Which vitamin can human skin produce after ultraviolet B exposure?

Human skin can produce vitamin D after exposure to ultraviolet B radiation.

UVB light acts on a cholesterol-related molecule in the skin called 7-dehydrocholesterol. This reaction forms previtamin D3, which changes into vitamin D3 and is then processed by the liver and kidneys into biologically active forms. Vitamin D helps regulate calcium and phosphate balance, supporting normal bone mineralization.

The amount produced varies with factors such as skin pigmentation, latitude, season, time of day, clothing, age, and sunscreen use. Sunlight exposure is therefore not a precise or universally reliable measure of vitamin D status. Dietary sources and supplements can also provide vitamin D.

Vitamin D is sometimes called a vitamin and sometimes described as a hormone precursor because the body makes it and then activates it through several organs. Excessive ultraviolet exposure can damage skin and increase skin-cancer risk, so producing vitamin D does not require intentional sunburning.