Is exposure to sunlight associated with cardiovascular benefits?

A good deal of attention has been paid to the potential benefits of limited exposure to sunlight as a means of producing vitamin D and which has a wide range of purported health benefits. This is important from a public health perspective because vitamin D deficiency has become a world-wide concern.

For instance, a global analysis of 7.9 million people between 2000 and 2022, revealed how 15.7% of participants had vitamin D levels below 30 nmol/L.¹ Moreover, values below 30 nmol/L are acknowledged as being too low with the potential to weaken bones.² In addition, other works has suggested that reduced levels of vitamin D are associated with a higher risk of death from any cause (i.e., all-cause mortality) as well as cardiovascular mortality and an elevated risk for some cancers.³ But if sufficient levels of vitamin D are necessary for optimal health, it seems reasonable to assume that supplementing with the vitamin would provide the same benefits. Only it doesn’t.

One of the largest ever randomised trials of vitamin D supplements failed to demonstrate improvements in health outcomes. For example, the VITAL trial randomised over 25,000 participants to vitamin D or placebo and followed them for 5.3 years. The results showed no difference in either major cardiovascular events or death from a range of cancers between the two groups.⁴

If it isn’t vitamin D, what else might be responsible for the health benefits associated exposure to sunlight? Furthermore, is the converse true, i.e., that avoidance of sunlight is potentially harmful? In 1990, Swedish researchers began a prospective study to examine the effect of sun avoidance on all-cause mortality. But what they found was something of a surprise.

Sun exposure reduces the risk of death

The Melanoma in Southern Sweden cohort study enrolled over 29,000 women who were followed over the next 20 years.⁵ Researcher categorised the women into three groups based on their habits around sunbathing and use of tanning beds. The three groups were sun avoiders; those with moderate sun exposure and a third group with the highest level of exposure. The team also collected a wide range of data such as smoking status, body mass index, levels of exercise etc. The results were astonishing: those classed as sun avoiders had a two-fold higher risk of dying.

What was more intriguing was how the risk of dying from causes such as cardiovascular disease decreased, as the amount of sun exposure increased. A further analysis of the data revealed that women with the lowest exposure to sunlight also had 41% higher risk of being diagnosed with hypertension.⁶ More surprising still, was how non-smokers who avoided the sun had a similar life expectancy to smokers with the highest sun exposure!

More recently, a team from Edinburgh university led by dermatologist Richard Weller, have undertaken an analysis of data for participants in the UK Biobank.⁷ Their analysis, which is currently available as a preprint, included over 376,000 individuals of white ancestry, followed over 12.7 years.

The results were similar to those observed in Sweden: those with the highest exposure to sunlight had a 14% lower risk of all-cause and cancer mortality and a 19% lower risk of cardiovascular mortality, compared to those with less active sun-seeking behaviour. As with the Swedish study, these findings were adjusted for a range of other factors which might have affected the result. Another interesting observation from the Edinburgh group’s study was the influence of latitude. The further south someone lived, the greater the health benefits.

But what is it about exposure to sunlight that reduces the risk of dying?

Cutaneous nitric oxide and sunlight

It has been known for many years that nitric oxide (NO) is present in the skin where it functions to control inflammation, helps with wound healing and forms part of the skin’s antimicrobial defence system. It turns out that NO is also rapidly released upon exposure to UV radiation and acts as a vasodilator⁸ and whole-body irradiation with UVA, causes a rapid and significant reduction in both systolic and diastolic blood pressure in healthy volunteers.⁹

Other evidence pointing to a possible solar-related anti-hypertensive effect, comes from an analysis of the season variation in blood pressure. It seems that on average, blood pressure readings are lower during the summer months.¹⁰ So could exposure to sunlight reduce blood pressure? This seems likely given the results of a recent pilot study by Korean researchers. This showed that 45 minutes of daily exposure to sunlight for three days in July, significantly reduced blood pressure.¹¹

In other work, higher levels of prenatal exposure to sunlight were associated with a reduced risk of hypertensive disorders in pregnancy.¹² The best possible evidence is derived from a randomised controlled trial. However, in one such trial where patients with stage 1 hypertension were exposed to 14 consecutive days of treatment with a UVA lamp, although blood pressure reductions were observed following irradiation, these were not sustained and had no effect on 24-hour ambulatory readings.¹³

Other potential health benefits of sunlight

Although many studies hint at a potential benefit from exposure to sunlight, much of this work has focused on vitamin D. Consequently, trying to untangle whether a health benefit is due to vitamin D or solar radiation itself is challenging. For example, a lower risk of breast and colorectal cancer have been associated with higher rather than lower levels of vitamin D. Similarly, the incidence of metabolic syndrome, Alzheimer’s disease and autism, all appear to be less prevalent in those with higher vitamin D levels. In addition, as with the VITAL trial, it seems that oral supplements of vitamin D are generally less effective at improving these diseases. It seems possible therefore, that vitamin D levels serve as a marker for increased sun exposure.

