GI 7: Small Intestine Digestion & Absorption (CHO + Fat)

Major Foods Classification and Ingestion

Carbohydrate, Fat, or Protein

- (Vitamins & Minerals are others nutrient groups)

Ingested as larger complex compounds (Polysaccharides, Triglycerides, Polypeptides)

In order to be assimilated into the body, larger complex compounds must be?

first broken down (Digested) into molecules capable of being absorbed

- Monosaccharides, Fatty Acids, & Amino Acids

- i.e. must be able to be transported from the lumen across the GI mucosal epithelium and into the blood or lymph by various substrate-specific transport processes

what is the major site of digestion & absorption?

small intestine

Intestinal epithelial cell loss + turnover

cell loss is high

- mainly due to the abrasion resulting from the movement of GI content through the tract

cell turnover is rapid

- replaced 3-6 days

- due to stem cells located in Villus Crypts that produce new Enterocytes and Mucus secreting Goblet Cells

- these cells constantly migrate upwards and replace cells lost at villus tips

hydrolysis

the basic process of digestion

Digestion of Carbohydrates

Enzymes return hydrogen and hydroxyl ions from water (hydrolysis) to polysaccharides separating the monosaccharides from one another

3 major sources:

• Sucrose: a disaccharide

• Lactose: a disaccharide

• Starch: a polysaccharide

hydrolysis of monosaccharides

Digestion of Carbohydrates: Mouth

enzyme: Salivary α amylase - ptyalin

product: maltose

% digested: 5% of starch

Digestion of Carbohydrates: Stomach

enzyme: Gastric enzymes

product: maltose

% digested and time: 30-40% of starch, 1 hour

Digestion of Carbohydrates: Small Intestine

enzyme: Pancreatic amylase

product: maltose

% digested: 100%

Digestion of Carbohydrates: Small Intestine - Brush Border

enzymes:

- Lactase: Lactose

- Sucrase

- Maltase

- α dextrinase

product: glucose

% digested:

- Glucose is 80% of final product

- Galactose and fructose are 10% each

Carbohydrate substrate

constitute 50-80% of a typical American diet (300-500 gms = 1,600 C/day)

includes:

- starch

- glycogen

- disaccharides

- monosaccharides

starches

50-60% of digested carbs

includes:

• amylopectin (Branched Glucose)

• amylose (Unbranched Glucose)

• Plants (Potatoes & Grains)

glycogen

branched glucose

- (Small Amt.) Animal Tissue

disaccharides

sucrose (Glucose-Fructose)

• 30-35% (Fruits; Table Sugar)

lactose (Glucose-Galactose)

• 7-10% (Dairy Products)

maltose (Glucose-Glucose)

• 3-5% (Many Foodstuffs)

monosaccharides

Single sugar molecules

- glucose and fructose

minor carbohydrates

• Alcohol

• Lactate

• Pectin

• Dextrin

• Pyruvate

cellulose

• Fiber = Bulk

• Not Digested

• Glucose β-1,4

• Bacteria

Carbohydrate Digestion Substrates Overview

Phases of CHO Digestion

Ingested Starches, Glycogen & Disaccharides must be hydrolyzed to Monosaccharides in order to be absorbed by enterocytes

Digestion occurs in two phases:

A. Luminal

B. Membrane

Luminal Phase of digestion of CHO

• begins in the mouth & stomach through the actions of Salivary ⍺-Amylase

• However, digestion mainly occurs in the small intestine via Pancreatic ⍺-Amylase action

final products of ⍺-Amylase action (luminal phase)

1. small oligosaccharides

• Maltose

• Maltotriose

• a-Limit Dextrin

2. Disaccharides

• Maltose

• Lactose

• Sucrose

Luminal phase of digestion of CHO does NOT produce?

