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Sun exposure may protect against a number of cancers, according to research based on a study of more than four million people in 11 countries.
Vitamin D produced in the skin as a result of exposure seems to reduce the risk of several solid cancers, especially stomach, colorectal, liver and gallbladder, pancreas, lung, female breast, prostate, bladder, and kidney cancers, say the authors (European Journal of Cancer 2007 May 30 doi: 10.1016/j.ejca.2007.04.018).
The researchers, who used registered cases of skin cancer as markers of sun exposure, found that the greatest protective effect was after non-melanoma skin cancer, which is associated with cumulative sun exposure, rather than melanoma, which is more related to sunburn.
The authors say that the protective role of the sun was prominent only in sunny countries: “The overall risk of all second solid primary cancers after any skin cancer was significantly decreased in sunny countries, whereas it was significantly increased in less sunny countries. This is the first report on such a finding, which needs a large study population,” say the authors from 13 cancer registries in Australia, Canada, Denmark, Finland, Iceland, Norway, Scotland, Singapore, Slovenia, Spain, and Sweden. Australia, Singapore, and Spain were deemed sunny.
The authors looked at 416134 cases of skin cancer and 3776501 cases of non-skin cancer as a first cancer. They looked at the number of second primary cancers after skin melanoma, basal cell carcinoma, or non-basal cell carcinoma and at skin cancers after non-skin cancers. They compared these to the expected numbers of solid cancers, using standardised incidence ratios—the ratio of observed to expected numbers—for each malignancy.
The standardised incidence ratios after skin melanoma for all second solid primary cancers, with the exception of skin and lip, were significantly lower for the sunny countries. The largest differences were after non-melanoma skin cancers. The quotient of the standardised incidence ratio for the sunny countries divided by the ratio for the less sunny countries for basal cell carcinoma was 0.65 (95% confidence interval 0.58 to 0.72) and for non-basal cell carcinoma was 0.58 (0.50 to 0.67). The standardised incidence ratio for melanoma in sunny countries was 1.03 (0.99 to 1.08), significantly lower than in the 1.14 (1.11 to 1.17) in less sunny countries.
A surprise finding was that the risk of internal solid cancers after the primary skin cancers was lower in the sunny countries but not in less sunny countries, the authors say.
“Assuming that cumulative sun exposure plays a major role in the development of non-melanoma skin cancers, it is difficult to understand why there is such a significant difference between sunny and less sunny countries,” they say.
One explanation may be that seasonal variations in the effective ultraviolet exposures lead to changes in serum vitamin D concentrations in less sunny countries: “It has been speculated that the seasonal variation in serum vitamin D might be important in the development of diseases related to vitamin D insufficiency. Our results support this idea,” they say.