Nutritional and Socioeconomic Factors in Relation to Prostate Cancer Mortality: a Cross-National Study

Background: Large international variations in rates of prostate cancer incidence and mortality suggest that environmental factors have a strong influence on the development of this disease. The purpose of this study was to identify predictive variables for prostate cancer mortality in data from 59 countries. Methods: Data on prostate cancer mortality, food consumption, tobacco use, socioeconomic factors, reproductive factors, and health indicators were obtained from United Nations sources. Linear regression models were fit to these data. The influence of each variable fit in the regression models was assessed by multiplying the regression coefficient b by the 75th (X75) and 25th (X25) percentile values of the variable. The difference, bX75 ˛ bX25, is the estimated effect of the variable across its interquartile range on mortality rates measured as deaths per 100 000 males aged 45‐74 years. Reported P values are two-sided. Results: Prostate cancer mortality was inversely associated with estimated consumption of cereals (bX75 ˛ bX25 = ˛7.31 deaths; P = .001), nuts and oilseeds (bX75 ˛ bX25 = ˛1.72 deaths; P = .003), and fish (bX75 ˛ bX25 = ˛1.47 deaths; P = .001). In the 42 countries for which we had appropriate data, soy products were found to be significantly protective (P = .0001), with an effect size per kilocalorie at least four times as large as that of any other dietary factor. Besides variables related to diet, we observed an association between prostate cancer mortality rates and a composite of other health-related, sanitation, and economic variables (P = .003). Conclusions: The specific food-related results from this study are consistent with previous information and support the current dietary guidelines and hypothesis that grains, cereals, and nuts are protective against prostate cancer. The findings also provide a rationale for future study of soy products in prostate cancer prevention trials. [J Natl Cancer Inst 1998;90:1637‐47] Prostate cancer is the most prevalent cancer among men and represents a large and growing health problem in the United States and other Western countries. Incidence and mortality rates vary widely across populations, with the highest rates in North America and northern Europe, intermediate rates in southern Europe and Latin America, and the lowest rates in Asia and Africa (1,2). Incidence increases markedly with age. Ecologic studies investigating the health experiences of migrants over time (3‐6) and cross-national comparisons (7‐10) suggest that environmental, rather than genetic, factors are responsible for large international variations in rates. Against the background of international variation, increases in prostate cancer mortality and incidence rates have been reported in the last three decades in the United States, Asia, South America, and central Europe. Prostate cancer incidence in the United States is currently increasing at a rate nearly three times as high as that of female breast cancer (11). The incidence of prostate cancer is extremely low in populations of western Africa, but in African-Americans, who originate from the same regions of Africa, the incidence is nearly 50% higher than that of U.S. whites (12), indicating a potentially strong environmental influence on the disease. Similar to epidemiologic studies of diet and breast cancer, studies of diet and prostate cancer have produced equivocal results (13). Ecologic studies and laboratory animal experiments tend to produce results strongly consistent with dietary hypotheses that propose a role for fat and animal product intake, whereas data from analytic epidemiologic studies are less consistent (8,9,14‐19). Possible explanations include the fact that data on diet in humans are derived from self-reports that are subject to measurement error and a variety of biases (20) and that the range of dietary fat intake (e.g., grams per person per day or as a percent of dietary energy) within particular study populations tends to be very narrow (21), hampering efforts to detect dietary effects. Also, the effect of particular dietary constituents may be modified by other factors, such as the antagonism of the effect of fat by fiber (e.g., through its influence on enterohepatic circulation) (22). This study was conducted to identify major predictors of prostate cancer mortality by use of data from a total of 59 countries on which we had complete data on most covariates of interest. In addition to a wide range of nutritional predictors (including alcohol), we examined the significance of estimated per capita tobacco consumption, a variety of indicators of socioeconomic status (SES), and reproductive factors.