Absorption of Macronutrients

Macronutrient absorption location

Small intestine is the primary site of macronutrient absorption.

Luminal digestion definition

Enzymatic breakdown in the intestinal lumen by salivary, gastric, and pancreatic enzymes.

Brush border digestion definition

Final enzymatic cleavage by enzymes anchored to enterocyte microvilli.

Major carbohydrate-digesting enzymes

Salivary amylase, pancreatic amylase, brush border disaccharidases (lactase, sucrase, maltase, isomaltase).

Glucose/galactose transporter

SGLT1 (Na⁺‑glucose dependent secondary active transport).

Fructose transporter

GLUT5 (facilitated diffusion).

Effect of glucose on water absorption

Glucose + Na⁺ cotransport enhances water absorption (“Gatorade method”).

Role of bile acids

Form mixed micelles to solubilize lipids for absorption.

Micelle absorption mechanism

Passive diffusion of FA, monoglycerides and, cholesterol across brush border.

Chylomicron assembly

Occurs in rough ER + Golgi; requires apoB‑48 and MTP.

Chylomicron secretion

Exocytosed into lacteals → lymph → thoracic duct → bloodstream.

Regional absorption: duodenum

Iron, calcium, carbohydrates, amino acids, water, Na⁺.

Regional absorption: jejunum

Most nutrient absorption (carbs, proteins, lipids).

Regional absorption: ileum

Bile acids, vitamin B12 (IF‑B12 complex).

Effect of ileal resection

Bile acid diarrhea(bile acid reabsorbtion) + B12 deficiency.

Soluble fiber definition + duties

Non‑digestible carbohydrates forming gels; hold water, soften stool, slow glucose absorption, lower cholesterol.

Key enzyme locations

Salivary glands (amylase + lingual lipase), stomach (pepsin, gastric lipase), pancreas (amylase, lipase-calipase, proteases,), brush border (disaccharidases, peptidases, enterokinase).