Vitamin D is both a nutrient we eat and a hormone our bodies make. It is a fat-soluble vitamin that  has several important functions. Perhaps the most vital, has long been known to help the body absorb and retain calcium and phosphorus; both are critical for building bone. That’s because your body can only absorb calcium when vitamin D is present

Nicknamed the sunshine vitamin, vitamin D is produced naturally by the body when it’s directly exposed to sunlight. 

Few foods naturally contain vitamin D, though some foods are fortified with the vitamin. For most people, the best way to get enough vitamin D is taking a supplement because it is hard to eat enough through food.