Digestion of Undigested Food: Fiber Fermentation, Bile Acid Recycling, and Fecal Excretion

Undigested carbohydrates (dietary fiber) reach the colon where they exert bulking and metabolic effects:

Fiber Fate

• Soluble fiber (pectin, β-glucan, inulin): Fermented by colonic microbiota → short-chain fatty acids (SCFAs): acetate, propionate, butyrate.
• Insoluble fiber (cellulose, hemicellulose, lignin): Minimally fermented; adds bulk, accelerates transit.

SCFA Utilization

• Butyrate: Primary energy for colonocytes; anti-inflammatory, anticarcinogenic.
• Propionate: Gluconeogenesis substrate in liver; inhibits cholesterol synthesis.
• Acetate: Peripheral energy; lipogenesis.

Bile Acid Cycle

Primary bile acids (cholic, chenodeoxycholic) synthesized in liver → conjugated → secreted → 95% reabsorbed in terminal ileum → portal return. Remaining 5% → colon → bacterial deconjugation/dehydroxylation → secondary bile acids (deoxycholic, lithocholic) → excreted or passive absorption.

Fecal Composition

~75% water; solids: bacteria (~30%), fiber, shed epithelial cells, inorganic salts, bile pigments (stercobilin → brown color), nitrogenous waste.