Foods

(8 servings daily)

Grains 6 Bread, 1 slice Whole-wheat bread,

1 slice

Cooked grain or cereal, 1�2 cup

Calcium-fortified cereal,

1 oz

Ready-to-eat cereal, 1 oz

Legumes,

nuts, and other

protein-rich

foods

5 Cooked beans, peas, or

lentils, 1�2 cup

Tofu or tempeh, 1�2 cup

Nut or seed butter, 2 tbsp

Nuts, 1�4 cup

Meat analog, 1 oz

Egg, 1

Cow’s milk or yogurt, 1�2 cup Calcium-fortified soy milk, 1�2 cup Cheese,

3�4 oz

Vegetables 4 Cooked vegetables, 1�2 cup

Bok choy, collards, broc-

coli, Chinese cabbage,

kale, mustard greens, or

okra; 1 cup cooked or

2 cups raw

Raw vegetables, 1 cup Calcium-fortified tomato

juice, 1�2 cup

Vegetable juice, 1�2 cup

Fruits 2 Medium fruit, 1

Cut-up or cooked fruit, 1�2 cup Fruit juice,

1�2 cup Dried fruit,

1�4 cup

Calcium-fortified fruit

juice, 1�2 cup

Figs, 5

Fats 2 Oil, 1 tsp

Soft margarine, 1 tsp

Mayonnaise, 1 tsp

LO 6.8: THE TAKE-HOME MESSAGE Some vegetarians abstain from all animal foods, whereas others may eat eggs and dairy products or even fish or poultry

in limited amounts. A balanced vegetarian diet may reduce the risk of heart

disease, high blood pressure, diabetes, cancer, stroke, and obesity. All vegetar-

ians must take care in planning a varied diet that meets their nutrient needs,

especially for protein, iron, zinc, calcium, vitamin A, vitamin D, vitamin B12, and

omega-3 fatty acids.

HEALTHCONNECTION (CONTINUED)

What Is a Vegetarian Diet? 239

Registered

Nurse and MD

M egan, a college sophomore,

has a micro-fridge in her dorm

room so she eats breakfast

in her room daily. Her lunch and din-

ner are eaten in the campus dining hall.

Even though she eats breakfast and

lunch before her classes, she is often

fatigued by midafternoon. Megan has

lost 5 pounds over the last 2 months,

and her worried parents convinced her

to make an appointment at the college

student health center. During the initial

screening, the health center registered

nurse (RN) uncovered that Megan had

recently become a vegan. Megan was

following a vegan diet that she found on

the Internet, which restricted her food

choices to only grains, vegetables, and

fruit. The nurse communicated this find-

ing to the doctor, and after his physical

examination of Megan, a blood test was

ordered to rule out any underlying health

issue. The test results were negative.

Based on the results of the labora-

tory test and the findings from the RN’s

initial screening, the doctor referred

Megan to the campus registered dietitian

nutritionist (RDN) to receive guidance

about her diet. The nurse called Megan

with the contact information for the RDN.

Megan’s Stats

❏ Age: 19

❏ Height: 5 feet 3 inches

❏ Weight: 110 pounds

❏ BMI: 19.3

Critical Thinking Questions

1. Why do you think Megan is hungry

and tired during the day?

2. Based on her food log, which nutrients

are likely to be low in Megan’s diet?

3. Based on her food log, which other

food group is Megan falling short of

daily?

NUTRITION in PRACTICE:

RDN’s Observation and Plan for

Megan

❏ Discuss the need to add vegan pro-

tein sources at each meal to meet

her daily protein needs. Add peanut

butter on her morning English muf-

fin. Top her salad bar lunch with the

chickpeas that are available in the

dining hall. Add tofu to her veggie stir-

fry, which is always available at the

salad bar.

❏ Discuss the need to consume three

servings of a vegan equivalent of dairy

foods in order to meet her calcium,

vitamin D, and vitamin B12 needs.

Drink a fortified soy beverage at each

meal.

A month later, Megan returns for a

follow-up visit with the RDN. By incorpo-

rating all of the RDN’s suggestions, she

was feeling less fatigued and hungry. She

had also gained 1.5 pounds. A review of

her food record shows that she is meet-

ing her daily protein needs and her diet

is more balanced. However, she is still

a little hungry late in the afternoon. The

RDN recommends that Megan add some

soy cheese with her afternoon popcorn

snack to curb her hunger.

Megan

MEGAN’S FOOD LOG

Food/Beverage Time

Consumed

Hunger

Rating* Location English muffin with jam and a banana

8:00 A.M. 5 In dorm room

Salad bar salad: lettuce, tomato, carrots, peppers, and cucumbers with Italian dressing. Diet soda.

