Vitamin D, calciferol, is a fat-soluble vitamin. It is found in food, but also can be made in your body after exposure to ultraviolet rays from the sun . 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.

The major biologic 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. 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. Vitamin D prevents rickets in children and osteomalacia in adults, which are skeletal diseases that result in defects that weaken bones .

What are the sources of vitamin D?

Food sources
Fortified foods are the major dietary sources of vitamin D . Prior to the fortification of milk products in the 1930s, rickets (a bone disease seen in children) was a major public health problem in the United States. Milk in the United States is fortified with 10 micrograms of vitamin D per quart , and rickets is now uncommon in the US .

One cup of vitamin D fortified milk supplies about one-fourth of the estimated daily need for this vitamin for adults. Although 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 . The table of selected food sources of vitamin D suggests dietary sources of vitamin D.

Exposure to sunlight
Exposure to sunlight is an important source of vitamin D. Ultraviolet (UV) rays from sunlight trigger vitamin D synthesis in the skin . Season, latitude, time of day, cloud cover, smog, and suncreens affect UV ray exposure . For example, in Boston the average amount of sunlight is insufficient to produce significant vitamin D synthesis in the skin from November through February. Sunscreens with a sun protection factor of 8 or greater will block UV rays that produce vitamin D, but it is still important to routinely use sunscreen whenever sun exposure is longer than 10 to 15 minutes. It is especially important for individuals with limited sun exposure to include good sources of vitamin D in their diet.

When can vitamin D deficiency occur?

A deficiency of vitamin D can occur when dietary intake of vitamin D is inadequate, when there is limited exposure to sunlight, when the kidney cannot convert vitamin D to its active form, or when someone cannot adequately absorb vitamin D from the gastrointestinal tract .

The classic vitamin D deficiency diseases are rickets and osteomalacia. In children, vitamin D deficiency causes rickets, which results in skeletal deformities. In adults, vitamin D deficiency can lead to osteomalacia, which results in muscular weakness in addition to weak bones .

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 such as confusion. High blood levels of calcium also can cause heart rhythm abnormalities. Calcinosis, the deposition of calcium and phosphate in soft tissues like the kidney can be caused by vitamin D toxicity .
Consuming too much vitamin D through diet alone is not likely unless you routinely consume large amounts of cod liver oil. It is much more likely to occur from high intakes of vitamin D in supplements. The Food and Nutrition Board of the Institute of Medicine considers an intake of 25 mcg for infants up to 12 months of age and 50 mcg for children, adults, pregnant, and lactating women to be the tolerable upper intake level . A daily intake above the UL increases the risk of adverse health effects and is not advised.