Adequate sunlight is an essential part of the chemical process that helps our body produce vitamin D. But factors such as the season of the year, latitude, time of day, cloud cover, smog, and sun screens all affect the amount of ultraviolet ray exposure we receive from the sun and can reduce our supply of vitamin D. In countries where exposure to the sun is reduced by head to feet clothing for religious or other reasons, vitamin D under nutrition is a problem. Elderly people who stay inside and have poor diets may also have a sub clinical deficiency of vitamin D.

Vitamin D can be made in our bodies - in the skin to be more precise - but only if sunlight is available to carry out the synthesis. Vitamin D, or calciferol, is a fat-soluble vitamin. It exists in several forms, each with a different biological activity. Vitamin D3 (cholecalciferol) is made by the body and is found in some foods. Vitamin D2 (ergocalciferol) is the form most often added to milk and other foods, and the form most often found in nutrition supplements.

Fortified foods are the major dietary sources of vitamin. One cup of vitamin D fortified milk, for example, 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.

In the body, vitamin D together with calcium plays a major role in bone development. Most nutritionists now acknowledge that the combination of vitamin D and calcium supplements can be quite helpful for preventing and treating osteoporosis. Women with severe osteoporosis have low levels of vitamin D. Supplementing with vitamin D alone may not be helpful, but the combination of calcium and vitamin D can slow down or even reverse osteoporosis.