Could Too Little Sun Cause Cancer?

Nov. 20, 2003 -- While more Americans are following the now familiar dermatologic mantra to avoid sun exposure and always wear sunscreen to reduce their risk of skin cancer, there's growing concern that this advice is contributing to another health problem -- a vitamin D deficiency.

This important nutrient is best known for building strong bones and teeth -- key to preventing osteoporosis -- but low levels have also been linked to an increased risk of type 1 diabetes, muscle and bone pain, and perhaps more frightening, a greater chance of cancers of the breast, colon, prostate, ovaries, esophagus, and lymphatic system.

"We all recognize that vitamin D is critical for bone health, but we have not appreciated, although we have known for at least 20 years, that almost every cell in the human body has receptors for activated vitamin D," says Michael Holick, MD, PhD, director of the Vitamin D Research Lab at Boston University Medical Center and considered by many to be the nation's leading authority on vitamin D. "We need adequate amounts to keep cell growth in check."

Americans Deficient

In other words, without enough vitamin D, cells can multiply too quickly and promote cancerous tumors. Yet between 20% and 80% of all Americans have low enough levels to classify them as vitamin D-deficient, says Holick, who also directs clinical medical research at Boston University.

One reason: Most of the body's vitamin D comes from sunlight exposure on bare, unprotected skin.

"When wearing sunscreen, you will absolutely prevent the synthesis of vitamin D -- even more so than you will prevent skin cancer," says Cedric Garland, DPH, professor of family and preventative medicine at the University of California at San Diego School of Medicine.

In this week's British Medical Journal, he writes that avoiding sun exposure is a bad strategy for overall cancer prevention. When weather permits, he recommends getting at least 10 to 15 minutes of sunlight each day on bare shoulders, arms, and legs -- without using sunscreen.

Unfortunately, now is an especially difficult time to avoid vitamin D deficiency for many Americans.

"From November through March, many people can't get enough vitamin D from sunlight, no matter how much exposure they have," Garland tells WebMD. "This is especially problematic east of the Mississippi River and from Philadelphia north, because there's a lot of sulfur coal in the air, producing what we call `acid haze,' a precursor of acid rain. It prevents ultraviolet vitamin D getting through the air on days where there's a lot of pollution."

Pollutants aside, Holick adds that his research indicates that during these winter months, there's insufficient vitamin D from sunlight in most of the country north of Atlanta. This may explain, at least in part, why some studies dating back to the 1940s find that after adjusting for other factors, people in New England states have a higher overall cancer death rate than those in sunnier climates. More recently, he says studies have specifically linked vitamin D deficiency, which can be detected with a blood test, to several non-skin cancers.

But the problems extend beyond cancer. A study to be published in next month's Mayo Clinic Proceedings suggests that vitamin D deficiency may be responsible for unexplained bone and joint pain.

And two years ago, Finnish researchers noted in The Lancet that people who got vitamin D supplements through adulthood were 80% less likely to develop type 1 diabetes than their non-supplemented peers.