The present Chinese population study was designed to investigate the association between soy consumption and survival from breast cancer. Study participants included 5,042 women in the Shanghai Breast Cancer Survival Study aged 20 to 75 years and diagnosed during the period March 2002 to April 2006. The women were followed until June 2009. Personal interviews were undertaken approximately six months after diagnosis to collect participant information concerning cancer diagnosis and treatment, lifestyle factors (including diet) since cancer diagnosis, and disease progression. The information was reassessed at three follow-up interviews which took place at 18, 36, and 60 months. Linkage with the Shanghai Vital Statistics Registry database was used to obtain survival information for study participants lost to follow-up. In addition, medical records were reviewed to verify breast cancer diagnosis and treatment information. Cox regression analysis, with adjustment for known clinical predictors and other lifestyle factors, was used to estimate associations. Intake of soy food was treated as a time-dependent variable. During a median follow-up period of 3.9 years (range = 0.5 to 6.2 years), 444 deaths and 534 recurrences or breast cancer-related deaths occurred in the 5,033 survivors who had been surgically treated. Soy food intake, as measured by either soy protein or soy isoflavone intake, was found to be inversely associated with death and recurrence. The hazard ratio (HR) associated with the highest fourth of soy protein intake was 0.71 (95% confidence interval (CI) = 0.54 - 0.92) for total mortality and 0.68 (95% CI = 0.54 - 0.87) for recurrence compared to the lowest quartile of intake. Multivariate-adjusted four-year mortality was 10.3% and the recurrence rate was 11.2% for participants in the lowest quartile of soy protein consumption compared to 7.4% mortality and 8.0% recurrence for those in the highest quartiles of soy protein consumption. The inverse association was found among participants with both estrogen receptor positive (ER+) and estrogen receptor negative (ER-) tumors and among both users and nonusers of tamoxifen. The authors concluded that among women with breast cancer, soy food consumption was significantly associated with decreased risk of death and recurrence.

The present Korean case-control study was designed to examine the associations between consumption of soy foods and breast cancer risk. A total of 1.399 newly-diagnosed breast cancer cases were recruited in the period 1998 to 2004 from a Seoul university hospital. The study also included 938 cancer-free controls selected from seven different clinical departments at the same time in the same hospital. The women were interviewed in detail concerning their diets and life-style factors. Multivariate odds ratios (mOR) and 95% confidence intervals (CI) concerning daily intakes of soybean, tofu, and soybean paste were calculated using unconditional logistic regression. Adjustment was made for known risk factors, including age, education, family history of breast cancer, history of benign breast diseases, smoking, alcohol intake, height, body mass index, number of full term deliveries, age at menarche, menopausal status, and use of hormone replacement therapy. Compared to those in the lowest fifth (Q1), women in the highest quintile (Q5) of soybean intake had lower odds of having the breast cancer (mOR = 0.70, 95% CI = 0.54-0.89). High tofu consumption also was found to have an inverse association with risk of breast cancer (Q5 vs. Q1; mOR = 0.67, 95% CI = 0.51-0.89). On the other hand, high soybean paste consumption was not found to have a significant association with breast cancer (Q5 vs. Q1; mOR = 0.85, 95% CI = 0.63-1.14). The association between soybean consumption and breast cancer was found to be stronger in postmenopausal women (mOR = 0.60, 95% CI = 0.39-0.91) than in premenopausal women (mOR = 0.76, 95% CI = 0.56-1.04). Conversely, the association between tofu intake and breast cancer risk also was found to be stronger in premenopausal women (mOR = 0.62, 95% CI=0.44-0.87) than in postmenopausal women (mOR = 0.78, 95% CI = 0.45-1.26). The authors conclude that high intakes of soy bean and tofu, but not soybean paste, have inverse associations with breast cancer risk.

