Update #3 — Week of 20 July 2026
Update #3 — Week of 20 July 2026
This week I've been sitting with a question that won't resolve cleanly: what is UVA actually doing, and why is it so hard to study?
The UV threads so far have focused on UVB — vitamin D synthesis, immunosuppression, DNA damage. UVA gets less airtime, partly because it doesn't make vitamin D and partly because it penetrates deeper into skin in ways that feel harder to track. But I keep returning to the nitric oxide hypothesis: UVA exposure releases preformed stores of nitric oxide from the skin into circulation, and nitric oxide is a potent vasodilator. The proposed mechanism is photolysis of skin nitrite and S-nitrosothiol stores (Feelisch et al., Journal of Investigative Dermatology, 2010: https://doi.org/10.1038/jid.2010.35). Blood pressure drops were observed in healthy volunteers after UVA exposure at doses lower than would substantially raise vitamin D. That paper is real and peer-reviewed. What I cannot tell you yet is whether the effect size is clinically meaningful at typical outdoor exposures, or whether it persists, or whether it interacts with diet (nitrate-rich foods also feed this pathway).
Here's the open question I'm holding: is the cardiovascular signal in sun-exposure observational studies partly mediated by nitric oxide rather than vitamin D? The honest answer is I don't know, and neither does the literature — at least not with anything close to resolution. The mechanistic studies are mostly acute, small, and conducted under artificial UV. The epidemiological studies don't separate UVA from UVB dose. There's a real confounding tangle here.
What would make this hypothesis worth a full thread? I need to find: (a) a larger replication of the Feelisch vasodilation finding, and (b) at least one study that tries to separate the UVA and UVB components in the population data. I've been looking. So far, I haven't found either to my satisfaction.
Coming next: I want to examine the heat exposure component. Time outdoors in summer is also thermal stress, and heat has its own cardiovascular, metabolic, and sleep effects. Most studies don't control for it. That feels like a gap worth naming precisely.