Nutrition 1

Chapters 8 and 9 Summary Outline

8.1 What Are Vitamins?

Vitamins are essential organic nutrients that do not provide energy and are required in small quantities

in the diet. Americans consume vitamins that are naturally present in foods, added to foods by

fortification, and supplied by supplements. Some foods are fortified with nutrients according to

government guidelines to promote public health. Others are fortified according to the manufacturer’s

perceptions of what will sell in the marketplace.

The bioavailability of a vitamin depends on how much can be absorbed and used by the body. The ability

to store and excrete vitamins helps to regulate the amount present in the body. Water-soluble vitamins

are generally excreted more easily than and not stored as well as fat-soluble vitamins.

Vitamins are needed to promote and regulate body processes that are essential for growth,

reproduction, and tissue maintenance. Each vitamin has one or more functions: Many are coenzymes,

some are antioxidants. More than one vitamin is typically needed to ensure the health of a particular

organ or system.

Vitamin deficiencies remain a major health problem worldwide, but severe deficiencies are rare in

developed countries. Recommended intakes for vitamins are expressed as RDAs or AIs. UL values

estimate the highest dose that is unlikely to cause toxicity. Food labels can help identify packaged foods

that are good sources of some vitamins.

8.2 Thiamin

The best food sources of thiamin are lean pork, legumes, and whole or enriched grains.

Thiamin is required for the formation of acetyl-CoA from pyruvate and for a reaction in the citric acid

cycle and is therefore particularly important for the production of ATP from glucose. It is also needed for

the synthesis of the neurotransmitter acetylcholine.

Thiamin deficiency causes nervous system abnormalities (dry beriberi) or cardiovascular problems (wet

beriberi). Deficiencies are common in people with alcoholism. No toxicity has been identified.

8.3 Riboflavin

Milk, meat, and whole and enriched grains are the best food sources of riboflavin.

Riboflavin coenzymes are needed for the generation of ATP from carbohydrate, fat, and protein.

Riboflavin deficiency is rarely seen alone because food sources of riboflavin are also sources of other B

vitamins and because riboflavin is needed for the utilization of several other vitamins. No toxicity has

been identified.

8.4 Niacin

Beef, chicken, turkey, fish, and whole and enriched grains are the best food sources of niacin. The amino

acid tryptophan can be converted into niacin, so tryptophan from dietary protein can meet some of the

niacin requirement.

Niacin coenzymes are important in the breakdown of carbohydrate, fat, and protein to provide energy

and in the synthesis of fatty acids and sterols.

Niacin deficiency results in pellagra, which is characterized by dermatitis, diarrhea, dementia, and

finally, if untreated, death.

Supplements of the nicotinic acid form of niacin can lower elevated blood cholesterol but frequently

cause toxicity symptoms such as flushing, tingling sensations, nausea, and a red skin rash.

8.5 Biotin

Liver and egg yolks are good sources of biotin.

Biotin is needed for the synthesis of glucose and fatty acids and for the metabolism of certain amino

acids.

An RDA has not been established for biotin because some of our requirement for this vitamin is met by

bacterial synthesis in the GI tract. However, an AI has been set. Toxicity has not been reported.

8.6 Pantothenic Acid

Pantothenic acid is abundant in the food supply and deficiency is rare.

Pantothenic acid is part of CoA, which is required for the production of ATP from carbohydrate, fat, and

protein and the synthesis of cholesterol and fatty acids. There is no RDA, but an AI has been established.

8.7 Vitamin B6

Food sources of vitamin B6 include chicken, fish, liver, and whole grains. Enriched grains are poor

sources.

Pyridoxal phosphate, the coenzyme form of vitamin B6, is needed for the activity of more than 100

enzymes involved in the metabolism of carbohydrate, fat, and protein. Vitamin B6 is a coenzyme for

transamination, deamination, and decarboxylation reactions and is therefore particularly important for

amino acid metabolism.

