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A Little Sun Exposure May Reduce Cancer Risks - 2002-02-17

Scientists say a little sun may reduce your risk of cancer and other diseases. Although excess sunlight boosts the chances of skin cancer, researchers say moderate amounts lower the risk of other malignancies because of the skin's natural ability to produce protective vitamin D from solar radiation. But vitamin D deficiencies are common.

Don't throw away your sunscreen lotion, but don't apply it immediately in the sun, either, because moderate exposure to sunlight boosts your well being. That's the message from the annual meeting of the American Association for the Advancement of Science in Boston.

"We evolved in sunlight and sunlight is probably critically important for good health," says Boston University physician Michael Holick. He says too little exposure to the sun causes the skin to produce insufficient vitamin D. A deficiency causes the skeletal deformity rickets and the brittle bone disease osteoporosis.

A recent study shows that children who did not receive a vitamin D supplement early in life had more diabetes than those who did. Dr. Holick says insufficient vitamin D also increases the chances of cancer, which is the uncontrolled division of cells into tumors. "Almost every cell in your body has the ability to activate vitamin D. It's the activated form of vitamin D that not only regulates calcium and bone health, but it tells the cells to stop overgrowing," he says. "We believe that's probably the principle mechanism that's involved in the increased risk of dying of cancer because of vitamin D deficiency."

The findings may explain why people in colder, northern climates have a higher chance of dying of cancer. Independent researcher William Grant compared U.S. cancer death figures between 1970 and 1994 with solar ultraviolet-B radiation data from a U.S. spacecraft. He found that areas of less sunlight had more breast, colon, prostate and 10 other cancers. "For the data set from 1970 to 1994, I've come up with a number of 23,600 people per year who died prematurely because of too little UVB radiation and or vitamin D," he says.

Some of the highest U.S. cancer mortality rates are among African Americans. Researcher Nina Jablonski of the California Academy of Sciences suggests that dark skin is one of the reasons, because the pigment naturally blocks much solar radiation. She says dark skin evolved to protect people in the very sunny tropics from the higher radiation levels, but is no benefit in grayer regions. "Populations that initially evolved around the equator, say, Africans or Indians and Sri Lankans, who now live in northern latitudes very often suffer from vitamin D deficiencies," she says. "So in a sense, we're paying the price for the fact that humans have been much more mobile, especially in the last few centuries."

Ms. Jablonski says vitamin D deficiencies are appearing even in parts of sunny Africa and the Middle East, wherever people are spending more time indoors and covering their bodies more than in the past.

She points out that, historically, the need for this nutrient may have been the factor limiting mankind's migration above 50 degrees north latitude until humans developed technology to fish and harvest other vitamin D-rich foods. It may also be responsible for women being the so-called fairer sex. "In every indigenous population we've examined, females are slightly more lightly colored than males, probably because they need to synthesize more vitamin D in their skin during critical periods such as pregnancy and lactation," she says.

Dr. Michael Holick says the best source of vitamin D is the sun because that which is produced in the skin goes directly to the blood, rather than through the digestive system and provides far more than foods and supplements do. He recommends that light skinned people in northern latitudes get 10 minutes of face and hand exposure three times a week before applying sunscreen, while dark skinned people need 10 to 20 times that amount of solar radiation.