The main source of vitamin D for humans is sunlight. When skin is exposed to ultraviolet (UV) rays from the sun, it synthesizes vitamin D, specifically in the form of vitamin D3 (cholecalciferol). This vitamin plays a crucial role in various bodily functions, including bone health, immune function, and the regulation of calcium and phosphorus levels. While vitamin D can also be obtained from certain foods, such as fatty fish, fortified dairy products, and egg yolks, sunlight remains the most effective and natural source. Adequate sunlight exposure is essential, especially in regions with limited sunlight during certain seasons.