Vitamin D: Hormone, Not Just a Vitamin

Calling vitamin D a "vitamin" is somewhat misleading. It functions more like a steroid hormone — your skin synthesizes it from cholesterol when exposed to UVB radiation, and its active form (calcitriol) binds to receptors in virtually every cell in the body, influencing the expression of over 1,000 genes.

Synthesis & Forms

When UVB light hits 7-dehydrocholesterol in the skin, it produces vitamin D3 (cholecalciferol). This is then converted in the liver to 25-hydroxyvitamin D (calcidiol) — the form measured in blood tests — and subsequently in the kidneys (and other tissues) to the active hormone 1,25-dihydroxyvitamin D (calcitriol).

Vitamin D2 (ergocalciferol) comes from plant and fungal sources and is less effective at raising blood levels than D3. Most supplemental forms and food fortification now use D3 for this reason.

Why Deficiency Is So Common

Vitamin D deficiency is widespread globally for several reasons: people spend more time indoors, sunscreen blocks UVB synthesis, darker skin pigmentation reduces production, and very few foods naturally contain significant amounts. Latitude matters too — above about 35° north (or below 35° south), UVB intensity is insufficient for vitamin D synthesis during winter months.

The D-K2 Connection

Vitamin D increases calcium absorption from the gut, but vitamin K2 is needed to direct that calcium into bones and teeth rather than soft tissues like arteries. Supplementing with vitamin D without adequate K2 may increase the risk of inappropriate calcium deposition. The calcium page covers this interplay in more detail.

Food Sources

Fatty fish (salmon, mackerel, sardines), cod liver oil, and egg yolks are the best natural food sources. UV-exposed mushrooms contain D2. Fortified milk, orange juice, and cereals provide smaller amounts. For most people at northern latitudes, supplementation during winter months is the most reliable way to maintain adequate levels.

As a fat-soluble nutrient, vitamin D should be taken with a meal containing some fat for optimal absorption.

External resources: NIH — Vitamin D Fact Sheet | GrassrootsHealth — Vitamin D Research