12:30 P.M. 5 Campus dining hall

Popcorn 3:00 P.M. 5 Studying in dorm room

Veggie stir-fry over rice. Diet soda.

6:30 P.M. 5 In dining hall

* Hunger Rating (1–5): 1 = not hungry; 5 = super hungry.

240 Chapter 6 | Proteins

Visual Chapter Summary

LO 6.1 Proteins Are Made of Amino Acids

Linked with a Peptide

Bond Proteins are made up of 20 amino acids, 11 nonessential and nine essential. Nonessential amino acids can be made in the body, but the essential amino acids must be consumed in the diet. A third category, conditionally essential, contains those amino acids that under certain conditions must be supplied by food.

Amino acids are composed of carbon, hydrogen, oxygen, nitrogen, and, in some cases, sulfur. Every amino acid contains a central carbon, an acid group (COOH), an amine group (NH2), a single hydrogen, and a unique side chain that gives each amino acid its distinctive qualities.

Amino acids are joined together by peptide bonds through condensation. Two amino acids joined together form a dipep- tide, three form a tripeptide, and a polypep- tide consists of many amino acids joined together. The unique sequence of amino acids in the chain is the primary structure of a protein. Hydrogen bonding between the carboxyl and amine groups of the amino acids causes the chain to twist and coil, forming the secondary structure. The hydrophobic side chains cluster together on the inside and combine to the globular tertiary structure. The quaternary struc- ture forms when two or more polypeptide chains bond. Heat, acids, bases, salts, or mechanical agitation denature proteins. The primary structure doesn’t change but the shape of the protein does, and it no longer functions.

LO 6.2 Protein Digestion Occurs in the Stomach and Small Intestine

NH2 OHC

Side

chain

Amine

group

Carboxyl (acid)

group

LO 6.3 The Metabolism of Protein Is Based on the Body’s Needs During protein synthesis, the cell derives amino acids from the amino acid pools throughout the body. Proteins are assembled by ribosomes in the cell cytoplasm, using instructions encoded in genes in the DNA in the cell nucleus and trans- ported to the cytoplasm by messenger RNA. Excess amino acids can be deami- nated and then converted into ATP, glucose, or fatty acids. Transamination is used to synthesize nonessential amino acids.

Amino acids

Capillary

Enterocytes

ATP

Amino acid pool

Protein turnover Gluconeogenesis Energy production Fat cells

The chemical digestion of protein begins in the stomach. Gastrin stimulates the release of HCl from the parietal cells and the inactive enzyme pepsinogen from the chief cells. HCl denatures the protein and converts pepsinogen to pepsin, which breaks polypep- tides into shorter chains. Cholecystokinin from the duodenum stimulates tryp- sinogen, carboxypeptidase, and chymotrypsinogen from the pancreas. These proteases hydrolyze the shorter chains into tripeptides and dipeptides. Dipeptidases and tripeptidases hydrolyze the tripeptides and dipeptides into single amino acids that are absorbed through the enterocytes via the portal vein to the liver. Absorbed amino acids are used to synthesize new proteins, or are converted to energy (ATP), glucose, or fat and stored in the adi- pose tissue.

Visual Chapter Summary 241

Nitrogen intake

Equilibrium

Nitrogen

excretion

LO 6.4 Protein Plays Key Roles in the Body Proteins provide structural and mechanical support and help maintain body tissues. Most enzymes and many hormones are proteins, as are antibodies and certain other chemicals involved in the immune response. Proteins help maintain fluid and acid–base balance, transport substances throughout the body, and act as channels in cell membranes. Proteins also provide energy, improve satiety, and help control appetite.

LO 6.5 Protein Needs Are Based on the Dietary Reference Intakes

and Determined by Nitrogen

Balance Studies For a healthy adult, the amount of dietary protein consumed every day should equal the amount of protein used. Nitrogen balance studies suggest an RDA for adults of 0.8 grams of pro- tein per kilogram of body weight daily. Men typically consume more than 100 grams of protein daily, and women more than 70 grams—in both cases, far more than is needed. The AMDR for protein intake is between 10 and 35 percent of total daily kilocalories.

LO 6.6 Protein Foods Are Evaluated by Their

Digestibility, Amino Acid

Score, and Essential Amino

Acid Content Protein quality is determined by the body’s ability to digest the protein and the essential or non essential amino acids that the protein contains, called the amino acid score. A protein’s digestibility and its amino acid score are combined to yield a protein digestibility corrected amino acid score (PDCAAS). The essential amino acid with the lowest score is called the limiting amino acid. Proteins with a higher PDCAAS are of higher quality. A complete protein, found in animal foods, quinoa, and soy, provides a complete set of the essential amino acids. Plant pro- teins are typically incomplete, as they are missing or low in one or more of the essential amino acids. Combining complementary protein sources can yield a high-protein meal. Healthy food sources of proteins include eggs, lean meats, low-fat or fat-free dairy products, quinoa, soy, other legumes, nuts, and seeds. Grains and vegetables also supply protein to the diet. Most people consume more than enough protein each day and thus protein supplements are not necessary.

