* *
*By William B. Grant, Ph.D.
SUNARC <http://www.sunarc.org>*
With all of the publicity that UV radiation (UVR) is an important cause of skin cancer, premature skin aging and cataract formation, one might think that avoidance of UVR would be the best policy. Not so fast. If protection against UVR were the most important thing, all humans would have very dark skin, since the melanin in dark skin protects against skin cancer and premature skin aging.
Skin pigmentation becomes paler the closer one's ancestors lived to the polar-regions, evidently to balance cutaneous production of vitamin D with protection against free radicals and DNA damage from UVR [Jablonski and Chaplin, 2000]. In addition, even a cursory look at the geographic variation in cancer mortality rates in the United States [Devesa et al., 1999] indicates that some environmental factor has to explain why mortality rates for a number of internal cancers are approximately twice as high in northeastern, highly-urbanized states than in southwestern, more rural states.
Diet and smoking are, of course, important risk factors for many types of cancer [Doll and Peto, 1981]. But in order for diet to explain the geographic variation in cancer rates, Americans would need to be eating drastically different diets by region. However, anyone who has travelled throughout the United States knows that the food choices do not vary much anywhere in the contiguous 48 states.
*The Risk of Cancer Lessens With More Sun Exposure *
The key to understanding this geographic pattern was provided by Cedric and Frank Garland in 1980 [ Garland and Garland , 1980]. They reasoned that sunlight, through the production of vitamin D, reduced the risk of colon cancer in the sunny areas compared to that in the darker areas. They performed an ecologic study of annual solar irradiance versus colon cancer mortality rates and found a strong inverse correlation, i.e. the more sunlight, the less cancer. (An ecologic study treats entire populations defined geographically as entities, with values for disease outcome and environmental or dietary factors averaged for each entity.)
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