Deaths from heart disease are orders of magnitude more common than deaths from skin cancer. So it makes sense to do things that reduce heart disease risk even at the cost of some increase in skin cancer risk. Sun exposure seems to have exactly this tradeoff: a modest but important reduction in heart disease risk accompanied by a modest increase in skin cancer risk. But the modest increase in skin cancer risk corresponds to a much, much smaller number of deaths. Some experts claim that the reduction in heart disease risk from sun exposure can be fully replicated by drugs that reduce heart disease. But that seems unlikely to me. We are evolutionarily adapted to a fair amount of sun exposure, and quite a few different bodily mechanisms may be predicated on sun exposure; anti-heart-disease drugs may only replicate or imitate a few of these mechanisms.

The importance of sun exposure is shown by the fact that people whose ancestors lived in northern climes are white. Enough people died from insufficient sun exposure that those of us with northern ancestry are preferentially descended from people whose skin let more sunlight in by being white. Note that this implies that those with ancestry from nearer the equator might need as much sun exposure as they can get, and would be ill-advised to use sunscreen that blocks out any physiologically useful component of sunlight. (That in turn would imply that sunscreen marketing to these folks would worsen racial inequality in health.)

I come to these views from reading the persuasive article “Is Sunscreen the New Margarine?” by Rowan Jacobsen. The bottom-line recommendation I take is that as long as you strenuously avoid getting a sunburn (even a mild one), and keep sunlight out of your eyes, sunshine on bare skin is good for you. (However, at the high end of this non-sunburn range, that much sun exposure may have long-run negative cosmetic effects—such as leathery skin—that you care about.)

I recommend that you read the whole article “Is Sunscreen the New Margarine?” but to convince you to take that recommendation, let quote some of the passages that most struck me as teasers. I’ll separate passages with my own added bullets.

• If there was one supplement that seemed sure to survive the rigorous tests, it was vitamin D. People with low levels of vitamin D in their blood have significantly higher rates of virtually every disease and disorder you can think of: cancer, diabetes, obesity, osteoporosis, heart attack, stroke, depression, cognitive impairment, autoimmune conditions, and more. The vitamin is required for calcium absorption and is thus essential for bone health, but as evidence mounted that lower levels of vitamin D were associated with so many diseases, health experts began suspecting that it was involved in many other biological processes as well.

• Yet vitamin D supplementation has failed spectacularly in clinical trials. Five years ago, researchers were already warning that it showed zero benefit, and the evidence has only grown stronger. In November, one of the largest and most rigorous trials of the vitamin ever conducted—in which 25,871 participants received high doses for five years—found no impact on cancer, heart disease, or stroke.

• These rebels argue that what made the people with high vitamin D levels so healthy was not the vitamin itself. That was just a marker. Their vitamin D levels were high because they were getting plenty of exposure to the thing that was really responsible for their good health—that big orange ball shining down from above.

• … today most of us have indoor jobs, and when we do go outside, we’ve been taught to protect ourselves from dangerous UV rays, which can cause skin cancer. Sunscreen also blocks our skin from making vitamin D, but that’s OK, says the American Academy of Dermatology, which takes a zero-tolerance stance on sun exposure: “You need to protect your skin from the sun every day, even when it’s cloudy,” it advises on its website. Better to slather on sunblock, we’ve all been told, and compensate with vitamin D pills.

• Weller’s doubts began around 2010, when he was researching nitric oxide, a molecule produced in the body that dilates blood vessels and lowers blood pressure. He discovered a previously unknown biological pathway by which the skin uses sunlight to make nitric oxide.

  It was already well established that rates of high blood pressure, heart disease, stroke, and overall mortality all rise the farther you get from the sunny equator, and they all rise in the darker months. Weller put two and two together and had what he calls his “eureka moment”: Could exposing skin to sunlight lower blood pressure?

  Sure enough, when he exposed volunteers to the equivalent of 30 minutes of summer sunlight without sunscreen, their nitric oxide levels went up and their blood pressure went down. Because of its connection to heart disease and strokes, blood pressure is the leading cause of premature death and disease in the world, and the reduction was of a magnitude large enough to prevent millions of deaths on a global level.

