But a clear view of the sun may be eclipsed by such messages about its risks. Sunshine doesn't just cause skin cancer. It is by far the biggest source of vitamin D, produced naturally in the skin by the same ultraviolet B rays that lead you to tan and burn. More than just a builder of strong bones, the sunshine vitamin has, over our eons on earth, insinuated itself into myriad cellular functions. A rising tide of basic research shows that it has regulatory effects on almost every system of the human body.
At the same time, what scientists call an impressive body of epidemiologic evidence suggests that a little unprotected sun exposure could be vitally important in preventing ailments from autoimmune disorders to cardiovascular disease and cancer. And in reducing mortality from them.
To do that, however, requires a shift in attitude toward sun exposure. Like so many issues in science and medicine, this one is complex, and passions run high on both sides. It may come down to this: Are you up to grasping the subtleties of sunlight?
That the human body might benefit from the sun's rays should come as no surprise. Our ancestors evolved in a world bathed in bright sunlight. The rays that shaped the biology of our oldest mammalian ancestors still glimmer deep in our genes.
These days, however, many of us spend our daylight hours indoors, in a kingdom of low-lux gloom. "Modern habits and work routines have curbed our exposure to sunlight," says Daniel Kripke, professor of psychiatry at the University of California at San Diego. "People don't walk as much as they used to, they don't work outside as much, they exercise indoors at gyms." Children spend more hours indoors playing on computers and fewer at recess.
When Kripke and his colleagues measured how much time people spend in the open in sunny San Diego, they were shocked by the results. On average, San Diegans enjoy less than an hour a day outside; many are in daylight for only 10 or 20 minutes.
Our low-light lifestyle may be undermining our health.
Patterns of Protection
A half-century ago, epidemiologists noticed a peculiar geographical pattern of death rates from cancer in the U.S.—higher in the north, lower in the south. Indeed, people living in the Northeast had as much as a twofold higher risk of dying from cancer than those in southern regions. Thus arose the hypothesis of a "sunlight effect," suggesting that direct sunshine might protect against cancer. But there was little clue of how this might work, and the link remained largely unplumbed.
At around the same time, evidence of the negative aspects of sun exposure was growing. In the 1960s, science learned that ultraviolet rays cause mutations in DNA. "This effectively pushed aside the emerging evidence about sunlight's possible benefits," says Edward Giovannucci, professor of nutrition and epidemiology at the Harvard School of Public Health.
Then, in 1980, brothers Frank and Cedric Garland, both epidemiologists at UCSD, revisited the topic and made a revolutionary suggestion. They studied the geographic distribution of colon-cancer deaths in the U.S. and found that cancer mortality rates were highest in places where populations were exposed to the least amount of natural light—in big cities and in rural areas at high latitudes. They proposed that the beneficial sunlight effect might be linked with vitamin D.
The sunshine vitamin had long been known for building strong bones. But how could it possibly prevent cancer?
More Vita in Vitamin D
We get a little vitamin D from diet—from fatty fish and egg yolks, for instance—and from fortified foods and supplements. However, our greatest source by far is exposure to sunlight. "The skin is incredibly efficient at making vitamin D," says Michael Holick, professor of medicine at Boston University and arguably the world's foremost expert on the vitamin. "Wearing a bathing suit at Cape Cod and exposing yourself to sunlight for 15 to 30 minutes until you have a slight pinkness is equivalent to 20,000 IU of vitamin D." (That's like drinking 200 glasses of milk.)
Perhaps it was by raising levels of vitamin D that abundant sunlight lowered the risk of cancer, the Garlands' thinking went. And conversely, perhaps sunlight deprivation and low levels of vitamin D in the blood jacked up risk of the disease.
Over the next two decades, scientists learned that most tissues and cells in the body have a receptor for vitamin D, including cells in the colon, breast, immune system, brain—even melanoma cells. This would help to explain why cells and tissues might respond to the presence of the vitamin.
