Vitamin D does not come from the SUN. It comes from our skin.

Vitamin D is not one chemical but many. The natural type is produced in the skin from a universally present form of cholesterol, 7-dehydrocholesterol. Sunlight is the key: Its ultraviolet B (UVB) energy converts the precursor to vitamin D3. In contrast, most dietary supplements are manufactured by exposing a plant sterol to ultraviolet energy, thus producing vitamin D2. Because their function is almost identical, D2 and D3 are lumped together under the name vitamin D — but neither will function until the body works its magic (see figure).

The sun's energy turns a chemical in your skin into vitamin D3, which is carried to your liver and then your kidneys to transform it to active vitamin D.

The first stop is in the liver, where vitamin D picks up extra oxygen and hydrogen molecules to become 25-hydroxyvitamin D, or 25(OH)D. This is the chemical that doctors usually measure to diagnose vitamin D deficiencies. But although 25(OH)D is used for diagnosis, it can't function until it travels to the kidney. There it acquires a final pair of oxygen and hydrogen molecules to become 1,25 dihydroxy vitamin D; scientists know this active form of the vitamin as 1,25(OH)2D, or calcitriol, but for ordinary folks the name vitamin D is accurate enough.