Vitamin D is a fat-soluble vitamin in a family of compounds that includes vitamins D-1, D-2, and D-3. It can affect as many as 2,000 genes in your body. Your body produces vitamin D naturally when it’s directly exposed to sunlight. Besides getting vitamin D through sunlight, you can also get it through certain foods and supplements to ensure adequate levels of the vitamin in your blood. Vitamin D has several important functions. Perhaps the most vital are regulating the absorption of calcium and phosphorous, and facilitating normal immune system function. Getting a sufficient amount of vitamin D is important for normal growth and development of bones and teeth, as well as improved resistance against certain diseases.