Tuesday, January 15, 2008

The Sun: Balancing Risks and Benefits

Few things in life fall neatly into the category of "good" or "bad" for our well being. It is often the case that something can be beneficial in one respect (e.g. enhance our mood), but it might also be potentially harmful in another respect (e.g. high in calories, etc.).

Take something as simple as physical exercise. Recall, for example, my earlier post about the benefits of physical exercise. But there is some risk of harm associated with physical exercise, especially contact sports (e.g. sprains and strains, stress fractures, etc.). Determining what constitutes a sensible course of action requires us to responsibly pursue these benefits, so that we minimize, where possible, these risks of harm (though we cannot eliminate them completely).

Another obvious example are medically prescribed drugs, for they could remedy one's alignment but it is also possible that they could cause an adverse reaction (thus creating more problems for us). But when the likelihood of the benefits far outweigh the potential harms, it is sensible to pursue them. Much of our day-to-day activity involves such judgements (though we not even consciously think of things in these terms). The decisions we make concerning getting to work (via car, bike, etc.) has potential benefits and harms. Ditto for the food we consume.

And so life involves, for us as both individuals and collectively as societies, a complex balancing exercise. And thus it is always prudent, when faced with an argument that focuses exclusively on one purported harm or benefit, to step back and adopt the "big picture" perspective concerning the potential benefits and harms, and strive for some proportionality. And so reflecting on what constitutes responsible "risk management" should increase the likelihood that our lives will, on balance, go better.

Today I came across a great example that illustrates the complexities of potential harms and benefits- exposure to the sun. "Addressing the health benefits and risks, involving vitamin D or skin cancer, of increased sun exposure" by Johan Moan et. al. is published in the latest issue of PNAS. Here is the abstract:

Solar radiation is the main cause of skin cancers. However, it also is a main source of vitamin D for humans. Because the optimal status of vitamin D protects against internal cancers and a number of other diseases, a controversy exists: Will increased sun exposure lead to net health benefits or risks? We calculated the relative yield of vitamin D photosynthesis as a function of latitude with a radiative transfer model and cylinder geometry for the human skin surface. The annual yield of vitamin D is 3.4 and 4.8 times larger below the equator than in the U.K. and Scandinavia, respectively. In populations with similar skin types, there are clear latitude gradients of all major forms of skin cancer, indicating a north–south gradient in real sun exposure. Surprisingly, the incidence rates of major internal cancers also increase from north to south. However, the survival prognosis also improves significantly from north to south. Reasons for these findings are discussed in view of the role of vitamin D. In Norway, melanoma rates increased by a factor of 6 from 1960 to 1990, while the prognosis improved in the same period. After 1990, melanoma rates have remained constant or even decreased in age groups <50 years, whereas the prognosis has not improved further. These data, together with those for internal cancers and the beneficial effects of an optimal vitamin D status, indicate that increased sun exposure may lead to improved cancer prognosis and, possibly, give more positive than adverse health effects

And here is a sample from the article:

There is a controversy as to whether increased sun exposure to Western populations would prolong or shorten lifetime expectancy, result in fewer or more cancer deaths, and, in general, lead to health benefits or risks (1, 2). For years, emphasis has been placed on the increasing time trends of incidence and mortality rates of cutaneous malignant melanoma (CMM) (3, 4) and, in contrast, on the protective role of vitamin D regarding many types of internal cancer and other diseases (5–7). Too much sun exposure has been blamed for the high and increasing incidence rates of CMM. However, solar radiation is a major, if not the main, source of vitamin D in humans. Therefore, a population's increased sun exposure leads to improved vitamin D status. The observation that the incidence and mortality of several types of internal cancers decreases with decreasing latitude in the United States and other countries initiated the research on vitamin D–cancer relationships in the 1980s and 1990s. However, in some cases, there is an inverse gradient of the rates of internal cancer with latitude (1), with the rates being higher in regions with high annual UV fluences (New Zealand and Australia) than in countries with low annual UV fluences (Northern Europe, Scandinavia, and the U.K.), despite the fact that the populations of these regions are closely related genetically or, at least, have similar skin types, which is important for the photosynthesis of vitamin D.

These issues have health consequences far beyond those of cancer because a number of diseases are associated with inadequate vitamin D levels or low sun exposure: neurological, cardiovascular, metabolic, immune, and bone diseases (2, 7). Evolutionary arguments involving skin color also should be taken into account. A dark skin color is found among Africans and, possibly, early hominids who lived close to the equator (8). This pigment may protect against skin cancer and folate photodegradation (8, 9). A white skin color developed later in our history, as humans left Africa and went north. Because dark skin needs about 6 times higher solar exposure for vitamin D photosynthesis than white skin (10, 11) and because the fluence rate of vitamin D-generating solar radiation decreases with increasing latitude (Fig. 1A), one can argue that skin whitening may be related to the need for vitamin D and the lack of sunshine at high latitudes.

....So far, epidemiological data for cancer argue for an overall positive role of sun-induced vitamin D. There may be more beneficial than adverse effects of moderately increased sun exposure, even for total cancer mortality (65). This message should be addressed to populations at risk for vitamin D deficiency. Trends need to be closely followed in the future. In view of the supposedly long latency times for cancer manifestation, decades are needed for final evaluation of the impacts of the antisun campaigns with respect to melanoma incidence, cancer prognosis, and other possible positive or adverse health effects. Authorities should pay attention not only to skin cancer research, but also to research on vitamin D–sun–health relationships occurring worldwide.