⍺-Amylase Digestion Does NOT Produce Any Absorbable Monosaccharides

• The end-products above remain undigested because ⍺-Amylase can only hydrolyze Interior ⍺-1,4 linkages

• Most end products possess either Terminal ⍺-1,4 linkages or Branch Point ⍺-1,6 Linkages

Membrane phase of digestion of CHO

products of Luminal Digestion are further hydrolyzed

- forms absorbable Monosaccharides

- done by digestive enzymes located on the mucosal brush border membrane

Membrane enzymes

are mostly Disaccharidases that cleave:

1. one or more ⍺-amylase digestion product

- ⍺-Dextrins

- Maltotroiose

- Maltose

2. ingested disaccharides

- Sucrose

- Lactose

- Trehalose

Membrane Digestion

Maltose, Lactose, Sucrose & Maltotriose and ⍺-Limit Dextrin must be further digested to monosaccharides incoordination with enzymes associated with the second phase of carbohydrate breakdown

⍺-amylase requires?

a near neutral pH (optimal 6.7)

- to retain activity

- disorders that impair the ability to neutralize acidic gastric chyme will also impair carbohydrate digestion

Digestion: Fat

mechanism: Bile and agitation

end product: Emulsified fat

Digestion: Emulsified Fat

mechanism: Pancreatic lipase

end product: Monoglyceride and free fatty acids

Digestion: Cholesterol Esters and Phospholipids

mechanism:

- Cholesterol ester hydrolase

- Phospholipase A2

end product:

- Monoglycerides

- free fatty acids

Digestion of the triglyceride molecule

the fat digesting enzymes return three molecules of water to the TG molecule, splitting the fatty acid molecule away from the glycerol

Fat digestion locations

1. oral cavity

2. stomach

3. duodenum

Fat digestion: Oral Cavity

Lingual Lipase

- secreted into Saliva

- initiates fat digestion

- Due to limited exposure time, actions are Non-Essential for complete fat digestion

Fat digestion: Stomach

Gastric Lipase

• secreted from the fundus

• has similar actions to pancreatic lipase.

• gastric lipase accounts for only 10% of dietary fat (triglycerides) digestion

• Gastric lipase activity is usually considered Non-Essential because of the normally excessive secretion of pancreatic lipase

Indirect Actions: Mechanical Breakdown & Mixing of lipid compounds

• important for lipid digestion

• increases Surface Area for breakdown by Lipase

- assists in the emulsfication of hydrophobic fats

when does gastric lipase become essential?

gastric contribution can be important under abnormally high acid output conditions (Zollinger-Ellison syndrome) that can cause pancreatic lipase inactivation

Fat digestion: Duodenum

Pancreatic Lipase

- 90% of enzymatic fat digestion

- digests triglycerides into fatty acids & 2-monoglyceride

- activity depends on Colipase. secreted from the pancreas and functions as follows:

• acts at the lipid-water interface of fat droplets to hydrolyze emulsified triglycerides

Sites of Dietary Fat Digestion Overview

Lipase Impairment

• Pancreatic Insufficiency: Lipase Deficiency

• H+ Hypersecretion: Lipase Inactivation

Review of Micelle Formation & Lipid Digestion

Fat digestion - Triglycerides

the major components of dietary fat

• comprised of three fatty acids esterified to glycerol

• Pancreatic Lipase digests Triglycerides into two Free Fatty Acids (FFA) and 2-Monoglyceride

Fat digestion - Cholesterol esters

• Consist of cholesterol esterified to a fatty acid.

• Cholesterol Ester Hydrolase hydrolyzes esters to Cholesterol & FFAs

• completes triglyceride breakdown by hydrolyzing 2-Monoglyceride to Glycerol & FFA

Fat digestion - Phospholipids

e.g. Lecithin

• comprised of glycerol esterified to two fatty acids and either choline (phosphatidylcholine) or inositol (phosphatidylinositol)

• Phospholipase A2 releases FFAs to yield lysophospholipids (Lysolecithin)

Summary of Fat Digestion

Intestinal Bacteria

- E. Coli

- Enterobacter aerogenes

- Clostridium perfringens (Gas Gangrene)

found:

- duodenum/jejunum - few if any

- ileum - few

- colon - large numbers

• Harmless Inside

• Harmful Outside GI Tract (Sepsis)

Intestinal Bacteria Produce?

• Vitamin K

• Vitamin B12

• Thiamine (B1)

• Riboflavin (B2)

• Folate

• Short-Chain FFA

• Pigments (Bile)

• Gases

• Amines

• Ammonia

Intestinal Bacteria Utilize?

• Cellulose

• Vitamin C

• Vitamin B12

• Choline