Vitamin D plays myriad roles in the body beyond metabolizing calcium. It stimulates the secretion of insulin, modulates the immune system, and helps to regulate all kinds of cells, keeping a check on how they grow and mature, how they proliferate, differentiate, and—when it's time—give up the ghost in a timely fashion. "There's good evidence that every cell and tissue in the body needs vitamin D for optimal function," says Holick.
Multiple sclerosis is an often-crippling disease thought to arise when immune cells attack the body's own nerve cells. As with cancer, early observers noted lower rates of the condition at sunnier latitudes. People who spend the first 10 years of their life at latitudes below 35 degrees have a 50 percent lower risk of developing MS.
In 2006, a study of 140 white men and women revealed that those with the highest levels of vitamin D were 62 percent less likely to suffer from MS than those with the lowest levels. A study of twins found that those who spent more time in the sun than their sibling from ages 6 to 15 were 40 percent less likely to get the disease later.
The studies bolster a growing body of experimental research suggesting that vitamin D regulates the immune system and suppresses autoimmune reactions. Still, it's possible that UV light confers protection apart from raised vitamin D levels.
So, too, hypertension and cardiovascular disease are more prevalent at higher latitudes, where winter light is low, and among those with dark skin pigmentation—all of which restrict skin manufacture of vitamin D. In a 1998 experiment, Holick and his colleagues treated hypertensive patients with exposure to ultraviolet light three times a week for nine weeks. By the end of the study period, the researchers reported in The Lancet, the patients' vitamin D levels increased by 180 percent and their blood pressure fell to normal.
A Link to Cancer
Perhaps most impressive is the burgeoning evidence of the role of sunlight and vitamin D in cancer. Recent epidemiological studies show that if you live at high latitudes, you're more likely to fall ill with cancers of the colon, pancreas, prostate, ovaries, and breast than are your counterparts at lower latitudes. You're also more prone to die from these cancers.
When Giovannucci and his team at Harvard looked at studies on the relationship of vitamin D levels to the risk of colorectal cancer, they found that raising blood levels of the vitamin would reduce the incidence rates of colorectal cancer by half. A similar investigation of breast cancer led by Cedric Garland demonstrated that women with the highest levels of vitamin D had the lowest risk of breast cancer.
Especially intriguing to Giovannucci is a metanalysis of previous studies released in September. European investigators pooled data from several randomized trials of vitamin D for treatment of osteoporosis, fractures, and other ailments, and then looked at overall mortality rates of those getting vitamin D versus those taking a placebo. "The results were remarkable," says Giovannucci. People taking supplements of 300 to 2,000 IU of vitamin D had a statistically significant reduction in mortality from any cause. The study also revealed no downside to taking vitamin D in such doses.
So convincing is the evidence of D's effects that some scientists say the single most effective thing you can do to protect yourself against cancer—apart from avoiding tobacco and excessive alcohol use—is to get enough of the vitamin.
Others urge more caution. Epidemiological evidence does not show cause and effect, just association. Some other factors could be responsible for the apparent relationship between vitamin D and cancer risk. Hopes for similar anticancer effects from nutrients, notably beta-carotene and vitamin E, did not pan out when the supplements were tested in randomized, placebo-controlled trials.
However, Giovannucci points out, the kind of data that support the health benefits of vitamin D are no different from those linking sun exposure with skin cancer. "There are no randomized trials that show that using sunscreen reduces the incidence of melanoma," he says. "It's all observational data and epidemiology. People who live in sunnier areas have more skin cancer. Conversely, people who live in sunnier areas have much less incidence of other cancers. But for some reason, people are more willing to accept the data on skin cancer as causal."
The evidence linking sun exposure with reduced risk of many cancers is not just epidemiological. There is experimental evidence, too. A 2006 study led by Holick compared tumor growth in mice that had low levels of vitamin D and mice with high levels of the vitamin. "The tumors took off in the vitamin-D-deficient mice," says Giovannucci. By the study's end, they were 80 percent larger than the ones in the D-sufficient mice.
