Which vitamin is synthesized in the skin upon exposure to sunlight?

The synthesis of vitamin D occurs in the skin as a direct result of exposure to sunlight, specifically ultraviolet B (UVB) radiation. When the skin is exposed to UVB rays, a chemical reaction takes place that converts 7-dehydrocholesterol, a compound found in the skin, into previtamin D3. This previtamin D3 then undergoes a thermal isomerization process to become vitamin D3 (cholecalciferol). Following this, vitamin D3 is further processed by the liver and kidneys to form the active hormone calcitriol, which plays a vital role in calcium metabolism, bone health, and immune function.

The other vitamins listed do not have this mechanism of synthesis in the skin. For example, vitamin A is primarily obtained through diet in the form of carotenoids and retinol, vitamin C is a water-soluble vitamin that must be consumed in food sources, and vitamin E also comes from dietary intake and does not involve skin synthesis. Therefore, vitamin D is unique in its ability to be produced by the skin when stimulated by sunlight, highlighting its importance in maintaining adequate levels through exposure to sunlight.