The Sun Under Your Skin: How UVA Releases Nitric Oxide and Lowers Blood Pressure

Key Takeaways:

• Human skin holds a reservoir of photolabile nitric oxide derivatives — nitrite and S-nitrosothiols — that ultraviolet A light decomposes into free nitric oxide without any enzyme involved.
• Whole-body UVA irradiation lowered systolic and diastolic blood pressure by about eleven percent for up to an hour, with forearm blood flow rising roughly a quarter and brachial artery vasodilation rising sharply.
• The effect is independent of vitamin D and of nitric oxide synthase, which is why sunlight carries a cardiovascular signal that a supplement capsule does not reproduce.

Sunlight and nitric oxide are connected by a chemistry that has nothing to do with vitamin D. The dermis and epidermis hold a large, quietly maintained store of nitric oxide derivatives — inorganic nitrite and S-nitrosothiols, including S-nitrosoalbumin — that sit in the tissue as photolabile cargo. When ultraviolet A wavelengths, the long band from roughly 320 to 400 nanometers, strike the skin, those molecules decompose photochemically and release free nitric oxide. No enzyme is required. In healthy volunteers, whole-body UVA exposure raised plasma nitrosated species, and systolic and diastolic pressure fell by about eleven percent over the following hour, tracking closely with a twenty-six percent rise in forearm blood flow and a marked increase in flow-mediated vasodilation of the brachial artery.

What makes this more than a curiosity is that the pathway runs around the two mechanisms most people assume are doing the work. When investigators irradiated skin with UVA alone — a band that produces essentially no vitamin D — blood pressure still fell, and forearm blood flow still rose even when nitric oxide synthase was blocked, pointing squarely at pre-formed cutaneous stores rather than enzymatic synthesis. Confocal imaging localized most of the light-sensitive nitric oxide pool to the upper epidermis. That helps explain something epidemiology has hinted at for decades: blood pressure and cardiovascular disease rise with latitude and in winter, and in a twenty-year Swedish cohort of nearly thirty thousand women, those who avoided sun exposure died sooner, driven mainly by cardiovascular and non-cancer causes. It is a reminder that blood pressure is a whole-system output, not a single organ’s decision.

How much sunlight do you need for the nitric oxide effect?

Less than a tan and far less than a burn. The dose that mobilizes cutaneous nitric oxide is modest and brief, and it is delivered by exposing surface area — forearms, lower legs, back — rather than by roasting the face, which is the site that carries most of the lifetime photoaging and carcinoma burden while contributing little to the vasodilatory effect. None of this revises what ultraviolet light does to collagen and to keratinocyte DNA; the molecular case for daily broad-spectrum sunscreen on the face, neck, and hands stands exactly as written, and the same Swedish cohort that found lower cardiovascular mortality in sun-exposed women also found more skin cancer in them. Tanning beds are not a shortcut; they are a carcinogen exposure with a cardiovascular rounding error attached. For anyone with a melanoma history, immunosuppression, or a very fair phenotype, the honest answer is that the risk arithmetic tips the other way and blood pressure should be managed by other levers. For everyone else, the target is calibrated: short, regular, unburned exposure — never a burn, and never total avoidance.

References:

1. Opländer, C., Volkmar, C. M., Paunel-Görgülü, A., van Faassen, E. E., Heiss, C., Kelm, M., Halmer, D., Mürtz, M., Pallua, N., & Suschek, C. V. (2009). Whole body UVA irradiation lowers systemic blood pressure by release of nitric oxide from intracutaneous photolabile nitric oxide derivates. Circulation Research, 105(10), 1031-1040.
2. Liu, D., Fernandez, B. O., Hamilton, A., Lang, N. N., Gallagher, J. M. C., Newby, D. E., Feelisch, M., & Weller, R. B. (2014). UVA irradiation of human skin vasodilates arterial vasculature and lowers blood pressure independently of nitric oxide synthase. The Journal of Investigative Dermatology, 134(7), 1839-1846.
3. Lindqvist, P. G., Epstein, E., Nielsen, K., Landin-Olsson, M., Ingvar, C., & Olsson, H. (2016). Avoidance of sun exposure as a risk factor for major causes of death: A competing risk analysis of the Melanoma in Southern Sweden cohort. Journal of Internal Medicine, 280(4), 375-387.