Amanda Schaffer was online on Aug. 16 to chat with readers about this article. Read the transcript.
This summer, science is grappling anew with the burning, beguiling power of the sun. Three investigations worth watching (from under a big leafy tree) focus on whether exposure to ultraviolet light is potentially addictive; why using sunscreen does not seem to be associated with a lower risk of melanoma, the most deadly form of skin cancer; and whether sun protection can come in pill form. All of which may influence what kind of sunscreen you buy, what else you do to protect yourself—and, arguably, why it feels so good to soak up those dangerous rays in the first place. A rundown:
1) Is ultraviolet light addictive?
A fundamental question in sun science is why we go on basking like lizards despite the obvious risks. Researchers have long suspected a connection between UV exposure and natural "feel good" molecules called beta-endorphins, which are also released during exercise. But for years, the evidence was ambiguous, as some scientists looking for it failed to find an endorphin surge in peoples' blood following exposure to ultraviolet light.
Last year, however, the field warmed up, so to speak. Steven Feldman, a dermatologist at Wake Forest University, gave eight frequent tanners a drug called naltrexone, which blocks the body's opioid receptors. These are sites in the body and brain where endorphins, as well as drugs like morphine and codeine, may attach. Feldman found that on naltrexone, half of the frequent tanners showed signs of withdrawal, like nausea and jitteriness, whereas none of the infrequent tanners did. He argues that with their opioid receptors blocked, the tanners "were deprived of their UV fix," because they'd developed a chemical dependency on the light.
David Fisher of the Dana Farber Cancer Institute in Boston thinks there may be an evolutionary rationale for the sun's draw. Maybe the endorphin release offered an adaptive advantage by reducing the pain associated with sunburn, or encouraged people living at high latitudes to spend time in the sun and thus avoid vitamin D deficiency. This spring, Fisher and his colleagues happened on a molecular connection between UV light, tanning, and endorphin release. In a paper published in March in Cell, they reported that UV damage directly causes the production of beta-endorphin in the skin.
No one knows exactly how this endorphin release might trigger chemical dependency—the molecules may or may not reach the bloodstream. But the door is now wide open to molecular sleuthing. Meanwhile, some indoor-tanning fans are also touting the endorphin findings, since it seems to prove chemically that tanning feels terrific (even as it kills you).
2) Does sunscreen protect against melanoma?
Epidemiological evidence suggests that sunscreen helps prevent squamous cell carcinoma, one of the less lethal forms of skin cancer. But the findings don't hold true for melanoma (subscription required), the really scary kind of the disease, which appears to be rising at a fast rate. In a couple of studies, sunscreen was actually correlated with higher melanoma risk.
What's going on? Until recently, most of the sunscreens on the market blocked or absorbed UVB but not UVA. (An exception is old-fashioned zinc oxide.) So some experts worry that UVA is the real culprit when it comes to melanoma. And by reducing sunburn, which is caused by UVB, these sunscreens may have allowed sunbathers to stay out longer, increasing their overall exposure to UVA and, possibly, their risk of melanoma.
The implicit lesson, then, seems to be about the importance of developing and using better UVA protection. Some new broad-spectrum sunscreens include UVA protection—Neutrogena and Banana Boat, for instance, are running aggressive ads this summer touting their formulations. (The Food and Drug Administration will soon weigh in with new rules for UVA labeling; in the meantime, for a helpful consumer guide, see here.) It's typically harder to protect against UVA than UVB, so better products are still needed. And in any case, the sun-cancer link is probably more complicated.
In the 1990s, Richard Setlow, a biophysicist at Brookhaven National Laboratory, took hybrid platyfish bred for cancer susceptibility, exposed them to different wavelengths of light, and determined that UVA and visible light were responsible for 90 to 95 percent of the fish's resulting melanomas. Setlow argues that UVA, with its longer wavelengths, is likely to penetrate deeper into skin, causing abnormalities that may eventually result in the cancer. But more recent studies don't support this. In 2004, researchers at George Washington University Medical Center experimented on mice to assess the relative importance of UVA and UVB and found that "only UVB-containing sources initiated melanoma." And this month, scientists at the University of Texas raised questions about whether UVB might be a bigger culprit than UVA even in Setlow's platyfish. It seems that UVA rarely induces melanoma, even in these fish, lead researcher David Mitchell told me. New insights into the DNA damage caused by different wavelengths of light further muddy the connection between UVA and melanoma.
So if UVA doesn't explain why UVB-protecting sunscreens seem powerless against melanoma, what might? It's possible that the lag time between sun exposure and tumor development is longer for melanoma than for other kinds of skin cancer, says Fisher, and so the protective effect of sunscreen slathered on in the 1980s and 1990s may eventually cause a decrease in melanoma, even if it hasn't yet. Spotty sunscreen use is probably a factor, too. Most people don't slop on the recommended amount (a shot glass's worth for an adult) and don't keep reapplying after swimming or sweating. This lack of diligence may have a disproportionate impact on melanoma rates compared with other forms of skin cancer.
3) Can you get sun protection from a pill?
Some dermatologists are betting on sun pills to make up for the flaws in topical creams and sprays. The most promising one—though not a sure bet by any means—is called Heliocare. Manufactured from the extract of a Central American fern, Heliocare is supposed to be taken once or twice a day as an adjunct to creams and sprays. It has some street cred because its active ingredient was championed by Thomas Fitzpatrick, a legendary Harvard dermatologist who helped to develop topical sunscreens in the 1960s. According to his son Scott, whose company distributes Heliocare to doctors in the United States, Thomas Fitzpatrick later came to believe that the topical sunscreens were insufficient, and so he supported the development of a pill and conducted some of the early research on the fern extract. (The elder Fitzpatrick passed away in 2003, so we can't ask him.)
Two small studies, published in 2004 in the Journal of the American Academy of Dermatology, suggest that Heliocare's active ingredient may help prevent sunburn and some of the DNA damage caused by ultraviolet light. In one paper, researchers measured the response of nine people to ultraviolet light before and after taking oral doses of the fern extract. Afterward, participants tended to experience less sunburn, and their skin samples showed fewer cyclobutane pyrimidine dimers, a form of DNA damage linked to skin cancer.
These studies were funded by I.F. Cantabria, the Spanish company that manufactures Heliocare, and they were probably too small to pick up on side effects. Still, Scott Fitzpatrick says, sales are booming, especially in sunny states like Texas and California. Other pills that promise to protect against sun damage are apparently popular in Europe. So expect to see more of these sun busters peeking out of a beach bag near you. Keep in mind, though, that even the best broad-spectrum sunscreen can't beat the old advice: Wear a hat, and keep out of the midday sun.