Which of the following vitamins acts as a hormone?

Concept: A hormone is defined by the way it works, not by where it comes from. A molecule is a hormone when it is made in one tissue, carried in the blood to a different tissue, and acts there by binding a specific receptor. A vitamin, by definition, is a micronutrient that the diet must supply, and most vitamins work locally as coenzymes or antioxidants at the exact place where they are used.

Application: trace what happens to this micronutrient in the body, step by step.

1. Ultraviolet-B light converts 7-dehydrocholesterol present in the skin into cholecalciferol, so the body itself synthesises the molecule in a definite tissue.

2. The liver adds a hydroxyl group to give 25-hydroxycholecalciferol (calcidiol), the main circulating store.

3. The kidney adds a second hydroxyl group to give 1,25-dihydroxycholecalciferol (calcitriol), which is the biologically active form.

4. Calcitriol is released into the blood and carried to the intestine, bone and kidney, that is, to tissues far away from the skin where synthesis began.

5. In those target tissues calcitriol binds the nuclear vitamin D receptor (VDR); the receptor then switches on genes for calcium-binding transport proteins, so calcium and phosphate absorption from the gut and their handling in bone and kidney are raised.

6. Made in one tissue, carried by blood, and acting on a distant tissue through a specific receptor: every part of the hormone definition is satisfied. Since calcitriol is derived from a sterol, it is grouped with the steroid hormones, which is why this micronutrient is described as a prohormone.

Cross-check: contrast this with what the other micronutrients in the list actually do.

• Vitamin E (a tocopherol) works inside the membrane in which it is dissolved, quenching peroxyl radicals there; nothing is exported to a distant tissue and no receptor is involved.

• The B-group vitamins become coenzymes such as NAD, FAD and TPP that occupy the active site of the enzyme they serve; they assist a chemical reaction rather than signal to a cell.

• Vitamin C (ascorbate) re-reduces the metal centres of hydroxylase enzymes during collagen synthesis, again acting as a chemical helper at the site of the reaction and not as a messenger.

Result: the micronutrient that satisfies the full hormone definition, and whose active form calcitriol acts through a nuclear receptor on distant target tissues, is Vitamin D.