242 Chapter 6 | Proteins

LO 6.7 Too Much or Too Little Protein Is Linked to Health

Problems A diet too high in protein is linked to health problems such as cardiovascular disease, kidney stones, osteoporosis, and some types of cancer. An excess of protein-rich foods can displace whole grains, fruits, and vegetables in the diet. Eating too little protein can also compromise bone health.

Diets that are inadequate in protein, kilocalories, or both lead to protein-energy malnutrition (PEM), two forms of which are marasmus and kwashiorkor. Marasmus is a severe wasting disease caused by insufficient intake of kilocalories. Kwashiorkor occurs when a person consumes sufficient kilo- calories but not sufficient protein. PEM significantly increases vulnerability to infectious disease. It is treated with careful refeeding.

LO 6.8 Vegetarian Diets Can Reduce the Risk of Certain

Diseases Healthy vegetarian diets can reduce the risk of heart disease, high blood pressure, diabetes, cancer, stroke, and obesity. All vegetarians must take care to eat a varied diet that meets all of their nutrient needs, especially for protein, iron, zinc, calcium, vitamin D, vitamin B12, vitamin A, and omega-3 fatty acids.

Check Your Understanding 243

Terms to Know ■ proteins

■ amino acids

■ amine group

■ side chain

■ peptide

■ dipeptide

■ tripeptide

■ polypeptide

■ peptide bonds

■ essential amino acid

■ nonessential amino acid

■ conditionally essential amino acid

■ primary structure

■ secondary structure

■ tertiary structure

■ quaternary structure

■ denature

■ amino acid pools

■ protein turnover

■ genes

■ ribosomes

■ transcription

■ messenger RNA (mRNA)

■ translation

■ transfer RNA (tRNA)

■ elongation

■ sickle cell anemia

■ deamination

■ urea

■ transamination

■ catabolic

■ anabolic

■ albumin

■ edema

■ acidosis

■ alkalosis

■ buffers

■ transport proteins

■ antibodies

■ immunity

■ allergen

■ glucogenic amino acids

■ amino acid score

■ limiting amino acid

■ nitrogen balance

■ protein digestibility corrected amino

acid score (PDCAAS)

■ complete protein

■ incomplete protein

■ isoflavones

■ protein-energy malnutrition (PEM)

■ kwashiorkor

■ marasmus

■ vegetarian

Check Your

Understanding

LO 6.1 1. Proteins differ from carbohy- drates and lipids because a. they contain carbon–carbon

bonds. b. they contain nitrogen. c. they contain carbon, hydro-

gen, and oxygen. d. only proteins vary in chain

length. LO 6.1 2. Which of the following non-

essential amino acids can also be considered a conditionally essen- tial amino acid? a. Alanine b. Serine c. Glutamic acid d. Proline

LO 6.2 3. The enzyme that begins the chemical digestion of protein in the stomach is a. carboxypeptidase. b. pepsin. c. alcohol dehydrogenase. d. ghrelin.

Mastering Nutrition Visit the Study Area in Mastering Nutrition to hear an MP3 chapter summary.

LO 6.3 4. Gluconeogenesis is stimulated when a. the diet is high in

carbohydrate. b. the diet is high in fat. c. the diet is low in protein. d. the diet is low in

carbohydrate. LO 6.3 5. Before excess amino acids can

be used for energy or stored as fat, they must be a. deaminated. b. digested. c. denatured. d. deactivated.

LO 6.4 6. Proteins play important roles in the body, but do not a. regulate fluid balance. b. enable movement. c. act as chemical messengers. d. constitute part of bile used to

emulsify fat. LO 6.5 7. Connie is a healthy 22-year-

old student who weighs 155 pounds. What is her Recom- mended Dietary Allowance (RDA) for protein? a. 56 grams per day b. 70 grams per day c. 86 grams per day d. 140 grams per day

LO 6.6 8. Protein is found abundantly in a. fruits and nuts. b. milk, eggs, meat, and beans. c. vegetables and whole grains. d. oils and sugars.

LO 6.7 9. Kwashiorkor is a type of PEM that develops when a. there is a severe deficiency

of protein in the diet but adequate kilocalories.

b. intake of both protein and kilocalories is inadequate.

c. there is an inadequate amount of animal protein in the diet but sufficient plant protein.

d. there are adequate amounts of both protein and kilocalo- ries in the diet.

