Vitamin D: the Secret Prohormone

Did you know that vitamin D is really not a vitamin but a prohormone?

That’s right. Technically vitamin D is a prohormone, not a true vitamin. Vitamins are nutrients that must be obtained from the diet because the body can’t make them. However, your body can synthesize vitamin D when your skin is exposed to sunlight - UVB radiation converts 7-dehydrocholesterol into vitamin D3.

Functionally: Once produced,vitamin D undergoes two hydroxylation steps:

• First, in the liver, it becomes 25-hydroxyvitamin D (calcidiol).
• Second, in the kidneys, it’s converted to 1,25-dihydroxyvitamin D (calcitriol) — the active form, which acts as a hormone.

Calcitriol, the active form, regulates calcium and phosphate metabolism, bone health, immune function, and gene expression in multiple tissues — all typical hormonal actions.

In summary:

Vitamin D (the nutrient form) is often called a vitamin for historical reasons.

But biologically, it’s more accurate to describe it as a prohormone whose active metabolite functions as a steroid hormone.