We explored the postulated association between soy foods and colorectal cancer incidence by analyzing 13 epidemiological studies: 3 ecological, 1 cohort, and 9 case control. Seven case-control studies evaluated the association between soy intake and colon or colorectal cancer (2,008 cases). Point estimates generally suggest an inverse association between higher soy consumption and colon cancer onset, although nearly all of the confidence intervals overlap 1.0. Two of the nine case-control studies focused on adenomas as the outcome (675 total cases), and results for these studies also showed inverse associations. Of the six case-control studies that evaluated the association between soy consumption and rectal cancer (732 cases), the point estimates generally suggest an inverse association with unfermented soy consumption and rectal cancer onset but not fermented soy products. These studies have many limitations, particularly with regard to dietary measurement issues, such as incomplete assessment of soy intake, inadequate quantification, and inappropriate time period for cancer prevention as well as inadequate adjustment for confounders. Most of these issues would contribute to underestimations of any association. In spite of the methodological issues, the available evidence is compelling enough to warrant further study utilizing stronger methodology.
