Fiber's Role in Digestion: Where Is It Absorbed?

Dietary fiber is a non-digestible carbohydrate (plant cell walls: cellulose, hemicellulose, pectin, lignin). Humans lack enzymes to break β-glycosidic bonds, so fiber passes through the small intestine unabsorbed.

Site of Action: Large Intestine (Colon)

• Bulking: Insoluble fiber (wheat bran, vegetables) holds water → softer, larger stool → faster transit.
• Fermentation: Soluble fiber (oats, legumes, psyllium, fruits) fermented by colonic microbiota → short-chain fatty acids (butyrate, propionate, acetate) → nourish colonocytes, lower pH, modulate immunity.
• Metabolic Effects: Slows glucose absorption, binds bile acids → lowers LDL cholesterol.

Recommended Intake

• Women: 25 g/day
• Men: 38 g/day
• (US average: ~15 g/day)

Increase gradually with adequate fluids to avoid bloating.