Nutrition

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Chapter 4:
Human Digestion and Absorption

FN 255

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Digestive System

  • Digestion
  • Process of breaking down foods into a form the body can use

  • Mechanical and chemical breakdown of food

  • Absorption
  • Uptake of nutrients from GI tract into blood or lymph

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Digestive Organ System

  • GI tract
  • Mouth, esophagus, stomach, small intestine, large intestine

  • Accessory organs
  • Liver, pancreas and gall bladder

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Pathway of Movement

Mouth

Esophagus

Stomach

Small Intestine

Large Intestine

4C. Or 1L. capacity

2-3 hrs.

3-10 hrs.

72 hrs or longer.

Duodenum 10 in.

Jejunum 4 ft.

Ileum 5 ft.

3.5 ft.

10 in.

Anatomy of the GI Tract

  • GI tract (alimentary canal)
  • Long hollow muscular canal

  • Four layers
  • Mucosa
  • Innermost layer-hollow area is the lumen
  • Submucosa
  • Contains blood vessels carrying nutrients
  • Muscle
  • Move food forward
  • Serosa
  • Outside layer-protects the tract

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Sphincters

  • Ring like muscles that control the flow of contents in the GI tract

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GI Motility

  • Peristalsis
  • Contractions propel food

  • Segmentation
  • Back and forth movement; breaks apart food

  • Mass movements
  • Peristalsis over widespread area of LI to help eliminate waste

  • Elimination

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1.unknown

Mouth (Oral Cavity)

  • Mastication - Chewing increases surface area
  • Mixed with saliva food becomes a bolus
  • Saliva
  • Lysozyme
  • Break down bacteria
  • Mucus
  • Lubricate and hold bolus together
  • Amylase
  • First step in CHO digestion - breaks down starch

  • Lingual lipase
  • Inactive until reaches stomach

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Esophagus

  • Transports bolus and liquid to stomach

  • Swallowing
  • Moves bolus from mouth to esophagus

  • Epiglottis
  • Prevents food from lodging in trachea

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Stomach

  • Bolus goes thru LES into stomach

  • Formation of chyme

  • Pyloric sphincter

  • Gastric inhibitory peptide hormone (GIP)
  • Slows release of chyme into SI

  • Lingual lipase becomes active

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Stomach Secretions

  • Parietal Cells
  • Hydrochloric Acid
  • Inactivates proteins
  • Destroys bacteria and viruses
  • Aids in mineral absorption
  • Converts pepsinogen into pepsin (digests protein)
  • Pepsinogen
  • Inactive enzyme

  • Intrinsic Factor (IF)
  • Needed for B12 absorption

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Stomach Secretions

  • Chief cells
  • Gastric lipase
  • G cells
  • Pyloric Region of Stomach
  • Gastrin
  • Stimulates HCL and pepsinogen secretion
  • ↑ LES pressure
  • Stimulated by protein, caffeine, ETOH, vagus nerve, distention of stomach

Foveolar cells (a.k.a. surface mucus cells, mucus neck cells)

  • Protects the stomach from being digested
  • Heavy NSAID use may inhibit mucus production

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Small Intestine

  • Most digestion and absorption occurs here

~95% of absorption takes place here

  • Muscle contractions mix food with digestive juices

  • Food remains for 3-10 hours

  • Sections:
  • Duodenum, jejunum, ileum
  • Ileocecal valve

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Physiology of the Small Intestine

  • Physiology of the SI
  • Wall is folded – many involutions
  • Villi projections located on the folds
  • Absorptive cells (enterocytes) located on villi
  • Microvilli located on the villi
  • Increase intestinal surface area 600 x

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Figure 04_14

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Small Intestine Cells and Secretions

  • Goblet cells
  • Mucus
  • Glycocolyx – enzyme containing proteins
  • On surface of microvilli enterocytes (brush border of SI)
  • Enzymes
  • Maltase, sucrase, lactase
  • Aminopeptidase, dipeptidase

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2.unknown

Other Secretions

  • Gallbladder

  • Pancreas
  • Enzymes, bicarbonate
  • Pancreatic lipase, amylase, proteases (i.e. pepsinogen, chymotrypsin)

Gastric Inhibitory Peptide (GIP) (Secretion stimulated by CHO and fat)

  • Secreted by small and large intestine
  • s gastric motility
  • Stimulates insulin secretion

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Large Intestine

  • Little digestion here
  • Indigestible food stuff
  • Absorption of water, some minerals, vitamins
  • Sodium, potassium, Vitamin K, Biotin, some SCFA
  • Bacteria
  • Formation of feces for elimination

  • Sections:
  • Cecum, ascending, transverse, descending and sigmoid

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Bacterial Flora

  • Beneficial bacterial flora
  • Controls pathogenic bacteria
  • Synthesize Vitamin K and Biotin
  • Aid in lactose digestion and fermentation

