Digestion and Absorption in Humans

Digestion and Absorption

• Digestion involves enzymes breaking down carbohydrates, proteins, and lipids.
• Absorption is performed by specialized epithelial cells.
• Most absorption occurs in the small intestine, with some in the stomach and none in the esophagus.
• The small intestine has a specialized structure to increase nutrient absorption.
• Mucosal folds triple the surface area.
• Villi increase the surface area tenfold.
• Microvilli increase the surface area twentyfold, resulting in a total of 600 folds.

Digestion and Absorption of Carbohydrates

• Carbohydrates are ingested primarily as starch, sucrose, lactose, and cellulose.
• Cellulose is not digested in the gut.
• Digestion begins in the mouth with salivary amylase (ptyalin).
• Salivary amylase breaks down starch into smaller glucose polymers.
• Pancreatic amylase further digests starch, breaking down 1-4 alpha glycosidic bonds, yielding maltose and maltotriose.
• Brush border enzymes (lactase, sucrase, maltase, and alpha-dextranase) completely hydrolyze disaccharides and glucose polymers into monosaccharides (glucose, fructose, and galactose) for absorption.

Digestion and Absorption of Proteins

• Protein digestion begins in the stomach with pepsin, aided by HCl.
• Pepsin breaks peptide bonds in proteins, producing polypeptides.
• Pancreatic proteolytic enzymes (endopeptidases and exopeptidases) further break down polypeptides into smaller peptides and amino acids.
• Brush border enzymes (peptidases) convert the smaller peptides into oligopeptides and amino acids.
• Di- and tripeptides are transported into the enterocyte by active transport.

Digestion and Absorption of Lipids

• Bile salts emulsify lipids into smaller droplets, increasing the surface area.
• Pancreatic lipase and co-lipase act on the water-oil interface of the triglycerides.
• This yields free fatty acids (FFAs), and monoglycerides.
• FFAs, monoglycerides, phospholipids, cholesterol, and bile salts form micelles.
• The micelles deliver lipids to the epithelial cells of the small intestine.
• The free fatty acids and monoglycerides are released into the enterocytes by passive diffusion.
• The reformed triglycerides combine with cholesterol and phospholipids to form chylomicrons.
• Short-chain fatty acids and glycerol pass directly into the blood capillaries.

Absorption of Water and Electrolytes

• Water is absorbed passively via osmosis, following the absorption of sodium (Na+).
• Sodium is actively transported out of the enterocytes.
• Electrolytes (like sodium) are absorbed actively in both the small intestine and colon.

Absorption of Vitamins

• Water-soluble vitamins are absorbed passively, with the exception of vitamin C, B1, and B12.
• Vitamin B12 requires intrinsic factor for absorption.
• Lipid-soluble vitamins (A, D, E, and K) are absorbed in the same way as lipids.

Absorption of Iron

• Iron is absorbed as ferrous (Fe++) ion.
• Acidic pH and vitamin C increase iron absorption.
• Phosphates, oxalates, phytic acid, and pancreatic juice inhibit iron absorption.

Other Important Points

• Absorption differs for various nutrients; for example, proteins and carbohydrates have enzymatic digestion in the small intestine, and lipids are emulsified and absorbed via micelles.
• Some nutrients require specific transporters or hormonal regulation for absorption.