How Sunlight Lowers CVD Mortality Through Nitric Oxide Release

Part 2 of the series: Beyond Vitamin D: The Hidden Lifesaving Benefits of Sunlight

Part 1: Sunlight Paradox: Why Sun Exposure Increases Cancer but Extends Life

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Introduction

Cardiovascular disease is the world’s number one killer.

It claims more than 18 million lives every year. It does not care about your diet fads, your step count, or your cholesterol medication. It is relentlessly, brutally efficient.

Now consider a question that sounds absurd on its face: What if a daily dose of sunlight worked as well as a low-dose antihypertensive?

What if the sun — the same sun we are told to avoid — was quietly protecting the hearts of those who spent time in it?

This is not a metaphor. This is not about vitamin D. This is about a molecule called nitric oxide, and the evidence that sunlight triggers its release directly from your skin into your bloodstream, lowering your blood pressure with every exposure.

In the first article of this series, Sunlight Paradox: Why Sun Exposure Increased Cancer but Extended Life, we examined the Swedish paradox: women who got more sun developed more skin cancer but lived significantly longer than those who avoided it.

The number one driver of that survival gap was cardiovascular disease. Sun avoiders died of heart attacks and strokes at substantially higher rates.

This article explains why.

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The Study That Proved It: UVA Lowers Blood Pressure in Humans

In 2014 — the same year the Swedish study was published — a team at the University of Southampton conducted an experiment that should have made headlines around the world. [1]

The design was elegant. They recruited 24 healthy volunteers and exposed them to a UVA lamp while measuring their blood pressure before, during, and after. Then they repeated the experiment with a crucial twist: they covered the lamp with a filter that blocked the UVA and visible light but let through exactly the same amount of heat.

If blood pressure dropped in both conditions, it was just warmth relaxing the blood vessels. If it dropped only with UVA, something else was happening.

The results were unambiguous.

UVA irradiation significantly lowered both systolic and diastolic blood pressure. The sham heat control had no effect. The drop was not enormous — about 5 mmHg — but it was real, sustained for an hour after exposure, and caused specifically by the light, not the warmth.

When the researchers measured the participants’ blood, they found the smoking gun: nitrite levels rose in the UVA group but not the control group. Nitrite is the circulatory storage form of nitric oxide. The UVA light was mobilizing nitric oxide from the skin into the bloodstream, where it dilated blood vessels and reduced pressure.

A 5 mmHg drop in systolic blood pressure may sound modest. At a population level, it is not. A sustained 5 mmHg reduction translates to a roughly 10% lower risk of stroke and a 7% lower risk of coronary heart disease. No prescription required. Just sunlight.

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How It Works: The Skin as a Nitric Oxide Reservoir

To understand why this happens, you have to rethink what the skin is.

The skin is not a passive wrapper. It is the body’s largest organ, and it stores an enormous amount of nitrate and nitrite. These compounds come from dietary sources — leafy greens, beetroot — and they circulate in the blood before being deposited in the skin.

When UVA and blue-violet light (specifically wavelengths around 320-460 nm) hit the skin, they photolyze these stored nitrite and nitrate molecules. The chemical bonds break. Nitric oxide (NO) is released. It diffuses into the small blood vessels just beneath the skin’s surface and enters the systemic circulation.

Once in the blood, nitric oxide does what it always does:

• Relaxes vascular smooth muscle, dilating arteries and arterioles
• Lowers peripheral vascular resistance, the main determinant of blood pressure
• Inhibits platelet aggregation, reducing the tendency to form dangerous clots
• Improves endothelial function, maintaining the health of the blood vessel lining

This entire pathway is completely independent of vitamin D. UVB makes vitamin D in the epidermis. UVA and blue light make nitric oxide from the dermis. They are separate wavelengths, separate mechanisms, separate benefits.

A vitamin D pill gives you one. Only the sun gives you both.

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From Blood Pressure to Population Data: The U.S. UVR Study

If sunlight lowers blood pressure through nitric oxide, the effect should be visible at the population level. Blood pressure should, on average, be lower in places with more UVR exposure — and this should translate into fewer cardiovascular deaths.