Also, Giovannucci sees a crucial difference in the premise behind use of vitamin D and that behind use of vitamin E and beta-carotene. Hopes for other nutrients were based on a misguided notion that providing people with a big dose of a single antioxidant would prevent cancer related to oxidative stress. "With vitamin D, it's more a matter of correcting a deficiency—one caused by insufficient sun exposure," he points out.
"The anti-sun message has been hammered into us for a long time," says Giovannucci. "The problem is that it has been oversimplified, and our scrupulous avoidance of the sun may have inadvertently led to widespread vitamin D deficiencies."
Holick is more emphatic. "The message from dermatologists never to expose yourself directly to sunlight is itself hazardous to your health. It has put the population at serious risk for vitamin D deficiency." Indeed, perhaps a billion people worldwide are not getting sufficient vitamin D.
At greatest risk are the frail elderly and dark-skinned people living at high latitudes or in cloudy climates. "But vitamin D deficiency is common throughout industrialized countries," Holick says.
So urgent is the issue that the NIH held a major conference on the topic in September of 2007. "There is still no consensus on the optimal amount," says Barbara Gilchrest, conference attendee and chief of dermatology at Boston University. "But there is consensus on how people should get more of the vitamin—through supplements and increased fortification in foods."
Holick agrees that the easiest solution would be for everyone to take supplements. (He and his family take 1,400 IU per day.) But he also encourages people to take advantage of the beneficial effects of sunlight. As it makes vitamin D from sunlight, the body produces at least three other photoproducts whose biologic function is unknown and which are not available in supplements.
"I don't advocate tanning," he's quick to say. "And I don't advocate sunburning—ever." But he does advise some unprotected sun exposure. "You can get sufficient vitamin D spring, summer, and fall by exposing your arms and legs to the midday sun for only 10 to 15 minutes," he advises. "Then you can apply sunscreen." Such "sensible" exposure can provide a cache of vitamin D that is stored in body fat and released in winter, when blood levels drop.
"If I were to follow that advice," Gilchrest responds, "I would burn every time." The idea of encouraging the public to stand, unprotected, in the midday sun has created a firestorm of controversy among dermatologists. "This is not a good public health message," Gilchrest charges.
She worries about fair-skinned people who tan poorly and freckle easily. "For them, even a little sun carries a much greater risk of skin damage and skin cancer than for someone olive-skinned or of African background. People with fair skin are already getting all the sun they need from their daily activities."
When nature endowed humans with the capacity for vitamin D photosynthesis, Gilchrest points out, the expected life span was less than 40 years; trading vitamin D production now for risks of photoaging and cancer later in life made sense. "But it is a poor exchange today, when life expectancy has doubled and one in three fair-skinned persons develops skin cancer."
Since discovery of the mutagenic effects of UV rays in the 1960s, dermatologists have worked feverishly to convey the risks of sun exposure, particularly for the fair-skinned. In Gilchrest's view, the message has made only slow inroads in the public mind, especially among the young, who embrace sunbathing and a "sexy" tan. Nearly a third of U.S. residents get sunburned at least once a year.
Gilchrest sees no need to change the sun exposure warning. "The message 'go out and sunbathe to boost your vitamin D levels' is going to be heard by young people—that 13-year-old getting ready for the prom—who are not likely to absorb the complicated details."
"You have to consider the whole picture," Giovannucci counters. "While melanomas account for the deaths every year of about 7,000 men in this country, some 295,000 die annually of all cancers. We estimate that by raising vitamin D levels, the cancer mortality rate can be lowered by 29 percent—that's 85,550 fewer deaths."
"As long as we're telling people to take only 400 IU—the bare minimum required to prevent rickets," he says, "we can't ignore sun exposure as a readily available, natural way to boost levels of vitamin D. As with everything, it's a matter of moderation and balance, weighing risk and benefit."
Is there a level of sunlight exposure sufficient to maintain healthy levels of vitamin D that will not raise skin cancer risk? The jury is still out. Whichever way the balance tips, it's vital to understand that the story of sunlight is now shaded in gray.