• Wouldn’t all those rays also raise rates of skin cancer? Yes, but skin cancer kills surprisingly few people: less than 3 per 100,000 in the U.S. each year. For every person who dies of skin cancer, more than 100 die from cardiovascular diseases.

  People don’t realize this because several different diseases are lumped together under the term “skin cancer.” The most common by far are basal-cell carcinomas and squamous-cell carcinomas, which are almost never fatal.

• Melanoma, the deadly type of skin cancer, is much rarer, accounting for only 1 to 3 percent of new skin cancers. And perplexingly, outdoor workers have half the melanoma rate of indoor workers. Tanned people have lower rates in general. “The risk factor for melanoma appears to be intermittent sunshine and sunburn, especially when you’re young,” says Weller. “But there’s evidence that long-term sun exposure associates with less melanoma.”

• Lindqvist tracked the sunbathing habits of nearly 30,000 women in Sweden over 20 years. Originally, he was studying blood clots, which he found occurred less frequently in women who spent more time in the sun—and less frequently during the summer. Lindqvist looked at diabetes next. Sure enough, the sun worshippers had much lower rates. Melanoma? True, the sun worshippers had a higher incidence of it—but they were eight times less likely to die from it.

  So Lindqvist decided to look at overall mortality rates, and the results were shocking. Over the 20 years of the study, sun avoiders were twice as likely to die as sun worshippers.

• When I spoke with Weller, I made the mistake of characterizing this notion as counterintuitive. “It’s entirely intuitive,” he responded. “Homo sapiens have been around for 200,000 years. Until the industrial revolution, we lived outside. How did we get through the Neolithic Era without sunscreen? Actually, perfectly well. What’s counterintuitive is that dermatologists run around saying, ‘Don’t go outside, you might die.’”

• Meanwhile, that big picture just keeps getting more interesting. Vitamin D now looks like the tip of the solar iceberg. Sunlight triggers the release of a number of other important compounds in the body, not only nitric oxide but also serotonin and endorphins. It reduces the risk of prostate, breast, colorectal, and pancreatic cancers. It improves circadian rhythms. It reduces inflammation and dampens autoimmune responses. It improves virtually every mental condition you can think of. And it’s free.

• … the current U.S. sun-exposure guidelines were written for the whitest people on earth.

  …

  African Americans suffer high rates of diabetes, heart disease, stroke, internal cancers, and other diseases that seem to improve in the presence of sunlight, of which they may well not be getting enough. 

• … early sunscreen formulations were disastrous, shielding users from the UVB rays that cause sunburn but not the UVA rays that cause skin cancer. Even today, SPF ratings refer only to UVB rays, so many users may be absorbing far more UVA radiation than they realize. Meanwhile, many common sunscreen ingredients have been found to be hormone disruptors that can be detected in users’ blood and breast milk. The worst offender, oxybenzone, also mutates the DNA of corals and is believed to be killing coral reefs. Hawaii and the western Pacific nation of Palau have already banned it, to take effect in 2021 and 2020 respectively, and other governments are expected to follow.

• … many experts in the rest of the world have already come around to the benefits of sunlight. Sunny Australia changed its tune back in 2005. Cancer Council Australia’s official-position paper (endorsed  by the Australasian College of Dermatologists) states, “Ultraviolet radiation from the sun has both beneficial and harmful effects on human health.... A balance is required between excessive sun exposure which increases the risk of skin cancer and enough sun exposure to maintain adequate vitamin D levels.... It should be noted that the benefits of sun exposure may extend beyond the production of vitamin D. Other possible beneficial effects of sun exposure… include reduction in blood pressure, suppression of autoimmune disease, and improvements in mood.”

• New Zealand signed on to similar recommendations, and the British Association of Dermatologists went even further in a statement, directly contradicting the position of its American counterpart: “Enjoying the sun safely, while taking care not to burn, can help to provide the benefits of vitamin D without unduly raising the risk of skin cancer.”

For annotated links to other posts on diet and health, see:

• Miles Kimball on Diet and Health: A Reader's Guide