Which vitamin is primarily produced in the skin upon exposure to sunlight?

Vitamin D is primarily produced in the skin upon exposure to sunlight, specifically from ultraviolet B (UVB) rays. When the skin is exposed to sunlight, it synthesizes vitamin D3 (cholecalciferol), which is then converted to its active form, calcitriol, in the liver and kidneys. This process is essential for maintaining adequate levels of calcium and phosphorus in the body, which are crucial for healthy bone development and maintenance.

While the body can produce some vitamins through other means, such as obtaining them from dietary sources, vitamin D is unique in that a significant portion of it is made through the skin in response to sunlight. This sunlight exposure is particularly important in regions or during seasons with limited sunlight, highlighting the need for adequate vitamin D.