Little proof sunbathing cuts heart attack risk
"Sunbathing 'can cut risk of heart attacks and strokes'," reports the Metro – but there is little hard evidence to back the claim.
New research has tried to explain previous observations that blood pressure is lower in the summer than in the winter in people with mildly high blood pressure. Could sunlight – specifically ultraviolet A (UVA) light – be responsible?
In a lab experiment, researchers found that short bursts of UVA exposure reduced blood pressure in healthy volunteers. Further experiments suggested that UVA exposure modified levels of nitric oxides in the blood by transferring stores from the skin into the blood. The nitric oxides may then cause blood vessels to expand, leading to a drop in pressure.
It's important to note that the study only involved healthy young adults. It is unclear what effects similar UVA exposure would have in those groups most at risk of the complications of high blood pressure – often unfit older adults.
There was also no long-term follow-up, which means we can't say if the temporary fall in blood pressure would really benefit people by reducing heart attacks or strokes. It also cannot be compared with the risks of skin cancer.
Until these questions are answered, the proven methods of keeping blood pressure at a healthy level are still through getting regular exercise and eating a healthy diet.
Where did the story come from?
The study was carried out by researchers from the University of Southampton and the University of Edinburgh.
It was funded by the Foundation for Skin Research, Chest Heart and Stroke Scotland, the Claire Wand Fund and the Faculty of Medicine at the University of Southampton.
The study was published in the peer-reviewed Journal of Investigative Dermatology.
The media has confusingly focused on comparing the rates of cardiovascular disease with skin cancer, implying that the benefits of sun exposure are worth the risk – this was not tested by the researchers and remains unproven. This was a very small study that has not shown what effects sun exposure would have on people with high blood pressure.
However, in fairness to the UK media, it appears that the tone of their reporting has been influenced by comments made by one of the lead authors of the study, Professor Martin Feelisch. Professor Feelisch is quoted in the Daily Express as saying that, "We think the benefits of exposure to the sun far outweigh the risks from skin cancer."
If accurately reported, this is a sweeping statement to make based on a small study that involved people who did not have high blood pressure and with no significant follow-up period.
What kind of research was this?
This was a small laboratory study on humans that investigated whether ultraviolet A (UVA) radiation can lower blood pressure, and whether it does this by causing nitric oxide to be released from stores in the skin.
Larger studies with a longer follow-up would be required to see if UVA irradiation lowers cardiovascular mortality, and what the ideal exposure would be to balance the potential benefits of sun exposure with the risks of skin cancer.
What did the research involve?
To measure the effect of UVA light on blood pressure, 24 healthy people (18 men and six women) without high blood pressure were studied.
While wearing only their underwear, the participants lay on their backs and were exposed to a UVA lamp.
Initially, an aluminium foil blanket was placed over them so that the UVA rays could not reach them, but still allowing their skin and body temperature to increase. The researchers referred to this as "sham irradiation".
The use of sham irradiation was to check whether exposure to heat or UVA was responsible for changes in blood pressure. The participants were monitored for 60 minutes.
They were then exposed to "active UVA radiation", where they were exposed to UVA light for 22 minutes without an aluminium foil blanket. This amount of UVA was described as being equivalent to the amount of natural sunlight exposure that you would get spending 30 minutes under the midday sun in southern Europe in a T-shirt and shorts.
The researchers measured blood pressure and heart rate every 10 minutes, and blood was taken every 20 minutes to measure the level of nitric oxide metabolites.
The researchers then investigated the effect of UVA light on blood flow in 12 healthy men. Blood flow was measured in the forearm after "sham" or "active" irradiation in a randomised cross-over study.
The researchers also looked at human skin under a microscope.
What were the basic results?
The researchers found that active UVA irradiation caused decreases in mean arterial pressure and diastolic blood pressure, and increased heart rate.
UVA irradiation caused the levels of nitrite to increase and the levels of nitrate to decrease. While spelt similarly, nitrite and nitrates are two different substances – a rise in the former and a decrease in the latter corresponds to an overall increase in nitric oxide blood levels. The change in blood pressure seen was the same when participants were put on a low- or high-nitrate diet.
UVA irradiation of the forearm caused increased blood flow. This was seen even when the enzyme nitric oxide synthase was inhibited – this is an enzyme the body uses to make fresh nitric oxide. Researchers interpret this as meaning that the pre-formed nitric oxide stores in the skin were being used.
Microscope studies confirmed that nitric oxide release occurs independent of the nitric oxide synthase enzyme, more nitric oxide is released with more UVA exposure, and the majority of the light-sensitive nitric oxide pool is in the upper epidermis (outer layer of the skin).
How did the researchers interpret the results?
The researchers said: "Our data provide mechanistic insights into an important function of the skin in modulating systemic [nitric oxide] bioavailability, which may account for the latitudinal and seasonal variations of [blood pressure] and cardiovascular disease."
Conclusion
This laboratory study has found that UVA exposure caused a short-term drop in blood pressure and an increase in blood flow. The researchers believe that this is as a result of nitric oxide being released from the skin into the bloodstream.
However, the researchers' assertion that this study "has significant public health implications" is open to question. Such a bold statement should probably only be made if further controlled studies are carried out to see whether UVA exposure can reduce the risk of death as a result of cardiovascular disease, and whether UVA exposure can reduce blood pressure in people with hypertension (all the people in this study had normal blood pressure). It would be important to assess what the optimum amount of sunlight exposure is to balance out the well-documented risks of skin cancer.
It would also be imperative to see if similar results are achieved when sunscreen is applied to the skin. If so, that would be a win-win – all the benefits without any of the risk.
Currently, the official advice regarding the prevention of high blood pressure and safe sunlight exposure remains unchanged.
