Vitamin D is essential for the regulation of the minerals calcium and phosphorus within the body, and plays an important role in maintaining the structure of bone. Sun exposure is an easy and reliable way for most people to get vitamin D but it is also contained in fatty fish, (tuna, mackerel, and salmon), cheese, egg yolks and foods fortified with vitamin D (e.g., some dairy products, orange juice, soy milk, and cereals). Vitamin D is used to treat many conditions including osteomalacia (softening of the bones) and osteoporosis (weak bones).