Physiology of Digestion and Food Absorption

Physiology of Digestion and Food Absorption

1. Digestive Process:

• Mouth:

• Chewing: Increases the surface area of food for digestive enzymes.

• Salivary amylase secretion: Amylase in saliva breaks down starch into shorter polychains (dextrin) and maltose.

• Sweet taste creation: Maltose creates a sweet taste when chewing starch for a long time in the mouth.

• Stomach:

• Chyme: The mixture of food and gastric juice when ground up.

• Esophageal sphincter: Helps move food into the stomach, preventing reflux.

• Hydrochloric acid (HCl) secretion:

• Helps unravel protein chains, making it easier for digestive enzymes to access peptide bonds.

• Activates pepsinogen into active pepsin.

• Creates a suitable pH for pepsin activity.

• Pepsin secretion: Breaks down proteins into shorter polypeptide chains.

• Temporary suspension of starch digestion: Due to the acidic pH in the stomach, amylase is inactivated.

• Fiber: Prolongs stomach distention and creates a feeling of fullness.

• Small intestine:

• Digestive enzyme secretion: Intestinal mucosal cells secrete enzymes for food digestion.

• Primarily digests carbohydrates: Hydrolyzes into disaccharides and monosaccharides.

• Absorption of Glucose, galactose, fructose:

• Glucose and galactose enter the small intestine through active transport.

• Fructose enters the small intestinal mucosal cells through facilitated diffusion.

• Protein absorption: Small intestinal cells absorb amino acids and a small amount of dipeptides.

• Fat digestion:

• Pancreatic lipase breaks down triglycerides into monoglycerides and 3 fatty acids.

• Bile salts aid in the absorption of long-chain fatty acids.

• Chyme passage time through the small intestine to the ileocecal valve: 3-5 hours.

• Large intestine:

• Ileocecal valve: Prevents backflow of feces into the small intestine.

• Fiber digestion: Bacteria in the large intestine ferment fiber and remaining starch in an anaerobic process, producing water, gas, and short-chain fatty acids (acetic, propionic, butyric acid).

• Short-chain fatty acid absorption: Absorbed through the colon.

• Chyme passage time through the large intestine: 24-48 hours.

• Lactase: If not absorbed into the bloodstream, it is transferred to the large intestine.

2. Absorption:

• Glucose absorption: Primarily occurs in the small intestine.

• Fat absorption: Requires bile salts for absorption of long-chain fatty acids.

• Protein absorption: Small intestinal cells absorb amino acids and a small amount of dipeptides.

• Monosaccharides are absorbed through the portal vein.

3. Metabolism:

• Liver: Liver cells capture fructose and galactose, converting them primarily into glucose.

• Carbohydrate metabolism products: CO2, H2O.

• Protein metabolism products: CO2, H2O, NH3.

• Fat: In the body, 99% of lipid reserves are in the form of triglycerides.

4. Notes:

• Fiber:

• Draws water, softening stool.

• Not completely digested, fermented by bacteria to produce short-chain fatty acids.

• Unbranched starch: Digested slowly and causes a milder increase in blood sugar compared to branched starch.

• Disaccharides: Provide an indirect source of glucose through fructose and galactose metabolism.

• Sugar: A “clean” energy source for the body.

• Protein:

• A “dirty” energy source.

• Does not directly become protein in the body.

• Eating protein provides amino acids.

• MCT: A supplement for fat in cases of malabsorption of long-chain fatty acids and liver and biliary diseases.

• Cow’s milk protein allergy: Due to the absorption of dipeptides by intestinal cells.

5. Length:

• Small intestine: 7m.

• Large intestine: 1.5m.

6. Digestive Enzymes:

• Amylase: Secreted in saliva.

• Lipase:

• Secreted from the salivary glands at the base of the tongue.

• Fat digestion is primarily by pancreatic lipase.

• Peptidase: Separates tripeptides and dipeptides into amino acids.

7. Start:

• Protein digestion: Stomach.

• Carbohydrate digestion: Mouth.

8. Function:

• Protein digestion: Primarily begins in the stomach.

• Fat absorption: After emulsification, fat comes into contact with lipase enzymes.

Note: This article is based on the provided information and may not be comprehensive or entirely accurate. Please refer to professional resources for a more comprehensive understanding of the physiology of digestion and food absorption.