It has been suggested that soy food and isoflavone intake may be protective against the risk of colorectal cancer. However, epidemiologic evidence remains sparse and inconsistent. We addressed this issue in the Fukuoka Colorectal Cancer Study.
The study subjects were the 816 incident cases of histologically confirmed colorectal cancer and 815 community controls. Intakes of soy foods and isoflavones were assessed by in-person interview using a computer-assisted dietary method. Logistic regression analysis was applied to estimate odds ratio (OR) and 95% confidence interval (CI) of colorectal cancer with adjustment for dietary intakes of calcium and n-3 polyunsaturated fatty acids as well as for body mass index, physical activity, alcohol use, and other lifestyle factors.
Energy-adjusted intakes of soy foods (dry weight) and isoflavones were inversely associated with colorectal cancer risk in men and postmenopausal women, but not in premenopausal women. The multivariate-adjusted OR for the highest versus lowest quintile was 0.65 (95% CI 0.41-1.03, p for trend = 0.03) for soy foods and 0.68 (95% CI 0.42-1.10, p for trend = 0.051) for isoflavones in men. The corresponding values for postmenopausal women were 0.60 (95% CI 0.29-1.25, p for trend = 0.053) and 0.68 (95% CI 0.33-1.40, p for trend = 0.049). The site-specific analysis showed inverse associations of soy foods (p for trend = 0.007) and isoflavones (p for trend = 0.02) with rectal cancer in men.
The findings add to epidemiologic evidence for protective effects of soy foods and isoflavones in colorectal carcinogenesis.