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Once thought of as only helping develop strong bones, vitamin D is now believed to serve many purposes in the human body. A deficiency of the vitamin has been linked to several diseases and disorders.

Yet most people don't get enough of the so-called "sunshine vitamin."

For years, Americans have been taught that as summer approaches, they should reach for sunscreen to protect themselves from a scorching burn - and the skin cancer it might trigger. But new research shows that by covering up, they may be sacrificing important vitamin D, which is made by the skin when it's exposed to sunlight.

So, ahead of the beach season, we sought some guidance about the "sunshine vitamin" from Dr. Elizabeth A. Streeten, assistant professor of medicine in the division of endocrinology, diabetes and nutrition at the University of Maryland School of Medicine. What does vitamin D do? New research seems to connect it with many aspects of good health besides good bones.

Vitamin D is important for the entire body. Vitamin D deficiency is associated with reduced bone strength and risk of fracture; a twofold increased risk of some cancers such as colon, breast and prostate; an increased risk of both Type 1 and Type II diabetes, worse control of diabetes for those who have it, decreased immune function and possibly also heart disease.

Vitamin D increases calcium absorption from the [gastrointestinal] tract and helps the bone become mineralized, or hardened. It also serves as a differentiating factor for cells, meaning that it helps to keep cells in their mature form and prevents them from mutating into cancer cells. Most vitamin D comes from sun exposure. How does that work, and how much sun is needed?

Ultraviolet B light contained in sunshine converts vitamin D precursors present in the skin to vitamin D. To become active, the body then converts vitamin D to 1,25-dihydroxyvitamin D, the only form that the body can use. This activation occurs via two-step process: the first in the liver and the second in the kidneys.

The truth on how much sun exposure is required is that we do not know exactly and there is significant variation among individuals. The recommendation is to get 15 minutes of sun at the peak of the day - 11 a.m. to 3 p.m. - to the face, neck and arms, three times a week. However, this amount of sun has not been proven to be enough.

It is also important to know that at our latitude, the sun is only strong enough for significant vitamin D production from June to August. At the equator, vitamin D can be made in the skin all year round. Also, more sun is needed to make vitamin D with increasing age and increasing amounts of skin pigment. Therefore, those at highest risk for vitamin D deficiency are African-Americans. How many people are deficient? Is it a growing problem or just a constant problem since there has been so much focus on using sunscreen and not enough focus on diet?

Vitamin D deficiency is defined as a blood level of [the stored form] 25-hydroxyvitamin D of less than 30 [nanograms per milliliter]. Up to 65 percent of Americans are vitamin D-deficient, with the highest levels in the elderly. However, studies have shown that up to half of young adults and children are also deficient.

We do not know if the prevalence of vitamin D deficiency is growing or not, since the accurate blood test to detect it is fairly new, about 10 years old. There is not enough vitamin D in foods for our needs. There is substantial vitamin D only in some fish, such as salmon, swordfish, tuna, sardines, and only tiny amounts in other foods. Although some of our food is supplemented, the amounts are very low - for example, 100 units per 8 ounces of milk. For the average adult to get enough vitamin D from milk, he or she would have to drink 8 cups of milk per day.

Therefore, to get enough vitamin D for our needs - 1,000 to 2,000 units daily for adolescents and adults - until more vitamin D is added to our food supply, everyone should take a vitamin D supplement, at least during September through May. If sunscreen with SPF 8 and over blocks the skin's ability to make vitamin D, should people just wear lower SPF lotion and reapply frequently? Does UVA or UVB protection make a difference?

UVB rays are what is needed for vitamin D production, whereas both UVA and UVB can cause sunburn and tan. One way to prevent sunburn and allow vitamin D synthesis is to put on sunscreen only after being out in the sun for 15 minutes. What about skin cancer? Is there a risk from being out in the sun even for a limited time or using lower SPF sunscreen?

There is no data available to show that small amounts of skin sun exposure - 15 minutes before applying sunscreen - increases the risk of skin cancer. Exceptions to this may be redheads and others with extremely fair skin who burn very easily. Certainly, we need to be concerned about skin cancer, and it is well established that sun exposure sufficient to produce sunburn, particularly blistering burns, increases the risk of skin cancer. Would tanning beds help, particularly in the winter, when sun isn't so strong? When should people turn to supplements or fortified foods?

My opinion is that everyone, children and adults alike, needs to take a vitamin D supplement above the latitude of Atlanta, Ga., during the months of September through April. There is not enough vitamin D in foods to fulfill our requirements except for the most enthusiastic milk drinkers.

Tanning beds are, indeed, a good way to make vitamin D. However, there is no data available to support its use being safe, for example, from the risk of skin cancer, for me to recommend it to the average person. There is data in small numbers of people with GI diseases, who sometimes cannot absorb enough vitamin D from their GI tracts, that gentle use - five minutes, three times a week - of a sunlamp called the Sperti vitamin D lamp improves their vitamin D production adequately to meet their needs. African-Americans, older people and babies are more at risk of being deficient. How should they be getting their vitamin D? You don't want to take a baby out into the sun unprotected, right?

For children, the amount of vitamin D in the typical children's multivitamin - 400 units - is sufficient for most. For adults over age 18 and probably also for adolescents, age 13 for example, the dose of vitamin D needs to be increased to 1,000 to 2,000 units a day. Up to 10,000 units of vitamin D daily has shown to be safe. For those who are concerned about getting too much, a simple blood test for 25-D can accurately measure whether their vitamin D stores are adequate. Anything else important to know about vitamin D?

I would again just emphasize that up to 10,000 units of vitamin D is safe, so toxicity is very rare and usually only seen with prescription strengths of vitamin D. There is one prescription strength vitamin D pill available and that contains 50,000 units of vitamin D per pill.

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