Vitamin D is a fat-soluble vitamin . It helps our body absorb calcium and phosphorus. Having the right amount of vitamin D, calcium, and phosphorus is very important for building and keeping strong bones.. Vitamin D is made by the body naturally when skin is exposed to sunlight. Vitamin D is used to treat and prevent bone disorders . It is also used in kidney disease to keep calcium levels normal and allow normal bone growth. Vitamin D drops are given to infants because breast milk usually has low levels of vitamin D.