Soy isoflavones and breast cancer risk among Chinese women in Singapore: A cohort analysis

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A large number of in-vitro experiments and studies in animals have shown that phytoestrogens such as isoflavones from soy foods may have a chemopreventive role in breast cancer. Although the collective evidence from numerous case-control studies has suggested that a high intake of soy products may reduce breast cancer risk, evidence from population-based prospective studies is very sparse. We examined the relationship between intake of soy isoflavones and breast cancer risk in the Singapore Chinese Health Study, a population-based prospective cohort of 63,257 middle-aged and older Chinese men and women enrolled between 1993 and 1998. Detailed information on current diet, assessed via a validated food frequency questionnaire, medical history and lifestyle factors were obtained through an in-person interview at enrollment. The cohort has been continuously followed, both actively through re-interviews and passively through computer linkage to population-based death and cancer registries. Loss to follow-up has been negligible. All breast cancer cases identified from the cancer registry were subsequently verified through manual review of pathological reports and medical records. As of 31 December 2005, 629 women had developed incident breast cancer among the 34,028 women participants. We used the Cox regression methods to examine soy intake in relation to breast cancer risk with adjustment for age at and year of study enrollment, dialect group, level of education, family history of breast cancer, age when period became regular, number of livebirths, body mass index and n-3 fatty acid intake. High (above median value for all female participants) versus low dietary soy isoflavones was associated with a statistically significant, 18% reduction in breast cancer risk (relative risk [RR], 0.82; 95% confidence interval [CI], 0.70-0.97). When subjects were stratified by duration of follow-up, the soy-risk association was strongest among those with the longest (>10 years) duration of follow-up (RR, 0.48; 95% CI, 0.30-0.79). Earlier, in a case-control study, we noted that the protective effect of dietary soy in Asian American women is stronger for adolescent versus adulthood consumption (Wu et al., 2006). Thus, the present findings are consistent with our previous observation, both indicating that time of dietary soy exposure is an important codeterminant of breast cancer risk, with earlier exposure relating to a greater degree of risk reduction.