Where does most of absorption in the small intestine occur, and why?

• A. First 1-2 meters of the small intestine, proximal jejunum
• B. Distal ileum
• C. Duodenum
• D. Jejunum midportion

Answer: (A)

The main site of nutrient absorption is the proximal small intestine, especially the jejunal region in the first 1–2 meters. This area has the greatest absorptive capacity because the mucosa is highly folded with many villi and microvilli, giving a very large surface area for contact with chyme. Enterocytes in this region express abundant transporters and enzymes (for sugars, amino acids, peptides, and fats) and there is rapid blood and lymphatic drainage to carry absorbed nutrients away, maintaining favorable gradients. The duodenum handles initial digestion and some absorption (notably iron and calcium), while the ileum specializes in bile acid and vitamin B12 absorption. Thus, the proximal jejunum in the first segment of the small intestine accounts for the bulk of absorption.