Last year, Outside contributing editor Rowan Jacobsen wrote a piece called “Is Sunscreen the New Margarine?” that undercut decades of advice from public-health experts and (more worryingly) my mother. The basic gist of the research he reported on: the risks of skin cancer, though real, are less serious than we think, while the benefits of sun exposure are far greater than we’ve realized. The piece went viral—but unlike a lot of viral content, it contained enough carefully argued science to make me seriously question my sun phobia.

One of the key planks of the argument in Jacobsen’s piece was that ultraviolet radiation from the sun triggers the release of nitric oxide from your skin into your bloodstream, where it has wide-ranging effects including lowering your blood pressure. This is an important point for two reasons: one is that high blood pressure, by some estimates, is among the leading causes of premature disease and death in the world, affecting more than a billion people; the other is the implication that there’s something about sunlight that can’t be replaced by popping vitamin D pills.

The popularity of vitamin D supplements is easy to understand, because there are so many health conditions that seem to correlate with vitamin D levels, including cancer and heart disease. But study after study has found that taking vitamin D supplements doesn’t change your risk of developing these conditions. An alternative explanation is that vitamin D is simply an indicator of how much sun you’re getting, and the health effects come from something else in sunlight.

As Jacobsen explained, the main proponent of the nitric oxide theory is a dermatologist in Scotland named Richard Weller, who in the 1990s first showed that your skin has a big reservoir of nitric oxide that can be activated by sunlight. He has co-authored several studies investigating this effect, including a 2018 paper which found that exposure to an ultraviolet lamp produced measurable changes in nitric oxide levels in the blood. But a smoking gun linking UV exposure to blood pressure in the real world has been missing.

That makes a newly published study in the Journal of the American Heart Association particularly interesting. Weller and colleagues from several universities teamed up with Fresenius Medical Care, a company that specializes in kidney disease, to analyze data from 342,000 patients at more than 2,000 Fresenius dialysis clinics across the United States.

Dialysis patients are a useful group to study because they have to visit the clinic frequently—typically three times a week—which means they have their blood pressure measured by a professional using a standardized protocol each time. The researchers took this blood pressure data over a three-year period and calculated monthly averages for each patient. Then they used data from the National Center for Atmospheric Research and the National Oceanic and Atmospheric Administration to calculate daily ultraviolet exposure and temperature for each one of the dialysis locations.

As previous studies have found, there was a pronounced seasonal variation in blood pressure: higher in the winter, lower in the summer. Here are the average values for black patients (red line) and white patients (blue line), which are analyzed separately for reasons discussed below:

As noted above, this seasonal pattern can’t be explained by vitamin D, because giving people vitamin D supplements doesn’t flatten the line out. Another possibility is temperature. When it’s warm, your peripheral blood vessels dilate to allow more blood to pass close to the surface of the skin, shedding heat and keeping you cool. These dilated blood vessels make it easier for the heart to pump blood, lowering your blood pressure.

Sure enough, the data analysis shows that some—but not all—of the blood pressure changes can be explained by warmer temperatures in the summer. But even after taking the effects of temperature into account, there’s still a correlation between UV exposure and blood pressure: more UV is associated with lower blood pressure, as Weller’s theory predicted.

It’s worth noting the higher blood pressure in black patients, which is a common observation. It’s hard to disentangle all the possible contributing factors, including socioeconomic factors, dietary patterns, and sensitivity to salt. But it seems likely that the skin itself may play a role: darker skin tends to be naturally protected against UV-induced damage. That’s great for avoiding sunburn, but may be a disadvantage for triggering nitric oxide-related benefits, especially at northern latitudes with weak sunlight. In keeping with that theory, the white dialysis patients seemed to get a greater benefit from a given increase in UV exposure than the black patients, as you’d expect if dark skin pigmentation were partly blocking the beneficial UV.

There are a few caveats worth noting. One is that the researchers didn’t actually measure UV exposure for the subjects. Presumably most of them spent the vast majority of their days inside, especially since they were sick enough to require dialysis—so there’s an assumption the everyone’s daily UV exposure correlated with the total amount of available UV at their location. We’re also assuming that the link between UV and blood pressure is the same in dialysis patients as it is in the general population.

And a final caveat worth noting is that Weller himself has launched a product called Dr. Weller’s Sunwell Sunscreen that promises to block the harmful effects of UV while still allowing sunlight to trigger nitric oxide and vitamin D production. So however pure his motives might be, he does stand to profit if the world wakes up to the importance of nitric oxide.

Where does this leave us? I’d recommend giving a listen to this Outside podcast episode from last summer, where Jacobsen reflects on the response to his original article. He’d just returned from a conference where scientists in the pro-sunlight camp were wrestling with what sort of public health recommendations were appropriate in order to balance the well-established risks of sun exposure with its still-hypothetical benefits. Opinions ranged from “Get 10 minutes of unprotected sun exposure a day” to “Just don’t burn, whatever you do, and you’ll be fine.” I still don’t know what the best answer is, but I’m hoping researchers will keep pushing towards a more definitive answer.

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