Vitamin D (also referred to as calciferol) is a fat-soluble vitamin that is naturally present in a few foods, added to others, and available as a dietary supplement. It is also produced endogenously when ultraviolet (UV) rays from sunlight strike the skin and trigger vitamin D synthesis. Unlike the other twelve vitamins, vitamin D is only conditionally essential in the diet, as with adequate skin exposure to the ultraviolet B (UVB) radiation component of sunlight there is synthesis of cholecalciferol in the deeper layers of the skin's epidermis. Find out about vitamin D, including what it does, how much you need, and how to ensure you get enough. Learn more about Vitamin D uses, effectiveness, possible side effects, interactions, dosage, user ratings and products that contain Vitamin D. Vitamin D is an essential micronutrient that helps your body absorb calcium, protects the health of your bones and teeth, reduces inflammation, regulates blood sugar, and boosts your immune response. More specifically, you need vitamin D so your body can use calcium and phosphorus to build bones and support healthy tissues. With chronic and/or severe vitamin D deficiency, a decline in calcium and phosphorus absorption by your intestines leads to hypocalcemia (low calcium levels in your blood). Vitamin D is a nutrient the body needs, along with calcium, to build bones and keep them healthy. The body can absorb calcium only if it has enough vitamin D. Calcium is a major part of bones. Vitamin D also has many other uses in the body. It supports immune health and helps keep muscles and brain cells working.