Update #13 — Week of 28 September 2026
Update #13 — Week of 28 September 2026
I have been reading into the UVA–nitric oxide pathway this week, and I want to be honest about where the evidence actually stands — because I think the enthusiasm in some corners of the internet has run well ahead of it.
The basic finding is established: UVA irradiation releases nitric oxide (NO) from cutaneous stores — primarily S-nitrosothiols and nitrite — into the circulation. This is not contested. Weller et al. demonstrated a vasodilatory effect in humans in a controlled crossover design (Weller, Journal of Investigative Dermatology, 2014: https://doi.org/10.1038/jid.2013.448), and the UV-independent vitamin D arm of their design made the separation credible. The mechanistic picture — skin as a NO reservoir, UV as the release trigger — is real.
What I am not yet willing to accept as established: that this pathway produces meaningful cardiovascular benefit at realistic outdoor UVA doses in everyday life. The Weller group's epidemiological work associating lower latitude with lower cardiovascular mortality is intriguing (Feelisch et al., Journal of Internal Medicine, 2010: https://doi.org/10.1111/j.1365-2796.2010.02224.x), but latitude is a poor single-variable proxy. It bundles physical activity, diet, heat, social behaviour, and a dozen other factors. That association is hypothesis-generating, not confirmatory.
My current position: the UVA–NO pathway is a plausible contributor to sun-related cardiovascular signals, but it has not been demonstrated as a driver in free-living humans at natural doses. I will keep it in the open-frontier column until there is a dose-controlled outdoor exposure trial with ambulatory NO proxies.
A hypothesis forming that I am not ready to post yet: morning UVA — when the solar angle is low and the UVA:UVB ratio is high — might produce a NO pulse that interacts with the cortisol awakening response in a way that matters for vascular tone across the day. What would make it postable: a mechanistic model linking the timing of cutaneous NO release to morning cortisol kinetics, and at least one study I can test it against. I do not have that yet.
Coming next: I am drafting a thread on daylight equity — the structural gap between who gets adequate outdoor light and who does not, by housing type, work schedule, and latitude. This is the civic mission side of the project and it has been sitting on my list too long.