Background: Studies finding an inverse correlation of ambient solar irradiance with cancer mortality were the first to suggest that sun exposure and probably, therefore, vitamin D might protect against some cancers. Such correlation has been shown in Asian populations in some studies. We analyzed the correlation between mortality and incidence from a number of cancers and ambient solar ultraviolet (UV) B irradiance in China.
Methods: Cancer mortality data were obtained from the Second National Death Survey conducted in a sample of 263 counties in China from 1990 to 1992. National cancer registration data 1998-2002 in China were used for estimation of cancer incidence. Satellite measurements of cloud-adjusted ambient UVB intensity at 305 nm were obtained from a NASA database and GIS methods used to estimate the average daily irradiance for the 263 counties in 1990. We estimated cancer mortality rate ratios per 10 mW/(nm m(2)) change in UVB by fitting a negative binomial regression model with mortality as the response variable and UVB as the independent variable, adjusted for sex, age, and urban or rural area.
Results: Mortality rates for all cancers and cancers of the esophagus, stomach, colon and rectum, liver, lung, breast, and bladder were inversely correlated with ambient UVB. This correlation was present in men and women and rural residents for all these cancers but not urban residents for cancers of the esophagus, colon and rectum and liver. Lung cancer mortality showed the strongest inverse correlation with an estimated 12% fall per 10 mW/(nm m(2)) increase in UVB irradiance even if adjusted for smoking. Only incidence rates for cancers of the esophagus, stomach, colon and rectum and cervix were inversely correlated with ambient UVB. Mortality and incidence from nasopharyngeal cancer increased with increasing UVB [respectively 27 and 12% per mW/(nm m(2))]. Mortality from cancer of the cervix also increased, but to a lesser extent and mortality from leukemia was not consistently correlated with UVB irradiance.
Conclusion: Mortality from all cancers together and most major cancers in China was inversely associated with solar UVB. These associations were similar to those observed in a number of populations of European origin. Incidence of some cancer types had the same correlation with UVB. They suggest the possibility that vitamin D may reduce the incidence or improve the outcome of cancer in Chinese people.