Vitamin D isn't just a vitamin — it functions as a hormone.

Unlike most vitamins, which the body must get entirely from food, vitamin D can be synthesized by the skin when it’s exposed to ultraviolet B (UVB) sunlight, and the body then converts it through the liver and kidneys into calcitriol, its active form. Calcitriol behaves like a classic steroid hormone: it travels through the bloodstream and binds to receptors in cells throughout the body, regulating calcium absorption, bone growth, and immune and cell function, well beyond what most dietary vitamins do.

This dual identity is why vitamin D deficiency is unusually common in places with long winters, limited sun exposure, or cultural practices involving full-body coverings, since sunlight synthesis can’t make up the gap and dietary sources like fatty fish, egg yolks, and fortified milk are comparatively sparse. It’s also why vitamin D is measured and dosed differently from most vitamins, with recommended intake expressed in international units (IU) rather than the milligrams or micrograms used for something like vitamin C.