In which part of the small intestine is iron primarily absorbed, and what factor facilitates this process?

• A. Jejunum; pancreatic enzymes.
• B. Ileum; bile salts.
• C. Duodenum; gastric acid and ferric-to-ferrous reduction.
• D. Colon; bacterial siderophores.

Answer: (C)

Iron is absorbed mainly in the duodenum, where the intestinal lining takes up iron most efficiently. For this to happen, iron must be in the soluble ferrous form (Fe2+). Gastric acid in the stomach helps solubilize iron and promotes its reduction from ferric (Fe3+) to ferrous (Fe2+) form. In the duodenum, ferric iron is further reduced to Fe2+ by brush-border reductases, and the Fe2+ is transported into enterocytes by the DMT1 transporter. Once inside, iron exits through ferroportin and is oxidized back to Fe3+ to bind to transferrin in the blood. This combination—duodenum as the site and gastric acid–driven ferric-to-ferrous reduction—best explains the absorption process.