A 2020 study published in the Journal of the American Heart Association tested exactly this hypothesis. [2]

The researchers used satellite data to measure ambient solar UVR across different regions of the United States. They then correlated these measurements with population-level blood pressure readings and cardiovascular mortality rates, carefully adjusting for temperature — because warmer places might have different lifestyles, and temperature itself affects blood pressure.

The findings were clear:

• Higher ambient solar UVR was associated with lower population-level systolic blood pressure.
• The blood pressure difference was independent of temperature. It was the light, not the heat.
• The reduction translated into a predicted lower cardiovascular mortality at higher UVR levels.
• The effect was consistent with the known mechanism — UVA-driven nitric oxide release.

This was not a small study. It covered entire regions. And it demonstrated that the blood pressure benefit observed in a laboratory experiment was visible in real-world populations, across different climates and demographics.

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The U.S. Prospective Cohort: UVR and Cardiovascular Death

The same year the JAHA population study was published, another major piece of evidence had already been in the literature for several years.

In 2013, researchers using the NIH-AARP Diet and Health Study — a massive prospective cohort of over 500,000 Americans followed for an average of 12 years — examined the relationship between ambient UVR exposure and cause-specific mortality. [3]

Unlike the Swedish study, this one used satellite-measured UVR exposure linked to each participant’s residential location. This was an objective, unbiased measure of how much solar radiation reached the places where people actually lived. The researchers did not have to rely on anyone’s memory of sunbathing habits. The satellites recorded it.

After adjusting for physical activity, BMI, smoking, diet, and numerous other confounders, the findings were striking:

• Higher ambient UVR exposure was associated with lower total mortality.
• The mortality reduction was driven partly by lower cardiovascular disease mortality.
• The association persisted after controlling for every lifestyle factor the researchers could measure.
• The authors noted that vitamin D alone could not explain the effect and raised the possibility of other pathways — including nitric oxide.

This was half a million Americans, tracked for over a decade, with satellite-verified UVR exposure. The outcome was not a blood pressure reading. It was death from cardiovascular disease.

The Danish Nationwide Study: Skin Cancer as a Sun Marker

If sunlight lowers blood pressure via nitric oxide, we should see evidence of cardiovascular protection in people who get the most sun. The Swedish study showed this. But it was not the only one.

In 2019, a team of Danish researchers published a study in the International Journal of Epidemiology that leveraged the power of nationwide health registries. [2] They followed over 4 million Danish adults and examined the relationship between having a history of non-melanoma skin cancer (BCC or SCC) and subsequent risk of cardiovascular events and death.

This requires explanation. Why use skin cancer as a sun marker?

Because non-melanoma skin cancer is a reliable, objective proxy for cumulative sun exposure. It is not dependent on people accurately recalling their sunbathing habits. A diagnosis of BCC or SCC is a biological record that significant UVR reached the skin over a lifetime. It is, in a sense, an unintentional biomarker.

The findings were striking. After adjusting for age and other confounders, people with a history of BCC or SCC had a significantly lower risk of major adverse cardiovascular events (MACE) — including heart attack and stroke. They also had lower all-cause mortality.

Crucially, this protective association was:

• Consistent across age groups
• Independent of sociodemographic factors like income and education
• Specific to CVD, the mortality benefit was driven primarily by fewer cardiovascular deaths

The Danish study did not prove causation. But when combined with the Southampton mechanistic experiment, the causal chain becomes coherent: Sun → UVA → skin nitrite photolysis → nitric oxide → vasodilation → lower blood pressure → fewer cardiovascular events over decades.

This is not correlation without mechanism. It is a correlation with a proven, biologically measured mechanism that operates every time skin is exposed to sunlight.

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The VTE Connection: A Finding That Exercise Cannot Explain

In the Swedish study, we noted that physical activity was a potential confounder. Perhaps women who spent more time outdoors were simply fitter, and fitness protects the heart.

The researchers addressed this with statistical adjustments for BMI, smoking, and education. The survival benefit persisted.

But there is another finding from the same Swedish cohort that argues even more powerfully against the “it’s just exercise” explanation.

In a separate analysis, the research group examined the link between sun exposure habits and venous thromboembolism (VTE) — blood clots that form in veins, typically in the legs, and can travel to the lungs as a pulmonary embolism. [3]

They found that women with the highest sun exposure had a 30-50% lower risk of VTE compared to sun avoiders.

