Vitamin D, calciferol, is a fat-soluble vitamin. Vitamin D exists in several forms, each with a different activity.
Some forms are relatively inactive in the body, and have limited ability to function as a vitamin.
The liver and kidney help convert vitamin D to its active hormone form. Vitamin D aids in the absorption of calcium, helping to form and maintain strong bones. It promotes bone mineralization in concert with a number of other vitamins, minerals, and hormones. Without vitamin D, bones can become thin, brittle, soft, or misshapen. One cup of vitamin D fortified milk supplies about one-fourth of the estimated daily need for this vitamin for adults.