Ultraviolet Radiation

UVB

UVB has two primary effects: stimulating vitamin D production and inducing melanogenesis (tanning). Observational studies of UVB dose and geography have linked solar UVB to reduced risk for about 20 types of cancer. Although some researchers have criticized the observational studies using UVB doses related to geography, such studies have been supported by those in which nonmelanoma skin cancer (NMSC) incidence or mortality rates were used as the index of individual or population UVB irradiance. In an ecological study of cancer mortality rates in Spain, NMSC mortality rates for 1978–93 were inversely correlated with 17 types of cancer, including cutaneous malignant melanoma (CMM). A meta-analysis of diagnosis of a second cancer after diagnosis of NMSC found that there was a reduced risk for half a dozen types [Page 1701]of cancer as long as smoking was included in the analysis.

UVB and vitamin D are also beneficial in increasing survival once cancer reaches a detectable stage. Studies in Norway, Massachusetts, and England found that patients diagnosed with several types of common cancers in summer or fall had higher intermediate-term survival rates than those diagnosed in winter or spring.

The mechanisms whereby vitamin D reduces cancer risk are well known. They include increased apoptosis (programmed cell death) of defective cells and increased cellular differentiation, attenuation of growth-inducing signals, absorption and metabolism of calcium, and reduction of angiogenesis and metastasis. Vitamin D improves immune system response to viral infections that are commonly associated with diseases in winter, and because viruses are implicated in several cancers, this effect appears to be another mechanism for cancer risk reduction. Dose–response relations for serum 25-hydroxyvi-tamin D3 (calcidiol) level have been developed for breast and colorectal cancer through meta-analy-ses of observational studies, with the finding that it takes about 1,500 international units (IU) of vitamin D3 per day to reduce the risk of colorectal cancer by 50 percent, but 4,000 IU per day for breast cancer. A study of male health professionals at Harvard University estimated a 29 percent reduction in male cancer deaths for 1,500 IU of vitamin D3 per day. Studies have also suggested that blacks have lower cancer survival rates than whites, all things being equal for cancer stage at discovery and treatment level, because blacks have lower calcidiol levels due to darker skin and lower oral intake of vitamin D3.

There are, however, several problems with the UVB/vitamin D/cancer theory. For example, prediagnostic calcidiol levels have little correlation with later diagnosis of prostate cancer, yet the geographic variations of prostate cancer mortality rates are consistent with a beneficial role of solar UVB, and frequent sunburning in youth is strongly correlated with reduced risk. Vitamin D has been found to enhance the immune system response to both bacterial and viral infections, and infectious diseases are most common in winter. Because the geographic variation of prostate cancer mortality rates in the United States appears to be related more to wintertime than to summertime solar UVB, it may be that prostate cancer is related largely to infectious diseases in youth. Other cancers with both some risk from viruses and a pronounced latitudinal gradient in mortality rate in the United States are bladder, gastric, ovarian, testicular, and thyroid cancer, as well as Hodgkin's lymphoma.

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