Where does phosphate absorption primarily occur?

• A. Stomach
• B. Small bowel
• C. Large intestine
• D. Liver

Answer: (B)

Phosphate absorption occurs chiefly in the small intestine, especially in the proximal sections (duodenum and jejunum). This is where enterocytes have the active transport system for phosphate: sodium-phosphate cotransporters on the apical membrane (such as NaPi-IIb) that use the sodium gradient to move phosphate into cells. Vitamin D increases the expression and activity of these transporters, enhancing absorption. There is some passive, paracellular absorption as well, but the active uptake in the small intestine dominates. The stomach has little role in phosphate absorption, the large intestine handles mainly water and electrolyte balance with minimal phosphate uptake, and the liver is not a site of dietary phosphate absorption. Thus, the small bowel is the primary location.