Vitamin B6 deficiency causes neurological symptoms, anemia due to impaired hemoglobin synthesis,

poor immune function, and elevated levels of homocysteine, which can increase the risk of

cardiovascular disease. High intakes from supplements can cause nervous system abnormalities.

8.8 Folate (Folic Acid)

Food sources of folate include liver, legumes, oranges, leafy green vegetables, and fortified grains.

Folate is necessary for the synthesis of DNA, so it is especially important for rapidly dividing cells.

Folate deficiency results in macrocytic anemia and can cause an increase in homocysteine levels. Low

levels of folate before and during early pregnancy are associated with an increased incidence of neural

tube defects in the offspring. A high intake of folate can mask the anemia caused by vitamin B12

deficiency. Low intakes of folate have been associated with an increased risk of cardiovascular disease

and certain types of cancer.

It is recommended that women of childbearing age consume 400 μg of folic acid from fortified foods

and supplements in addition to the folate found in a varied diet.

8.9 Vitamin B12

Vitamin B12 is found almost exclusively in animal products.

The absorption of vitamin B12 from food requires adequate levels of stomach acid, intrinsic factor, and

pancreatic secretions.

Vitamin B12 is needed for the metabolism of folate and fatty acids and to maintain the insulating layer

of myelin surrounding nerves.

Vitamin B12 deficiency increases homocysteine levels and can result in anemia and permanent nerve

damage. Pernicious anemia is an autoimmune disease that results in severe vitamin B12 deficiency due

to an absence of intrinsic factor. Vitamin B12 deficiency may also occur in vegans, who consume no

animal products, and in older individuals with low stomach acid due to atrophic gastritis.

8.10 Vitamin C

The best food sources of vitamin C are citrus fruits.

Vitamin C is necessary for the synthesis and maintenance of collagen and for the synthesis of hormones

and neurotransmitters. Vitamin C is also a water-soluble antioxidant. Antioxidants protect the body from

reactive oxygen molecules such as free radicals. These molecules are generated from normal body

reactions and come from the environment. They cause damage by stealing electrons from DNA,

proteins, carbohydrates, and unsaturated fatty acids.

Vitamin C deficiency, called scurvy, is characterized by poor wound healing, bleeding, and other

symptoms related to the improper formation and maintenance of collagen.

Vitamin C supplements are common and are frequently used to reduce the symptoms of the common

cold.

8.11 Choline and Other Vitamin-Like Compounds

There are a number of substances, such as carnitine, lipoic acid, and inositol, that perform vitamin-like

functions in the body, but are not classified as vitamins because adequate amounts can be synthesized.

Choline is considered an essential nutrient but is not currently classified as a vitamin. Choline can be

synthesized in the human body, but the amounts are generally not enough to meet needs. The best

dietary sources of choline are egg yolks and meats.

9.1 Fat-Soluble Vitamins in Our Diet

Vitamins A, D, E, and K are soluble in fat, which affects how they are absorbed, transported, stored, and

excreted. Deficiencies of vitamins A and D are common in the developing world. In the United States,

the risk of deficiencies of fat-soluble vitamins is increasing due to low intakes of fruits and vegetables

and limited sun exposure.

9.2 Vitamin A Sources and Functions

Vitamin A is found both preformed as retinoids and in precursor forms called carotenoids. The major

food sources of preformed vitamin A include liver, eggs, fish, and fortified dairy products. Carotenoids

are found in plant foods such as yellow-, orange-, and red-colored fruits and vegetables and leafy green

vegetables. Some carotenoids are precursors of vitamin A. The most potent is β-carotene.

Preformed vitamin A and carotenoids are bound to proteins in food. To be absorbed they must be

released and incorporated into micelles. Fat malabsorption and very-low-fat diets reduce vitamin A

absorption.

