In which segments of the small intestine does the majority of nutrient absorption occur, and what are key absorption sites?

• A. Jejunum and Duodenum; Duodenum for iron and calcium absorption; Jejunum for most sugars, amino acids, and many minerals
• B. Ileum and Colon; Colon absorbs sugars
• C. Duodenum only; all nutrients absorbed there
• D. Jejunum only; Iron absorbed there

Answer: (A)

Most nutrient absorption happens in the small intestine, with the bulk occurring in the duodenum and jejunum. These sections have a highly folded lining with many villi and microvilli, which dramatically increases surface area and hosts a wide array of nutrient transporters.

The duodenum is a key site for iron and calcium absorption. Iron uses specific transporters in enterocytes and is highly dependent on an acidic environment and regulatory proteins, while calcium absorption is an active, vitamin D–dependent process aided by calbindin in the duodenal cells.

The jejunum takes on the majority of absorption for many nutrients, including most sugars and amino acids, as well as many minerals. Its tall villi and dense network of transporters like SGLT1 for glucose and various peptide and amino acid transporters make it the workhorse for digested nutrients.

The ileum handles other tasks, such as reabsorbing bile acids and vitamin B12, while the colon mainly reclaims water and electrolytes with limited nutrient absorption.

So, the best answer reflects that the duodenum and jejunum are the primary absorption sites, with the duodenum notably absorbing iron and calcium and the jejunum handling most carbohydrates, amino acids, and many minerals.