Which vitamin does human skin produce when exposed to sunlight?

Human skin produces vitamin D when exposed to sunlight, specifically through ultraviolet B (UVB) radiation.

UVB light acts on 7-dehydrocholesterol in skin cells, first forming previtamin D3 and then vitamin D3, also called cholecalciferol. The vitamin is not fully active immediately: the liver and kidneys modify it into forms the body uses to help regulate calcium and phosphate, minerals essential for bones and many cellular functions.

Sunlight is only one source. Vitamin D can also come from foods, fortified products, and supplements. The amount made in skin varies with latitude, season, time of day, cloud cover, skin pigmentation, age, clothing, and sunscreen use. Glass blocks most UVB, so sitting in sunlight behind a window is not an efficient way to produce it.

Vitamin D is often confused with vitamin A or vitamin C because all are associated with health and nutrition, but they have different biological roles. Excessive sun exposure can damage skin, so more sunlight is not automatically better; dietary intake or medical advice may be appropriate when deficiency is a concern.