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One of the persistent puzzles of cardiovascular medicine has been finding the reasons why high blood pressure, heart attacks, and strokes are more common in winter, and more common the farther one travels away from the equator. High blood pressure and heart attacks are much more common in Scotland than in Spain, and in Finland than in Greece. Even when people eat healthy diets high rates of high blood pressure and cardiovascular disease persist in cold, dark, damp parts of the world, leading researchers at the University of Southampton in England working with scientists at the University of Edinburgh in Scotland to investigate a possible connection between sunshine and blood pressure.
Twenty-four healthy volunteers were recruited to participate in the study. They were asked to submit to two different sessions in which they received enough UVA (ultraviolet A) light on their forearms from tanning lamps used for 20 minutes at a time to cause a mild sunburn. (The purpose of the dosage was not to induce sunburn, but to be sure the volunteers received enough light to have an effect.)
The researchers ran two different sessions to determine whether light or heat might be lowering blood pressure. In one session, the volunteers were exposed to both the UVA radiation and the heat from the lamps. In the other, the volunteers were exposed to strong light containing UVA rays but not the heat from the lamp. The researchers measured their blood pressure before and after exposure to light, and took skin samples to measure nitrates, nitric oxide (a free radical made from nitrates that dilates arteries and lowers blood pressure), and vitamin D.
The researchers found that:
- Exposure to strong light containing UVA radiation lowered blood pressure, with or without heat.
- Vitamin D levels not change during the UVA treatment sessions and so were not causing the lowering of blood pressure.
- Nitric oxide levels rose in the skin of the arm without a corresponding increase in the activity of an enzyme that makes it, that is, shining intense UVA light on the skin released this artery-opening chemical from already-existing stores.
This unexpected effect of sunlight could explain why people who live in cold, damp, climates in countries closer to the poles have higher rates of hypertension, myocardial infarction (heart attack), and cerebrovascular accidents, including brain aneurysms and strokes. The authors of the study recognize that it is important to avoid sunburn to minimize the risk of skin cancer, but some exposure to the UVA rays of the sun is necessary not only for the body's production of vitamin D but also for the maintenance of healthy blood pressure. Spending time in the sun, whether at work or play or simply sunbathing at the beach, or daily exposure to a sun lamp, at levels that do not, unlike this experiment, cause burning of the skin, may be essential to cardiovascular health.