Vitamin D (calciferol) is a fat-soluble vitamin. It is found in food and also can be made in your body after exposure to ultraviolet rays from the sun. The liver and kidney help convert vitamin D to its active hormone form (Nutr Rev 1998; 56:S4.)
The primary function of vitamin D is to maintain normal blood levels of calcium and phosphorus. Vitamin D aids in the absorption of calcium, helping to form and maintain strong bones. Without Vitamin D, bones can become thin, brittle, soft, or misshapen.
Although in the United States milk is fortified with Vitamin D, dairy products made from milk such as cheese, yogurt, and ice cream are generally not fortified with Vitamin D. Only a few foods naturally contain significant amounts of Vitamin D, including fatty fish and fish oils.
Individuals who have reduced ability to absorb dietary fat may need extra Vitamin D because it is a fat-soluble vitamin. Older Americans are thought to have a higher risk of developing Vitamin D deficiency, and individuals with limited sun exposure need to include good sources of Vitamin D in their diets.
There is a high health risk associated with consuming too much Vitamin D. Vitamin D toxicity can cause nausea, vomiting, poor appetite, constipation, weakness, and weight loss. It can also raise blood levels of calcium, causing mental status changes and heart rhythm abnormalities.