VITAMIN D3
Vitamin D is a fat-soluble vitamin that is found in food and can also be made in your body after exposure to ultraviolet (UV) rays from the sun. Sunshine is a significant source of Vitamin D because UV rays from sunlight trigger Vitamin D synthesis in the skin.
The major biologic function of Vitamin D is to maintain normal blood levels of calcium and phosphorus. By promoting calcium absorption, Vitamin D helps to form and maintain strong bones. Vitamin D also works in concert with a number of other vitamins, minerals and hormones to promote bone mineralization. Without Vitamin D, bones can become thin and brittle. Sufficient Vitamin D prevents rickets in children and osteomalacia in adults, two forms of skeletal diseases that weaken bones.
Moreover, older skin is less effective in making the vitamin even when it’s exposed to sunlight. Sunscreens are another problem: they filter out much of the ultraviolet radiation that produces Vitamin D.
Theoretically, you could make up for a shortage of sunshine-generated Vitamin D with diet. The problem is that precious few foods contain the vitamin. Essentially, it’s limited to several types of saltwater fish. So decades ago, health officials in many Northern countries decided to fortify foods with Vitamin D. In the United States, milk — but not other dairy foods — was chosen. An 8-ounce glass of milk is supposed to contain 100 IU of Vitamin D, although surveys have shown that the actual amount can be a great deal less.