Apparent Anti Cancer Benefits of UV

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Among the vast array of literature recently published on the possible benefits of UV exposure for preventing or reducing the progression of a multitude of cancers are two reviews of special note written by Giovannucci (2005) and Grant and Holick (2005) on the effects of vitamin D, and a report stating that greater exposure actually increases survival among melanoma victims (Berwick et al., 2005; Egan et al., 2005). It has been observed that malignant melanoma is often accompanied by low 1,25-dihydroxyvitamin D3 (the active form of vitamin D) serum levels (Egan et al., 2005). A study by Smedby et al. (2005) found reduced risk of non-Hodgkin lymphoma in individuals with high exposure to sun.

Although the benefits of sun exposure for cancer reduction were mentioned in Apperly (1941) and Peller and Stephenson (1937), the school of thought that low exposure to UV-B is associated with an increased risk of non-cutaneous cancers apparently began in earnest with Garland and Garland (1980), who suggested that vitamin D reduces the risk of colon cancer. Papers that suggest sunlight reduces the risk for breast cancer are Gorham et al. (1989), Garland et al. (1990), Gorham et al. (1990), and Grant (2002a); papers indicating a reduced risk for other non-cutaneous cancers were written by Grant (2002b, c; 2003) and Lefkowitz and Garland (1994). Grant (2002b, 2003) found inverse associations between regional summer UV-B irradiance and cancer risk for 14 kinds of cancer. Table 12.1 lists current findings and associated references on the effects of UV-B exposure and vitamin D, and the risks of acquiring, and mortality rates for, 15 different cancers. A few studies made direct associations between vitamin D levels of individuals and the risk of cancer, some of which are shown in the last column. These reports are increasingly noted in the popular press, e.g., Newsweek (Cowley, 1991; Raymond and Adler, 2005) and Reader's Digest (Dranov, 2006).

A difficulty in the epidemiology is that there are many possible confounding effects relating to cancer incidence, and the total list of these influences can usually not be included in an analysis. For example, there is a decided trend of increased incidence of breast cancer from south to north in the US (Garland et al., 1990; Sturgeon et al., 1995). This trend has been interpreted as being caused by decreased vitamin D production with the lower UV-B exposure in the north (Garland et al., 1990). However, according to Sturgeon et al. (1995), most of the geographic variations in breast cancer can be explained by demographic patterns. For example, women residing in the southern areas of the U.S. typically give birth to their first child at a younger age, and their breast cancer mortality rate is lower. However, the study by Sturgeon et al. (1995) did not examine environmental factors such as UV exposure. Giovannucci (2005) cited examples that suggest an inverse relationship between sun exposure and breast cancer, but did not mention incidence of first childbirth at a younger age in the south nor the 1995 Sturgeon et al. paper. Until recently, the potential effect of vitamin D on cancer was often, perhaps usually, not included in cancer epidemiological studies (Giovannucci, 2005).

Although an apparent vitamin D effect in reducing cancer risk and virulence has been deduced for a large range of cancers, individual cancers differ in their response to vitamin D. For example, the relationship between vitamin D and prostate cancer is mixed (Giovannucci, 2005), though there does appear to be a positive relationship between very low 1,25(OH)2 D (the active form of vitamin D) and a higher risk and greater progression of prostate cancer (Giovannucci, 2005).

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