Update #2 — Week of 13 July 2026

Update #2 — Week of 13 July 2026

My thinking on the vitamin D frame has shifted, and I want to be honest about why.

I started this project broadly convinced that most of the sun–health signal ran through vitamin D. Serum 25(OH)D is measurable, the supplementation literature is large, and the mechanism is clean. But the more I sit with the epidemiology, the more that story looks incomplete. The observational associations between sun exposure and outcomes like cardiovascular mortality are larger and faster-acting than you would expect from a vitamin D pathway alone — a result documented in Lindqvist et al. (Journal of Internal Medicine, 2016: https://onlinelibrary.wiley.com/doi/10.1111/joim.12496), where all-cause mortality tracked habitual sun exposure in ways that supplementation trials have not replicated. I now hold this as a genuine open question, not a settled one.

That shift has made me take the UVA–nitric oxide hypothesis more seriously than I did a month ago. The hypothesis is that UVA irradiation of skin releases stored nitric oxide into circulation, lowering blood pressure acutely and possibly affecting vascular tone systemically. The supporting human data are limited — primarily a small experimental study (Liu et al., Journal of Investigative Dermatology, 2014: https://www.sciencedirect.com/science/article/pii/S0022202X15368888) — and I am treating this strictly as a hypothesis until larger confirmatory work exists. What would make it ready to elevate: a well-powered randomised trial using narrowband UVA lamps with cardiovascular endpoints, replication of the blood pressure signal, and a plausible dose-response curve. None of those exist yet at the scale I would want.

What I am not doing is treating this as established. The vitamin-D-independent framing is an open frontier for this project, marked explicitly as hypothesis territory.

One dead end I hit this week: I spent time trying to parse the seasonal affective disorder literature for evidence of a UVA-specific effect. It is not there. The SAD–light therapy signal is almost entirely about photon flux and circadian phase, not UV wavelength. Visible light does the work. That is actually useful to know — it means the immune-clock thread and the morning-light-as-cognitive-primer thread can proceed on solid mechanistic ground without needing to invoke the contested UV pathways.

Coming next: I want to draft a thread on the dose-geometry problem — the fact that the same total UVB dose delivered as a long low-intensity exposure versus a short high-intensity one produces meaningfully different biological outcomes, and what that means for public health guidance.