Adverse effects of exposure to sunlight

No discussion of the benefits of exposure to sunlight would be complete without recognising the potential downsides. Solar UV radiation is a known carcinogen and which damages cellular DNA.¹⁴ Although the body has evolved several effective mechanisms to repair and remove damaged DNA,¹⁵ the non-melanoma skin cancers, squamous cell carcinoma and basal cell carcinoma arise from cumulative exposure to UV radiation.

Conclusion

There are likely to be some health advantages to sunlight exposure, but according to dermatologist, Richard Weller, we don’t currently know how much time is needed to gain any health benefits, because current sun advice relates to production of vitamin D and which is probably not relevant. Some in vitro work does indicate that exposure to low dose artificial light, replicating daylight exposure to UK sunlight, induces the production of NO but with a negligible increase in DNA damage.¹⁶ Thus it seems that some exposure to sunlight may have a blood pressure lowering effect, but in the absence of clear advice, navigating the risks and benefits of sun exposure remains challenging.

Rod Tucker is a Pharmacist and independent researcher

References

1. Cui A, Zhang T, Xiao P, et al. Global and regional prevalence of vitamin D deficiency in population-based studies from 2000 to 2022: A pooled analysis of 7.9 million participants. Front Nutr. 2023 Mar 17; 10:1070808.
2. National Institutes of Health. Vitamin D. Available online at: https://ods.od.nih.gov/factsheets/VitaminD-Consumer/#:~:text=Levels%20of%2050%20nmol%2FL,and%20might%20cause%20health%20problems [Accessed May 2024]

3. Dobnig H, Pilz S, Scharnagl H et al. Independent association of low serum 25-hydroxyvitamin d and 1,25-dihydroxyvitamin d levels with all-cause and cardiovascular mortality. Arch Intern Med. 2008 Jun 23;168(12):1340-9.
4. Manson JE, Cook NR, Lee IM et al. VITAL Research Group. Vitamin D Supplements and Prevention of Cancer and Cardiovascular Disease. N Engl J Med. 2019 Jan 3;380(1):33-44.
5. Lindqvist PG, Epstein E, Landin-Olsson M et al. Avoidance of sun exposure is a risk factor for all-cause mortality: results from the Melanoma in Southern Sweden cohort. J Intern Med. 2014 Jul;276(1):77-86.
6. Lindqvist PG, Landin-Olsson M, Olsson H. Low sun exposure habits is associated with a dose-dependent increased risk of hypertension: a report from the large MISS cohort. Photochem Photobiol Sci. 2021 Feb;20(2):285-292.
7. Stevenson AC, Clemens T, Pairo-Castineira E et al. Higher ultraviolet light exposure is associated with lower mortality: an analysis of data from the UK Biobank cohort study. MedRxiv. doi:https://doi.org/10.1101/2023.07.11.23292360
8. Mowbray M, McLintock S, Weerakoon R et al. Enzyme-independent NO stores in human skin: quantification and influence of UV radiation. J Invest Dermatol. 2009 Apr;129(4):834-42.
9. Opländer C, Volkmar CM, Paunel-Görgülü A et al. Whole body UVA irradiation lowers systemic blood pressure by release of nitric oxide from intracutaneous photolabile nitric oxide derivates. Circ Res. 2009 Nov 6;105(10):1031-40.
10. Kollias A, Kyriakoulis KG, Stambolliu E et al. Seasonal blood pressure variation assessed by different measurement methods: systematic review and meta-analysis. J Hypertens. 2020 May;38(5):791-798.
11. Park JW, Kim KA, Lee MG et al Effect of Short-Term Sunlight Exposure on Blood Pressure and Pulse Rate in Vitamin D3-Insufficient, Prehypertensive Patients: A Pilot Study. Complement Med Res. 2021;28(3):206-215.
12. Lu D, Li M, Gan Y, Yu G et al. Prenatal exposure to solar radiation and hypertensive disorders of pregnancy. BJOG. 2022 Feb;129(3):393-401.
13. Weller RB, Macintyre IM, Melville V et al. The effect of daily UVA phototherapy for 2 weeks on clinic and 24-h blood pressure in individuals with mild hypertension. J Hum Hypertens. 2023 Jul;37(7):548-553.
14. de Gruijl FR. Skin cancer and solar UV radiation. Eur J Cancer. 1999 Dec;35(14):2003-9.
15. Kciuk M, Marciniak B, Mojzych M et al. Focus on UV-Induced DNA Damage and Repair-Disease Relevance and Protective Strategies. Int J Mol Sci. 2020 Oct 1;21(19):7264.
16. Hazell G, Khazova M, O’Mahoney P. Low-dose daylight exposure induces nitric oxide release and maintains cell viability in vitro. Sci Rep. 2023 Sep 28;13(1):16306.

Rod Tucker

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