Life comes down to a struggle between risk and benefit. Although not always consciously, we evaluate our diet, our cosmetics, our medications, household chemicals and activity levels on the basis of whether they are good or bad for us. Mention sun exposure, and the conflict comes down to the 鈥渂ad,鈥� namely skin cancer, and the 鈥済ood,鈥� usually ascribed to the enhanced production of vitamin D. Skin cancer is bad, but why is vitamin D good? For one, it is required for the proper absorption of calcium and a lack can lead to soft bones, in extreme cases to the characteristic bow legs of rickets. But there may be more to vitamin D.
Epidemiological investigations have shown that people with high blood levels are generally healthier, particularly when it comes to cardiovascular disease. It is interesting to note that the heart disease rate in Australia is lower than in northern climates, and that in Britain the risk increases as one travels north even when lifestyle factors are taken in to account. But here is a curiosity. Numerous studies have been carried out with vitamin D supplements without noting an effect on cardiovascular disease. Could it be that high blood levels of vitamin D are just a marker for sun exposure and that the cardiovascular benefits are actually due to some other feature of sunlight? British dermatologist Richard Weller makes a case for nitric oxide, a chemical that can be released by the action of ultraviolet light on substances such as nitrates that are stored in the skin.
Nitric oxide is a gas, and lasts only a few seconds after it is produced in the inner lining of blood vessels by the action of enzymes on the amino acid arginine. During its brief existence, though, it acts as an important signalling molecule causing smooth muscles around blood vessels to relax. This results in an increased blood flow and a lowering of blood pressure. Indeed, the classic drug to treat angina, nitroglycerine, works by releasing nitric oxide, and Viagra鈥檚 performance is due to its ability to increase signalling and improve blood flow through the nitric oxide pathway.
It was back in 1996 that Weller discovered that sunlight had the ability to convert nitrates in the skin to nitric oxide, a discovery that took on greater meaning with the awarding of the 1998 Nobel Prize in Physiology or Medicine to Robert F. Furchgott, Louis J. Ignarro and Ferid Murad 鈥渇or their discoveries concerning nitric oxide as a signalling molecule in the cardiovascular system.鈥� Weller began to wonder whether nitric oxide produced by sun exposure might explain the lower blood pressure in populations living closer to the equator and why the average blood pressure in the U.K. is lower in summer than in winter.
To investigate, Weller exposed volunteers to ultraviolet light and measured blood pressure and nitric oxide levels. To eliminate the possibility of vitamin D playing a role, he used long wavelength UVA that does not produce vitamin D. Nitric oxide levels increased and blood pressure decreased with an exposure equivalent to about thirty minutes of sunshine in Edinburgh in the summer. The effect wasn鈥檛 dramatic, but could be significant in a large population. Just heating the skin had no effect, so UVA is needed to produce nitric oxide.
In another experiment, the performance of cyclists was enhanced in response to irradiation with UVA but only if they took nitrate supplements. The theory is that increased nitric oxide release dilates blood vessels and allows more oxygen to reach the muscles. Nitrates occur naturally, and are particularly high in celery, red beet root, lettuce and spinach, vegetables that have been associated with lower blood pressure. In light of Weller鈥檚 studies, this may be a consequence of the combination of nitrates and exposure to sunlight. He also identifies studies that showed Scandinavian women who spent more time sunbathing lived longer. Of course that may also have been due to less stress, different diets or activity levels. And then there is a Danish study that showed that people with non-melanoma skin cancer were much less likely to have a heart attack. Could skin cancer be a marker for a longer life?
As I said, life is a struggle between the good and the bad, but it isn鈥檛 easy to identify what is good and what is bad. Maybe someone should investigate if nudists live longer.