This is a highly specific clue. VTE is not a condition that physical activity is known to prevent in any straightforward, dose-dependent way.

Marathon runners get DVTs. Fit young athletes get pulmonary emboli. The risk factors for VTE are largely genetic, related to immobility, surgery, pregnancy, or cancer — not lack of exercise.

But VTE is directly and biologically linked to the nitric oxide pathway. Nitric oxide is a potent inhibitor of platelet aggregation. It keeps blood platelets from becoming sticky and clumping together unnecessarily. It also improves the health of the endothelial lining of veins, reducing the surface upon which clots can nucleate.

When the sun triggers nitric oxide release from the skin, it not only lowers blood pressure. It is making the blood slightly less likely to clot inappropriately.

The fact that VTE — a condition with no obvious link to exercise — was lower in sun-exposed individuals is a strong argument that the sun is exerting a specific, physiological effect. The effect is not being outdoors. It is being in the sunlight.

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The Evidence Hierarchy for CVD Mortality

The case for sunlight protecting against cardiovascular death is not built on a single study. It is built on converging evidence from different research designs, in different populations, using different exposure measures — all pointing toward the same conclusion.

Study	Exposure Measure	Outcome	Key Strength
Southampton (2014)	Controlled UVA irradiation	Blood pressure drop	Proves mechanism; causal evidence in humans
NIH-AARP U.S. (2013)	Satellite-measured UVR	CVD death	Objective exposure; 500K+ people; prospective
Swedish MISS (2014)	Self-reported sun habits	CVD death	Competing risk analysis; dose-response gradient
Danish Cohort (2019)	NMSC diagnosis (biological marker)	MI, all-cause death	Hard endpoints; 4 million people
JAHA U.S. (2020)	Satellite-measured UVR	Population BP; predicted CVD mortality	Temperature-adjusted; population-level

The hierarchy builds from mechanism to outcome:

1. Southampton proves sunlight lowers blood pressure through nitric oxide.
2. JAHA shows the effect is visible across entire populations, independent of temperature.
3. NIH-AARP links satellite-measured UVR directly to fewer cardiovascular deaths in half a million Americans.
4. Swedish MISS shows sun-avoiding women die of CVD at higher rates, with a dose-response curve.The
5. Danish Cohort uses a biological marker of sun exposure and finds lower heart attack and death rates in 4 million adults.

No single study is definitive on its own. But together, they form an evidence base that is difficult to dismiss.

Why This Matters for Public Health

Blood pressure is the single most important modifiable risk factor for cardiovascular disease worldwide. The global burden of hypertension is staggering: over 1.3 billion people, most of whom are not at target blood pressure despite medication.

Now consider the modern lifestyle. We commute in cars with UV-filtering glass. We work in offices lit by LEDs that emit no UVA and minimal blue light. We exercise in gyms or in the shade. We apply sunscreen that blocks UVA before going outside. We spend approximately 90% of our time indoors.

From a nitric oxide perspective, this is a population-wide deficiency state.

We have systematically removed a natural, daily, side-effect-free vasodilator from our physiology. Not because it was harmful. Because we did not know it was there.

The Southampton researchers estimated that the blood pressure-lowering effect of sunlight could account for a significant portion of the seasonal variation in blood pressure — higher in winter, lower in summer — that has been observed for decades. The latitude gradient in hypertension — higher blood pressure further from the equator — also aligns with this mechanism.

This does not mean sunlight replaces antihypertensive medication for people who need it. It does mean that public health advice to avoid the sun at all times may be inadvertently contributing to the burden of cardiovascular disease, particularly in high-latitude, low-sunlight regions.

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The Mechanism in Summary

Component	Detail
Trigger	UVA and blue-violet light (320-460 nm)
Site of action	Skin dermis — stored nitrate and nitrite
Chemical reaction	Photolysis of nitrate/nitrite to nitric oxide (NO)
Route to circulation	Diffusion into dermal blood vessels
Physiological effects	Vasodilation, reduced blood pressure, inhibition of platelet aggregation, improved endothelial function
Clinical outcomes	Lower risk of hypertension, myocardial infarction, stroke, and venous thromboembolism
Independence from vitamin D	Complete — this is a separate wavelength, separate mechanism, separate benefit

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Looking Ahead

The nitric oxide pathway explains why the Swedish sun avoiders died of cardiovascular disease at higher rates. It explains the Danish link between skin cancer and CVD protection. It explains the VTE finding. It explains the seasonal and latitudinal patterns in blood pressure.

