What vitamin is produced by the skin upon exposure to sunlight?

Vitamin D is the vitamin produced by the skin after exposure to sunlight. Specifically, ultraviolet B radiation converts 7-dehydrocholesterol in skin cells into previtamin D3, which then becomes vitamin D3, or cholecalciferol.

The process is only the beginning. The liver converts vitamin D3 into calcidiol, the main blood-storage form, and the kidneys convert it into calcitriol, the biologically active form. Calcitriol helps the intestines absorb calcium and phosphate, supporting bones, teeth, muscles, and other body functions.

Sunlight is not equally effective everywhere. Latitude, season, time of day, cloud cover, air pollution, clothing, sunscreen, skin pigmentation, and age all influence vitamin D production. Ordinary window glass blocks most UVB, so sitting in sunlight indoors does not provide the same effect. Excessive ultraviolet exposure also increases skin-cancer risk, so sunlight is not a universally appropriate treatment for deficiency.

Vitamin D can also come from foods and supplements. Fatty fish, fortified milk, some cereals, and certain mushrooms are dietary sources. Vitamin D2 is mainly associated with fungi, while human skin primarily makes vitamin D3.