The present Chinese prospective study was designed to investigate the impact of soy intake on risk of breast cancer. Soyfood is a rich source of isoflavones, a class of phytoestrogens with established anti-estrogenic and anti-carcinogenic effects. However, population studies examining the association between soy or soy isoflavone intake and risk of breast cancer have been inconclusive. The study included 73,311 women who participated in the Shanghai Women's Health Study. A validated food frequency questionnaire was employed to determine usual dietary intake at baseline and during follow-ups. After an average follow-up period of 5.7 years (415,534 person-years), 441 incident cases of breast cancer were identified. Total soy protein (p trend =0.004) and isoflavone (p trend =0.01) intake were both found to be associated with lower risk of breast cancer. An inverse dose-response relationship was also found for the intake of all the individual soy food items evaluated in the study, including tofu (p trend =0.02), fresh soybean (p trend<0.001), dried bean (p trend =0.002), soy products (p trend<0.001), soy bean sprout (p trend =0.07), and soy milk (p trend<0.001). The inverse association was found to be stronger among pre-menopausal women (p interaction = 0.01). When adolescent soy consumption was considered, pre-menopausal women with the highest soy protein intake during both adolescence and adulthood had a more than 50% lower risk (RR=0.47, 95% CI: 0.28-0.79) compared to women with the lowest soy intake in both periods. The authors conclude that this large population-based prospective study provides thus far the strongest evidence that soy consumption reduces the risk of breast cancer.

The current Japanese case–control study was designed to evaluate the association between soybean consumption and breast cancer risk. Elements of the Japanese diet that might contribute to the relatively low breast cancer rates in Japan have not been clarified in detail. The study focused on soybean products since they are widely consumed in Japan. Data from the hospital-based epidemiologic research program at Aichi Cancer Center (HERPACC) was used. A total of 167 breast cancer cases and 854 controls confirmed as cancer-free were included. Odds ratios (OR) and 95% confidence intervals (95% CI) were calculated using multiple logistic regression analysis. High intake of soybean products was found to be associated with lower risk of breast cancer in premenopausal women. Compared with women in the lowest third, the adjusted ORs for highest tertile intake of tofu was 0.49 (95% CI = 0.25–0.95). A significant reduction in risk of premenopausal breast cancer was also found for increasing consumption of isoflavones (OR=0.44; 95% CI = 0.22–0.89 for highest vs. lowest tertile; P for trend=0.02). The authors conclude that they found a statistically significant inverse association between tofu or isoflavone intake and risk of breast cancer among premenopausal women but not among postmenopausal women.

The current population-based case-control study investigated the associations between tofu consumption and risk of breast cancer in three Asian-American populations. Rates of breast cancer among Asian Americans are lower than those of U.S. whites but considerably higher than prevailing rates in Asia. The high consumption of soy in Asia compared to its lower consumption among Asian Americans has been suggested as a partial explanation of this trend. The study included Chinese-, Japanese-, and Filipino-American women in Los Angeles County, San Francisco-Oakland, and Oahu, Hawaii. Cases included 597 Asian-American women diagnosed with incident, primary breast cancer during 1983-1987 and 966 population-based controls. The controls were matched to the cases based on age, ethnicity, and geographical area of residence. The women were interviewed using a common questionnaire which measured frequency of intake of 90 food items. The present analysis compared the usual adult intake of soy (estimated primarily from tofu intake) of the breast cancer cases to that of the control women. After adjusting for age, ethnicity and present geographical area, intake of tofu was found to be more than twice as high among Asian-American women born in Asia (62 times per year) than those born in the U.S. (30 times per year). Among migrants, intake of tofu was found to decrease with years in the U.S. Risk of breast cancer was found to decline with increasing frequency of tofu consumption after adjusting for age, ethnicity, present geographical area, and migration history. The adjusted OR associated with each additional serving per week was 0.85 (95% CI = 0.74-0.99). The apparent protective effect of high tofu intake was observed in both premenopausal and postmenopausal women and the association remained after adjusting for selected dietary factors and menstrual and reproductive factors. However, note that this study was not designed specifically to investigate the role of soy intake and the assessment of soy intake may be incomplete. The authors comment that they cannot discount the possibility that soy intake is a marker of other protective aspects of the Asian diet or lifestyle.