Vitamin D is a group of fat soluble secosteroids (a hormone) responsible for increasing intestinal absorption of calcium, magnesium, phosphate, and many other necessary biological effects. It is found in fatty fish, egg yokes, and dairy protein as vitamin D3, and in dark leafy vegetables in a form called D2. D2 must be converted to D3 in the body. The recommended daily intake is 400IU to 2000IU, if severely deficient. Vitamin D3 also can be made in the body from sunlight. One of the most important roles of vitamin D is maintaining and strengthening the immune system. It also strengthens teeth and bones, helps protect against certain cancers nutritionally, provides nutritional support for the brain, assists in nerve repair, boosts your mood, and possibly helps lower your risk of degenerative diseases.