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Vitamin D is a misleading name: more than a classic vitamin, it is a prohormone that the body can make by itself from a cholesterol derivative and sunlight. It all begins in the skin, where 7-dehydrocholesterol —the precursor— absorbs ultraviolet B radiation and is transformed into previtamin D3, which rearranges with heat into vitamin D3 (cholecalciferol). That D3, however, still does nothing: it is a precursor that needs two activations.
The first occurs in the liver, which converts D3 into 25-hydroxyvitamin D, the storage form and the one measured in blood tests to assess vitamin D status. The second, more tightly regulated, occurs in the kidney, which produces the active hormone, calcitriol or 1,25-dihydroxyvitamin D, responsible for regulating calcium, phosphorus, bone, and numerous immune functions.
The supply of the skin precursor depends on factors that often work against it: at high latitudes and in winter, the angle of the sun filters out almost all the useful UVB; more pigmented skin needs much more exposure to produce the same amount; age reduces the concentration of 7-dehydrocholesterol in the skin; and sunscreen, clothing, and indoor living block synthesis. This is why vitamin D deficiency is so common despite having the precursor available, and why supplementation with D3 —which bypasses the skin— is such a widespread strategy.
There are two supplementable forms. D3 (cholecalciferol), of animal or lichen origin, is identical to the one the skin makes. D2 (ergocalciferol) comes from the ergosterol of irradiated fungi and yeasts. Both raise 25-hydroxyvitamin D, but most studies find that D3 does so more effectively and in a more sustained way, which is why it is usually preferred. The existence of plant-based D2, however, makes it useful for vegan diets when lichen-derived D3 is not available.
Active vitamin D increases calcium absorption, but where that calcium ends up depends on other nutrients. Vitamin K2 helps direct calcium toward bone and away from the arteries, and magnesium is a cofactor for the enzymes that activate vitamin D itself. That is why vitamin D is rarely thought of on its own: its benefit depends on a small team of nutrients that work with it.
Educational information, not medical advice.