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Right Now | Solar Tonic

Too Much Sunscreen?

September-October 2005

Photograph by Getty Images


Photograph by Getty Images

For many summers, people have slathered and sprayed on sunscreens and fretted about SPF factors while scrambling to protect themselves from ultraviolet-B (UV-B) radiation, a component of sunlight. Dermatologists have linked UV-B exposure to the development of skin cancer, including lethal melanomas. But according to a new theory, sealing our skins off from the sun may cause more cancer deaths than it prevents.

 Associate professor of medicine Edward Giovannucci notes that UV-B radiation, the source of suntan and sunburn, is also the component of sunlight that enables human skin (particularly lighter-pigmented skin) to synthesize the “sunshine vitamin”—D—used by every type of cell in the human body. Animal research has associated a lack of vitamin D with multiple sclerosis, osteoporosis, and pathological processes that underlie several forms of cancer, including those of the colon, breast, prostate, and digestive tract, such as stomach cancer. “If you look at these cancers as a group,” says Giovannucci, who is also a professor of nutrition and epidemiology at the Harvard School of Public Health, ”you’ll see that 30 people die of these cancers for every one who dies of skin cancer.”

In a recent review article in Cancer Causes and Control, Giovannucci describes animal and in vitro studies that demonstrate vitamin D’s anti-cancer effects against a wide spectrum of malignancies. “There are a lot of potential mechanisms,” he says. “Adding vitamin D to cells reduces cell proliferation and increases cell differentiation. The cells become more normal and less likely to divide. It might also be important for cell adhesion, making malignant cells less likely to spread. Vitamin D can help forestall cancer progression and metastasis.”

Epidemiological data indicate that colon cancer rates are higher in the Northeast, where people absorb less sunlight. Overweight and obese individuals also have lower levels of vitamin D and higher rates of cancer. (The vitamin is fat-soluble and may be captured in fat cells, becoming less available for biological processes elsewhere in the body. In one study that exposed slim and obese people to the same amounts of UV-B, the heavier people nonetheless ended up with less vitamin D in their bloodstreams.)

Darker-skinned people generate little or no vitamin D from sun exposure, and have much lower levels of circulating vitamin D, as well as higher rates of vitamin D deficiency. Research has shown signs of rickets in African-American children. In general, African Americans have higher rates of total cancer mortality, and many studies have documented prostate cancer rates in African-American men that are double those of their white counterparts. In the Health Professionals Study, whose sample is large enough to control for many variables that could affect cancer prevalence in blacks and whites, “The only factor we found that showed a significant difference was vitamin D levels,” says Giovannucci. There may be an evolutionary explanation for the differential. “As darker-skinned people migrated north, it could be that the lighter-skinned individuals [who generate vitamin D in response to sunlight] survived,” he says.

Because the atmosphere filters out much UV-B radiation, even light-skinned people fail to produce much vitamin D between November and March in higher latitudes, where winter sunlight passes through much more atmosphere at its lower angle to earth. The vitamin-generating mechanism depends on having the sun “high in the sky”—as in summer, or at midday. For the same reason, the skin can create vitamin D at the equator all year round. Mountain dwellers also get more UV-B, because the atmosphere is thinner at high altitudes. Used properly, however, sunscreen preparations completely block UV-B.

And UV-B radiation is by far the most important source of vitamin D. The vitamin occurs in a very few dietary items—like fatty fish and vitamin D-fortified milk—but 20 minutes on a sunny beach can produce 10,000 international units of the vitamin, while a glass of milk yields only 100. “To get the optimal benefit from dietary intake would probably mean consuming about 2,000 international units per day,” Giovannucci says. “However, most vitamin D supplements provide 400 units or less. Our estimates regarding the threshold of vitamin D toxicity are unreasonably low and based on old, poorly designed studies. The average doctor will scare patients away from taking 2,000 units a day, calling that toxic. Actually, it seems there is almost zero potential for vitamin D toxicity at that level.”

Giovannucci acknowledges that the evidence for these theories is still weak: there has not been a good double-blind controlled study, lasting perhaps 20 years, that compares people getting sun exposure to a placebo group. “But almost every other bit of evidence suggests that vitamin D is beneficial,” he says. “More sun, and higher rates of vitamin D, correlate with fewer cancers. It might ultimately prevent only a fraction, perhaps 30 percent, of those cancers it seems to affect. But that would still be vastly more cases than any skin cancers it causes. I don’t recommend that people go out and get sunburned—use common sense. But if the studies hold up, vitamin D will be a relatively important factor, since it affects such a large number of cancers. It may be time to rethink the message we are sending about sunlight.”

~Craig Lambert

 

Edward Giovannucci e-mail address: edward.giovannucci@channing.harvard.edu