Which vitamin's metabolism is influenced by skin exposure to sunlight?

• A. Vitamin A
• B. Vitamin B12
• C. Vitamin C
• D. Vitamin D

Answer: (D)

Vitamin D's metabolism is significantly influenced by skin exposure to sunlight because it is synthesized in the skin when ultraviolet (UV) rays from the sun convert 7-dehydrocholesterol, a compound naturally present in the skin, into vitamin D3 (cholecalciferol). This process requires direct sunlight, particularly UVB radiation. Once formed, vitamin D3 is then further metabolized by the liver and kidneys into its active form, calcitriol, which plays a crucial role in calcium and phosphate homeostasis, as well as in various bodily functions, including immune response and cellular growth.

Other vitamins, such as A, B12, and C, do not rely on sunlight for their metabolism. Vitamin A is primarily obtained from dietary sources and does not involve a sun-dependent synthesis process. Vitamin B12 absorption occurs in the gastrointestinal tract and is generally derived from animal-based foods. Vitamin C is a water-soluble vitamin that must be acquired through diet, as the body does not synthesize it, and it has no involvement with sunlight exposure. Thus, timing the synthesis of vitamin D with adequate sun exposure is essential for maintaining its levels in the body.