What causes this deficiency?The main reason is reduced exposure of the skin to the sun. Vitamin D is referred to as the sunshine vitamin because it can be made by human skin from cholesterol but only if the skin is exposed to sufficient ultraviolet light. However, Americans are making less and less vitamin D because they are spending less time in the sun.

The growing popularity of video games and TV among American kids has resulted in many kids spending less time outdoors than in the past. Those who go in the sun are now more likely to wear strong sunscreens, which block the ultraviolet rays needed by the skin to make vitamin D.Among adults, and particularly women, vitamin D deficiency can be a physician-caused illness. Many doctors and most dermatologists caution their patients to avoid the sun and, when not possible, use clothes and sunscreens to protect the skin. Avoiding excessive ultraviolet radiation and sunburn is good advice, but a small amount of ultraviolet radiation is needed to make vitamin D.

How much sun do you need?You need only 10-15 minutes of direct sun on your arms or legs several times a week to make enough vitamin D to meet your body?s requirement.However, people who live north of Atlanta on the East Coast, or Los Angeles on the West Coast, will have a hard time making enough vitamin D for at least several months each year. Why? Because the sun?s rays are too weak in December to penetrate even pale skin deep enough to trigger vitamin D synthesis. The problem is even worse for those with darker skin because melanin (like sunscreens) keeps the ultraviolet rays from penetrating the skin.Can You Get Enough Vitamin D from Food?Yes and no. Of course vitamin D can also be obtained from food or as supplements.There are challenges to this.? Except for fatty fish and fortified nonfat milk, there are few reliable sources of vitamin D.? Some other foods may be fortified with vitamin D but are not required to be.? Many kids and teenagers drink soft drinks instead of milk or fortified soy milk and rarely eat fish. If they do not take a vitamin D supplement or get regular sun exposure throughout the year, they may develop vitamin D deficiency.Two servings of nonfat milk or fortified soy milk supply about 200 IU (5 mcg) of vitamin D, which, along with 2-3 servings of fatty fish a week, should supply sufficient vitamin D for kids and younger adults even if they get little sunshine. Older adults need 2-3 times as much vitamin D as do younger people. In addition, their skin is much less able to make vitamin D when exposed to the sun. Some experts believe older adults need at least 1,000 IU of vitamin D per day.What Happens When You Don?t Get Enough Vitamin D?A lack of vitamin D reduces calcium absorption and impairs bone remodeling. Therefore, older people with low vitamin D levels in their blood are at increased risk of broken bones. In addition a lack of vitamin D can lead to osteomalacia, which can cause the bones to ache and weaken muscles. There is also growing evidence a lack of vitamin D increases the risk of colon, breast and prostate cancers. Adults deficient in vitamin D may also be more prone to hypertension and heart failure.In kids a lack of vitamin D leads to rickets. This causes the bones of the legs to bow either inward or outward. In addition, multiple sclerosis and type 1 diabetes are far more common far from the equator than close to it, so a lack of vitamin D may be increasing the risk of these diseases as well.By James Kenney, PhD, RD, LD, FACN.