Which water-soluble vitamins do not require Na+ co-transporters for absorption?

• A. Vitamin E and Vitamin D
• B. Vitamin C and Vitamin B6
• C. Vitamin B12 and Folic acid
• D. B1 and choline

Answer: (C)

Absorption of water-soluble vitamins can be either sodium-dependent or independent. The vitamins that do not require Na+ co-transporters are absorbed through mechanisms that don’t rely on sodium gradients.

Vitamin B12 is absorbed in the ileum after binding intrinsic factor, then taken up by receptor-mediated endocytosis. This pathway does not involve Na+-dependent transporters. Folate is absorbed mainly via proton-coupled folate transporters in the proximal small intestine and, in other tissues, by the reduced folate carrier; these routes are not Na+-dependent.

In contrast, vitamin C uptake uses sodium-dependent vitamin C transporters (SVCT1/2) for its absorption, so it does involve Na+ co-transport. Fat-soluble vitamins, while not the focus here, are absorbed with fats rather than through sodium-dependent transporters.

Thus, the pair that truly doesn’t require Na+ co-transporters for absorption is vitamin B12 and folic acid.