But the story does not end with the heart.

The same nitric oxide that dilates blood vessels also plays a role in glucose metabolism. The same sun that lowers blood pressure may also influence how the body handles sugar. And the metabolic benefits of sunlight — independently of vitamin D — are the subject of the next article in this series.

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Key Takeaways

• Cardiovascular disease was the number one driver of excess deaths in sun avoiders in the Swedish study — not skin cancer.
• A controlled experiment proved UVA light lowers blood pressure in humans, while heat alone does not. The effect is mediated by nitric oxide released from the skin.
• The skin is a massive nitric oxide reservoir. UVA and blue light photolyze stored nitrate and nitrite, releasing NO into the bloodstream.
• This pathway is entirely independent of vitamin D. UVB makes vitamin D. UVA and blue light make nitric oxide. Both matter. A pill gives you only one.
• A U.S. satellite study of over 500,000 Americans linked higher ambient UVR exposure to lower cardiovascular mortality, after adjusting for lifestyle confounders.
• A population-level U.S. study found lower blood pressure and predicted lower CVD mortality in regions with higher solar UVR, independent of temperature.
• A Danish nationwide study of 4 million adults found that a history of non-melanoma skin cancer — a biological marker of sun exposure — was associated with significantly lower cardiovascular mortality.
• The evidence hierarchy spans five studies across different populations, exposure measures, and outcomes — all converging on the same conclusion.
• Venous thromboembolism risk was 30-50% lower in sun-exposed women. VTE is not prevented by exercise, pointing to a sun-specific, blood-mediated protective mechanism.
• Nitric oxide inhibits platelet aggregation and improves vascular health, explaining the VTE and broader CVD protection.
• Modern indoor life represents a population-wide nitric oxide deficiency, with implications for global hypertension burden.
• The sun is not just a vitamin D factory — it is a natural, daily vasodilator that we have inadvertently removed from our physiology.

The next article in this series will explore how sunlight influences metabolic health — including type 2 diabetes and metabolic syndrome — through mechanisms that go beyond both vitamin D and nitric oxide.

Don’t Get Sick!

About Dr. Jesse Santiano, MD

Dr. Santiano is a retired internist and emergency physician with extensive clinical experience in metabolic health, cardiovascular prevention, and lifestyle medicine. He reviews all medical content on this site to ensure accuracy, clarity, and safe application for readers. This article is for educational purposes and is not a substitute for personal medical care.

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References:

[1] Liu D, Fernandez BO, Hamilton A, et al. UVA irradiation of human skin vasodilates arterial vasculature and lowers blood pressure independently of nitric oxide synthase. Journal of Investigative Dermatology. 2014;134(7):1839-1846. doi:10.1038/jid.2014.27

[2] Weller RB, Wang Y, He J, et al. Does incident solar ultraviolet radiation lower blood pressure? Journal of the American Heart Association. 2020;9(5):e013837. doi:10.1161/JAHA.119.013837

[3] Lin SW, Wheeler DC, Park Y, et al. Prospective study of ultraviolet radiation exposure and mortality risk in the United States. American Journal of Epidemiology. 2013;178(4):521-529. doi:10.1093/aje/kws589

[4] Brøndum-Jacobsen P, Nordestgaard BG, Nielsen SF, Benn M. Skin cancer as a marker of sun exposure associates with lower risk of myocardial infarction, hip fracture, and all-cause mortality. International Journal of Epidemiology. 2019;48(4):1096-1107. doi:10.1093/ije/dyz038

[5] Lindqvist PG, Epstein E, Olsson H. Does an active sun exposure habit lower the risk of venous thrombotic events? A D-lightful hypothesis. Journal of Thrombosis and Haemostasis. 2009;7(4):605-610. doi:10.1111/j.1538-7836.2009.03312.x

Disclaimer:
This article is for educational purposes and is not a substitute for professional medical advice, diagnosis, or treatment. Always consult your physician before making health decisions based on the TyG Index or other biomarkers.

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