  • Probiotic
  • Live bacteria
  • Prebiotic
  • Non digestible carbohydrates that promote growth of bacteria - inulin, fructans and resistant starch

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Accessory Organs

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Accessory Organs

  • Liver
  • Synthesis of proteins
  • Synthesis of bile salts
  • Detoxification of hormones, drugs, toxins

  • Excretory functions
  • Enterohepatic circulation of bile salts

  • Water soluble nutrients released into portal vein to liver
  • Hepatic veins release nutrients to general circulation

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Enterohepatic Circulation

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Accessory Organs

  • Gallbladder
  • Concentration and storage of bile
  • Hormones
  • Cholecystokinin (CCK) stimulates contraction
  • Pancreatic polypeptide inhibits contraction

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Accessory Organs

  • Pancreas
  • Endocrine function
  • Insulin and Glucagon

  • Exocrine function
  • Bicarbonate and digestive enzymes

  • Enzymes
  • Proteases (trypsin, chymotrypsin, carboxypolypeptidase, etc.), amylase, lipase

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Modes of Absorption

  • Primarily occurs in small intestine
  • Water, some minerals, and fiber reach large intestine
  • 5% of CHO, PRO, and FAT reach Lg Int

  • Passive
  • Facilitated
  • Active
  • Endocytosis

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Modes of Absorption

  • Passive
  • Higher to lower concentration ([ ])
  • Permeability of substance thru wall of SI
  • [ ] is high in lumen, low in enterocytes
  • [ ]drives nutrient into cells

  • Fats, water, some minerals are examples

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Modes of Absorption

  • Facilitated diffusion
  • Higher to lower concentration + carrier protein

  • Carrier protein shuttles substance into enterocyte
  • Difference in [ ] in lumen and cell not large enough to drive absorption by diffusion alone
  • NO energy is expended

  • Fructose is an example

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Modes of Absorption

  • Active absorption
  • Lower concentration to higher + energy

  • Carrier protein shuttles substance into enterocyte
  • Allows for absorption against a [ ] gradient

  • Energy IS needed – ATP

  • Glucose, amino acids are examples

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Absorption

  • CHO absorption
  • Glucose and galactose absorbed by active transport
  • Sodium dependent glucose transporter
  • ATP provided by sodium-potassium pump

  • Fructose absorbed by facilitated diffusion

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Absorption

  • Protein absorption
  • AA, small peptide fragments (di- and tripeptides) absorbed by active transport via carrier

  • Endogenous protein (digestive cells, enzymes) also enter the lumen for absorption

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Absorption

  • Fat absorption
  • Fat insoluble in water

  • Undergoes series of physical/chemical transformations

  • Aggregates into large oily TG droplets in upper SI that float in chyme

  • Bile emulsifies lipid droplets and facilitates absorption of fats through formation of micelles

  • Micelles reach luminal membranes of epithelial cells
  • MG and FFA passively diffuse from micelles thru lipid component of epithelial cell membrane

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Moving Nutrients

  • Cardiovascular system
  • Water-soluble nutrients are absorbed into bloodstream through capillary beds in the villi
  • Proteins, CHO, vit C, B vit’s, SCFA, MCFA
  • Lymphatic system
  • Fat-soluble nutrients are absorbed into lymphatic system through lacteals in the villi
  • LCFA, MCFA, Sterols, other lipids, Fat-soluble vitamins
  • Transported in lipoproteins through lymph
  • Enter bloodstream through thoracic duct

Water Absorption

  • Most occurs in small intestine
  • Small amount in stomach
  • 70-90% in small int
  • 10-30% in Large int
  • 95% of fluid that enters small intestine is absorbed
  • Fluid from beverages, foods (3L), and digestive secretions (7L)
  • ~ 500 ml of fluid from small intestine pass to the colon
  • ~350 ml of fluid absorbed by the colon

Total loss of fluid from GI tract only ~150 ml/day

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Sites of Absorption

Digestive Problems, p.138-145

  • Heartburn and GERD
  • Ulcers
  • Gallstones
  • Food Intolerances
  • Celiac Disease
  • Constipation
  • Diarrhea
  • Irritable Bowel Syndrome
  • Inflammatory Bowel Disease
  • Hemorrhoids

Hormone Stimulus for Secretion Secreted by Action

Gastrin Food in stomach, esp.

protein, caffeine, ETOH

Pyloric region of stomach

Upper duodenum

HCL secretion

Pepsinogen secretion

Secretin Acid chyme, partially

digested protein

Duodenum, jejunum Pancreas to produce bicarbonate

Cholecystokinin

(CCK)

Food, esp. fat and protein

in duodenum

Duodenum, jejunum Contraction of gallbladder

Secretion of pancreatic enzymes

Inhibits stomach motility

Gastric Inhibitory

Peptide (GIP)

Protein and fat in chyme

Small intestine Inhibits stomach motility

Insulin secretion