The Hidden Thing Sunlight Does to Your Heart

A question the data kept raising

People who live in sunnier countries have lower rates of cardiovascular disease. They also live longer, on average. For a long time, the medical world assumed this was about vitamin D — sun exposure produces vitamin D, and vitamin D supports heart health. It seemed like a clean explanation.

Weller was not convinced. If vitamin D was the whole story, then vitamin D supplements should produce the same benefit. But study after study has shown that supplements do not lower cardiovascular mortality in the same way that sun exposure appears to. Something else was happening in the skin.

What the skin releases when light hits it

The answer turned out to involve a molecule called nitric oxide. Your skin stores large amounts of it in a form called nitrite and nitrosothiols — compounds that sit just under the surface, stable and waiting. When UV-A light from the sun reaches your skin, it breaks these compounds down and releases nitric oxide directly into your bloodstream.

Nitric oxide relaxes the walls of blood vessels. This makes the vessels wider, reduces resistance, and lowers blood pressure. The effect begins within minutes of sun exposure. It does not require vitamin D to be produced first. It is a completely separate pathway — one that only works when actual light hits actual skin.

UV-A, not UV-B

This distinction matters more than it might seem. UV-B — the wavelength responsible for vitamin D production — is highly seasonal. In Norway, meaningful UV-B is only available from roughly May to September, and only around midday. The rest of the year, it is too weak to trigger anything in the skin.

UV-A behaves differently. It is present at lower intensities throughout most of the year. It reaches you on overcast days. It passes through some light clothing. It is the longer, more persistent wavelength of sunlight. And it is UV-A that drives the nitric oxide release Weller's team identified. This means the cardiovascular effect of sunlight may be available across more of the year than most people assume.

What the numbers show

Weller's team used data from the UK Biobank — one of the largest health databases in the world, with over 376,000 participants followed for an average of nearly 13 years. They compared people who actively sought out sun exposure with those who tended to avoid it, controlling for income, smoking, diet, and other factors.

The results were striking. People with more active sun exposure had 14 percent lower all-cause mortality — meaning they were less likely to die from any cause during the study period. Cardiovascular mortality was 19 percent lower. Cancer mortality was 14 percent lower. These were not small effects.

Melanoma risk was somewhat higher in the sun-seeking group, as you would expect. But the reduction in cardiovascular and cancer deaths far outweighed it. The overall conclusion: in low-sunlight countries, the benefits of regular UV exposure appear to exceed the risks.

Why a supplement cannot do this

This is the part that changes how you think about vitamin D pills. Supplements can raise your circulating vitamin D levels — that part works. But they cannot trigger nitric oxide release from the skin. That mechanism requires a physical signal: light hitting tissue.

Weller has pointed out that multiple large trials of vitamin D supplementation have failed to reduce cardiovascular mortality in the way that sunlight exposure does. His interpretation is that vitamin D may often be acting as a marker — a sign that someone gets enough sun — rather than the cause of the benefit itself. The actual driver, at least for the heart, may be nitric oxide.

What this means in practice

Weller is not saying to abandon sun protection. He is saying that regular, moderate sun exposure is not a risk to be minimised — it is a health behaviour, like exercise or sleep, that the body depends on. And like exercise, the benefit comes from consistency, not from occasional intensity.

A small, daily drop in blood pressure may not feel like much. But a 3-millimetre reduction in systolic blood pressure — roughly what regular sun exposure can produce — reduces the risk of cardiovascular events by around 10 percent at a population level. Over a lifetime, that compounds.

For someone living at a northern latitude, the practical takeaway is this: use the light you have. Step outside when the sun is there, even briefly. Expose your arms and face when the weather allows — not just in summer, but on the mild days of spring and autumn too, when UV-A is still present even if UV-B is not. Your skin is doing something useful every time.

Go Deeper

If you want to explore this topic further:

• Nitric oxide and sunlight — Wikipedia: Nitric oxide
• UVA and blood pressure — PubMed: "UVA irradiation of human skin vasodilates arterial vasculature and lowers blood pressure independently of nitric oxide synthase" (pubmed.ncbi.nlm.nih.gov/24445737)
• UK Biobank sun exposure study — medRxiv: "Higher ultraviolet light exposure is associated with lower mortality" (medrxiv.org/content/10.1101/2023.07.11.23292360v1)
• Risk-benefit analysis — medRxiv: "Risk-benefit balance of habitual ultraviolet exposure for cardiovascular, cancer, and skin cancer mortality" (medrxiv.org/content/10.64898/2026.01.08.26343592v1)
• Weller research overview — University of Edinburgh: Professor Richard Weller (inflammation-research.ed.ac.uk/research/research-groups/professor-richard-weller)