Vitamin D is a fat-soluble vitamin. Its primary function is to maintain normal blood levels of calcium and phosphorus. It also aids in the absorption of calcium, which helps to form and sustain strong bones. A Vitamin D deficiency can cause the bones to become thin and brittle. Prolonged deficiency of vitamin D results in changes in the bones of children and adults, and possible hearing loss with aging. In addition, rickets (in children) and osteomalacia (in adults) where bones are malformed and weak from poor calcium and phosphorus deposition.

Vitamin D is found in food, but food is not the primary source for Vitamin D. The sun is actually the primary source. Exposure to the sun creates a synthesis from dehydrocholesterol to Vitamin D in the skin. Usually, exposure to sunlight for about 15 minutes on the face and hands is enough. Sunscreens with a sun protection factor of 8 or greater will block UV rays that produce vitamin D, however it is important to use sunscreen when exposure is longer than 10 to 15 minutes.

In addition to sun block cloud cover can also interfere with the production of Vitamin D in the skin. People who live in places that have little sunlight during the winter months at are a higher risk of Vitamin D deficiency during that time.

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

Fortified foods are the major dietary sources of vitamin D. One cup of Vitamin D fortified milk supplies about one-fourth of the estimated daily need for this vitamin for adults. Only a few foods naturally contain significant amounts of vitamin D, including fatty fish and fish oils.

Being a fat-soluble vitamin, vitamin D is the most toxic of vitamins because the potential to do damage is at levels not much higher (4 times) than the Recommended Dietary Allowance (RDA).

Excessive doses of vitamin D can increase calcium loss from bone. Large doses of vitamin D are also linked to increased risk for premature heart attack and atherosclerosis. Vitamin D toxicity develops over time and there is a wide variation among individuals in their tolerance to toxicity.

Prolonged exposure to sunlight does not cause Vitamin D toxicity. Consuming too much vitamin D through diet alone is also not likely unless you routinely consume large amounts of fish liver oil. It is more likely to occur from high intakes of vitamin D in supplements.

Vitamin D supplements are sometimes recommended for exclusively breast-fed infants because human milk may not contain adequate vitamin D. The Institute of Medicine states, "With habitual small doses of sunshine breast- or formula-fed infants do not require supplemental vitamin D." Mothers of infants who are exclusively breastfed and have a limited sun exposure should consult with their pediatrician on whether a Vitamin D supplement is right for them. Since infant formulas are routinely fortified with vitamin D, formula fed infants usually have adequate dietary intake of vitamin D.

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© Melt Magazine 2003