Background: Although isoflavones, such as those found in soy, have been shown to inhibit breast cancer in laboratory studies, associations between consumption of isoflavone-containing foods and breast cancer risk have been inconsistent in epidemiologic studies. We evaluated the relationship between isoflavone consumption and breast cancer risk among women in the Japan Public Health Center-Based Prospective Study on Cancer and Cardiovascular Diseases (JPHC Study).

Methods: In January 1990, 21 852 Japanese female residents (aged 40-59 years) from four public health center areas completed a self-administered questionnaire, which included items about the frequency of soy consumption. Through December 1999 and 209 354 person-years of follow-up, 179 women were diagnosed with breast cancer. Cox proportional hazards regression was used to estimate the relative risks (RRs) and 95% confidence intervals (CIs) for breast cancer in relation to consumption of miso soup, soyfoods, and estimated isoflavones. All statistical tests were two-sided.

Results: Consumption of miso soup and isoflavones, but not of soyfoods, was inversely associated with the risk of breast cancer. The associations did not change substantially after adjustment for potential confounders, including reproductive history, family history, smoking, and other dietary factors. Compared with those in the lowest quartile of isoflavone intake, the adjusted RRs for breast cancer for women in the second, third, and highest quartiles were 0.76 (95% CI = 0.47 to 1.2), 0.90 (95% CI = 0.56 to 1.5), and 0.46 (95% CI = 0.25 to 0.84), respectively (P(trend) =.043). The inverse association was stronger in postmenopausal women (P(trend) =.006).

Conclusion: In a population-based, prospective cohort study in Japan, frequent miso soup and isoflavone consumption was associated with a reduced risk of breast cancer.