How is folic acid absorbed in the small intestine?

• A. Diffusion in the colon
• B. Active transport in the stomach
• C. Carrier-mediated process, primarily in the proximal part of the small intestine
• D. Carrier-mediated in the distal ileum

Answer: (C)

Folate absorption relies on a saturable, carrier-mediated mechanism rather than simple diffusion. The main site is the proximal small intestine—the duodenum and proximal jejunum—where a folate transporter on the enterocyte surface brings folate into cells. After luminal folate is converted to the appropriate mono-glutamate form by brush-border enzymes, it is transported across the enterocyte into the bloodstream via basolateral carriers.

The colon is not a primary site for folate uptake, and absorption in the stomach is not a significant contributor. The distal ileum is more associated with vitamin B12 absorption rather than folate. So, the description of a carrier-mediated process occurring primarily in the proximal part of the small intestine best fits folic acid absorption.