Vitamin D helps your body absorb calcium and phosphorus, which are essential for bone growth and development. It also supports immune system function. Research suggests vitamin D may help reduce. Vitamin D (calciferol) keeps bones strong and supports your immune system. People ages 1 to 70 need 15 mcg (600 IU) per day; people 70+ need 20 mcg (800 IU). Vitamin D is involved in many bodily functions and plays a vital role in immune health and maintaining healthy bones and teeth. Vitamin D is produced when you expose your skin to sunlight and it is found in some foods, such as fatty fish and fortified dairy products. Learn how to spot vitamin D deficiency symptoms, what foods are high in vitamin D, how much to take daily, and whether D2 or D3 is best for you. 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. 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. 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). Cholecalciferol is vitamin D3. Vitamin D helps your body absorb calcium. Vitamin D is also essential for maintaining a healthy immune system. The function of the immune system is to prevent or limit infection. Cholecalciferol is used as a dietary supplement in people who do not get enough vitamin D in their diets to maintain adequate health. Vitamin D is essential for the bones and teeth, the immune system, brain health, and for regulating inflammation. The body produces vitamin D as a response to sun exposure. Find out about vitamin D, including what it does, how much you need, and how to ensure you get enough.