What vitamin does the sun's ultraviolet rays help the skin produce?

The sun’s ultraviolet rays help the skin produce vitamin D. More precisely, ultraviolet B (UVB) radiation changes a cholesterol-related substance called 7-dehydrocholesterol in skin cells into previtamin D3, which then becomes vitamin D3, or cholecalciferol. The liver and kidneys later process it into active forms used by the body.

Vitamin D helps regulate calcium and phosphate, making it important for bones, teeth, muscles, and several other biological functions. People can also obtain it from foods, fortified products, and supplements, so sunlight is not the only source. The amount produced by skin varies with latitude, season, time of day, cloud cover, skin pigmentation, age, clothing, and sunscreen.

A common misconception is that any ultraviolet light works equally well. UVB is the key range, roughly 290–315 nanometres; UVA is not the main trigger for vitamin D production. Ordinary window glass blocks most UVB, so sunlight reaching someone indoors through a closed window does not provide the same vitamin-D-producing exposure.