In the body, the retinoids, which include retinol, retinal, and retinoic acid, are needed for vision and for

the growth and differentiation of cells. Retinol is transported in the blood and can be converted into

retinal or retinoic acid. Retinal binds to opsin in the eye to form rhodopsin. After light strikes rhodopsin

to begin the visual cycle, a nerve impulse is sent to the brain so the light is perceived. Retinoic acid

affects cell differentiation by altering gene expression. It is needed for healthy epithelial tissue and

normal reproduction and immune function. β-Carotene functions as an antioxidant, a role that is

independent of its conversion to vitamin A.

9.3 Meeting Vitamin A Needs

Vitamin A deficiency is a world health problem that increases the frequency of infectious disease and

causes blindness and death in hundreds of thousands of children. Mild vitamin A deficiency causes night

blindness. More severe deficiency interferes with cell differentiation. This impairs immune function and

growth and causes the epithelial surface of the eye to become hard and dry, leading to infection and

blindness.

The need for vitamin A can be met with preformed or provitamin A. Preformed vitamin A can be toxic at

doses as low as 10 times the RDA and can increase the risk of bone fractures and birth defects.

Carotenoids are not toxic, but a high intake can give the skin an orange appearance.

9.4 Vitamin D

Vitamin D can be made in the skin by exposure to sunlight, so dietary needs vary depending on the

amount synthesized. Vitamin D is found in fatty fish, cod liver oil, and fortified milk.

Dietary vitamin D as well as vitamin D synthesized in the skin must be modified by the liver and then the

kidney to form active vitamin D. Active vitamin D promotes calcium and phosphorus absorption from

the intestines and acts with parathyroid hormone to cause the release of calcium from bone and calcium

retention by the kidney. These roles are essential for maintaining proper levels of calcium and

phosphorus in the body.

Vitamin D deficiency in children results in a condition called rickets; in adults, vitamin D deficiency

causes osteomalacia. The risk of vitamin D deficiency is increased by factors that reduce the amount of

sunlight that reaches or penetrates the skin such as living at latitudes greater than 40 degrees north or

south, working and playing indoors, wearing concealing clothing, the presence of pollution and tall

buildings, having dark skin, and using sunscreen.

The RDA for vitamin D assumes limited sun exposure because the amount that is synthesized in the skin

is extremely variable. Vitamin D supplements are recommended for a number of groups including

breast-fed babies, people who do not drink milk, older adults, and individuals with dark skin

pigmentation or limited sun exposure. Too much vitamin D is toxic, but synthesis of vitamin D from

exposure to sunlight does not produce toxic amounts.

9.5 Vitamin E

Vitamin E is found in nuts, plant oils, leafy green vegetables, and fortified cereals.

Vitamin E functions primarily as a fat-soluble antioxidant. It is necessary for reproduction and protects

cell membranes from oxidative damage.

Vitamin E deficiency can cause hemolytic anemia and neurological problems.

Many people take vitamin E supplements. There is little risk of toxicity, but there is also little

documented evidence of any benefit from supplements.

9.6 Vitamin K

Vitamin K is found in plants and is synthesized by bacteria in the gastrointestinal tract.

Vitamin K is a coenzyme essential for the formation of blood-clotting factors as well as proteins needed

for normal bone mineralization and regulation of cell growth.

Deficiency causes bleeding, low-bone density, and an increased risk of atherosclerosis. Because vitamin

K deficiency is a problem in newborns, they are routinely given vitamin K injections at birth.

9.7 Vitamin/Mineral Supplements

Some groups require supplements to meet the need for certain nutrients, because they have low

intakes, increased needs, or excess losses. This includes dieters, vegans, infants and children, pregnant

women and women of childbearing age, older adults, individuals with dark skin pigmentation, and

people who have restricted diets, are taking medications, or who smoke or use alcohol.

Appropriate supplement use can promote health, but supplements should never be considered a

substitute for other good health habits and they should never be used instead of medical therapy to

treat a health problem.