A meta-analysis of soyfoods and risk of stomach cancer: the problem of potential confounders.

It has been suggested that consumption of soyfoods may be associated with a reduction in risk of various cancers, including nonhormonally dependent cancers. The purpose of this meta-analysis was to examine the relationship between fermented and nonfermented soyfoods and risk of stomach cancer. We searched the reference lists of English language publications of diet and stomach cancer studies that were conducted in Asia or among Asians living in the United States or elsewhere between 1966 and 1999. All of the analytic epidemiological studies that obtained individual data on intake of soyfoods and presented risk estimates of the association between intake of soyfoods and risk of stomach cancer were identified and included in this review. Our pooled analysis of 14 studies with data on fermented soyfoods yielded an odds ratio/relative risk of 1.26 (95% confidence interval, 1.11-1.43) in association with high intake of such foods. In contrast, our pooled analysis of 10 studies with data on nonfermented soyfoods found an odds ratio/relative risk of 0.72 (95% confidence interval, 0.63-0.82) in association with high intake of these foods. However, further analyses suggest that fermented and nonfermented soyfoods may be associated with salt and fruit/vegetable intake, respectively; salt and fruit/vegetable intake are directly associated with stomach cancer risk. In almost all of the studies we reviewed, the possible confounding role of salt, fruit/vegetable, and other dietary factors had not been considered in the soyfood analyses. In conclusion, the role of soyfoods in the etiology of stomach cancer cannot be determined with confidence until the roles of potential confounders, including salt, fruit/vegetables, and other dietary factors, are more adequately adjusted for.