The primary source of vitamin D for humans is sunlight. When the skin is exposed to ultraviolet B (UVB) rays from the sun, it synthesizes vitamin D, which is essential for maintaining healthy bones and teeth, as well as supporting immune function. While certain foods, such as fatty fish, egg yolks, and fortified dairy products, can provide vitamin D, the most effective way to achieve sufficient levels is through regular sun exposure. However, factors like skin tone, geographic location, and the time of year can influence the amount of vitamin D the body can produce.