Define dietary fibre. Classify it and describe its physiological effects.

Definition

• Formerly called roughage and dismissed as inert; now recognised as nutritionally important

Classification

Physiological Effects

• Increases stool bulk and water content → softer stools, easier passage
• Shortens intestinal transit time → less contact between carcinogens and mucosa
• Binds bile acids → more cholesterol is diverted to bile acid synthesis → plasma cholesterol falls
• Slows gastric emptying and glucose absorption → a flatter post-prandial glucose curve and lower glycaemic index
• Increases satiety and lowers energy density → helps weight control
• Fermented by colonic bacteria to short-chain fatty acids — acetate, propionate and butyrate, which is the preferred fuel of the colonocyte
• Acts as a prebiotic, supporting a favourable gut microbiota

Recommended Intake

• 25–40 g/day in adults, or about 14 g per 1000 kcal
• The traditional Indian diet, rich in whole grains and pulses, is usually adequate; refined flour and polished rice reduce it sharply

Deficiency and Excess

Applied Aspects

• Isabgol (psyllium husk) is a soluble fibre widely used in India as a bulk laxative and to lower cholesterol
• Fibre must be increased gradually and with adequate water, or it worsens constipation and causes bloating
• Phytate in bran binds iron, calcium and zinc, and can contribute to deficiency in populations already at risk — a genuine concern in Indian diets
• The evidence that fibre prevents colorectal cancer is weaker than once believed; its benefits for constipation, cholesterol and glycaemic control are firmly established
• Low-fibre diets are indicated in acute diverticulitis, inflammatory bowel disease flares and before colonoscopy