LO 6.8 10. Which of the following is a characteristic of a lacto-vegetar- ian diet? a. Calcium is likely to be defi-

cient in this diet. b. This diet is likely to be low in

fiber. c. The risk of developing heart

disease is reduced on this diet. d. This diet includes eggs and

dairy.

244 Chapter 6 | Proteins

Answers

1. (b) Proteins differ from carbohy- drates because they contain nitrogen found in the amine group. All three macronutrients contain carbon, hydrogen, and oxygen. The chains of glucose units or fatty acids vary in length as do chains of amino acids.

2. (d) Proline can be a conditionally essential amino acid under certain conditions.

3. (b) Pepsin is the active form of the enzyme that begins protein diges- tion. Carboxypeptidase and alcohol dehydrogenase are other digestive enzymes. Ghrelin is a hormone that stimulates appetite.

4. (d) If an individual does not eat an adequate amount of carbohydrate, the body can break down proteins to create glucose.

5. (a) Before amino acids can be con- verted to glucose or fatty acids, or enter the energy cycle, the amine group must first be removed through deamination. Denaturing is the pro- cess of unfolding or changing the shape of proteins.

6. (d) The body needs adequate amounts of protein to maintain fluid balance, to provide struc- tural and mechanical support for movement, and as hormones, which act as chemical messengers. Protein is not part of bile used to emulsify fat.

7. (a) Adults need 0.80 gram protein per kilogram of body weight daily. To calculate Connie’s daily protein needs, convert her body weight to kilograms and multiply by 0.8 gram per kg of body weight. She needs 56 grams per day.

8. (b) Animal foods and some plant- based proteins, such as beans and nuts, are protein rich. There is some protein in vegetables, but little in fruits. Oils and sugars do not contain protein.

9. (a) Kwashiorkor occurs when protein is deficient in the diet even though kilocalories may be adequate. Marasmus occurs when kilocalories are inadequate in a person’s diet. Pro- tein from animal sources is not nec- essary because people can meet their protein needs from a combination of plant proteins.

10. (c) Consuming a lacto-vegetarian diet reduces the risk for heart dis- ease. Because a lacto-vegetarian diet includes dairy products, it is likely to provide sufficient calcium. Because it is a vegetarian diet, it is likely to provide sufficient dietary fiber. Eggs are excluded on a lacto- vegetarian diet.

Answers to True or

False?

1. True. Proteins are the only macronu- trients that contain nitrogen.

2. False. Of the 20 amino acids that make up protein, nine are considered essential and 11 are nonessential.

3. False. Chemical digestion of pro- tein begins in the stomach with the enzyme pepsin, which is secreted by the chief cells lining the stomach.

4. True. When proteins arrive in the stomach, hydrochloric acid uncoils them, revealing the peptide bonds that connect the amino acids.

5. True. Amino acids can be con- verted to glucose through gluconeogenesis.

6. False. The primary function of pro- tein is to build new tissues and repair proteins that have been degraded or sloughed off in the body.

7. False. Growing children are in a state of positive nitrogen balance, which means that more nitrogen is being retained by the body than excreted in the urine.

8. False. Animal proteins are consid- ered complete proteins because they contain all nine essential amino acids.

9. False. Protein itself doesn’t raise blood cholesterol levels. It depends on the type of protein food you consume.

10. True. A diet that is inadequate in pro- tein results in kwashiorkor, charac- terized by edema and body wasting.

Web Resources

■ For information on specific genetic disorders, including those that affect protein use in the body, visit the National Human Genome Research Institute at www.nhgri.nih.gov

■ For more information on protein bars and supplements, visit the Center for Science in the Public Interest at www.cspinet.org

■ For more information on vegetarian diets, visit the Vegetarian Resource Group at www.vrg.org

References

1. Marieb, E. N., and K. Hoehn. 2016. Human Anatomy and Physiology. 10th ed. San Fran- cisco: Benjamin Cummings.

2. Ibid. 3. Marieb. 2016. Human Anatomy and Physiology. 4. National Human Genome Research Insti-

tute. 2014. Learning About Sickle-Cell Disease. Available at www.genome.gov. Accessed February 2017.

References 245

5. Berg, J. M., J. L. Tymoczko, and L. Stryer. 2013. Biochemistry. 7th ed. New York: W. H. Freeman and Company.

6. Kranz, S., et al. 2017. High-Protein and High-Dietary-Fiber Breakfasts Result in Equal Feelings of Fullness and Better Diet Quality in Low-Income Preschoolers Compared with Their Usual Breakfast. Jour- nal of Nutrition pii: jn234153. doi: 10.3945/ jn.116.234153.

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