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.