Epidemiology of soy exposures and breast cancer risk

Abstract

Most of the early studies published on soy and breast cancer were not designed to test the effect of soy; the assessment of soy intake was usually crude and few potential confounders were considered in the analysis. In this review, we focused on studies with relatively complete assessment of dietary soy exposure in the targeted populations and appropriate consideration for potential confounders in the statistical analysis of study data. Meta-analysis of the 8 (1 cohort, 7 case -control) studies conducted in high-soy-consuming Asians show a significant trend of decreasing risk with increasing soy food intake. Compared to the lowest level of soy food intake (p5 mg isoflavones per day), risk was intermediate (OR ¼ 0.88, 95% confidence interval (CI) ¼ 0.78 -0.98) among those with modest (B10 mg isoflavones per day) intake and lowest (OR ¼ 0.71, 95% CI ¼ 0.60 -0.85) among those with high intake (X20 mg isoflavones per day). In contrast, soy intake was unrelated to breast cancer risk in studies conducted in the 11 low-soy-consuming Western populations whose average highest and lowest soy isoflavone intake levels were around 0.8 and 0.15 mg per day, respectively. Thus, the evidence to date, based largely on case -control studies, suggest that soy food intake in the amount consumed in Asian populations may have protective effects against breast cancer.

Effect of dietary soy intake on breast cancer risk according to menopause and hormone receptor status

“…The effect of soy on breast cancer risk did not differ by ER/PR status, but the estimated isoflavone intake showed an inverse association with breast cancer risk only among postmenopausal women with ER þ /PR þ tumors. To date, the findings on the effect of soy on breast cancer risk in epidemiological studies have been inconclusive (Trock et al, 2006;Wu et al, 2008b). Large differences in study results exist between the Asian and Western populations; Wu et al (2008b) found a significant trend of decreasing risk with increasing soy consumption from a meta-analysis of the eight studies conducted in Asians, who are large consumers of soy.…”

“…Ecological and epidemiological studies have suggested that high soy consumption might explain the low incidence of breast cancer in Asian countries compared with Western countries (Pisani et al, 2002;Wu et al, 2008b). Soybeans are the predominant source of isoflavones.…”

“…Furthermore, several studies have reported possible synergistic effects of these components (Omoni and Aluko, 2005;Kim et al, 2008). However, the association between soy consumption and breast cancer risk remains inconclusive in epidemiological studies (Trock et al, 2006;Wu et al, 2008b). It has been suggested that the effect of soy consumption on breast cancer etiology may depend on several factors, including hormonal environment (Yamamoto et al, 2003;Hirose et al, 2005;Hwang et al, 2006), receptor status (Linseisen et al, 2004;Suzuki et al, 2008;Zhang et al, 2009), timing of soy exposure (Wu et al, 2002), amount and profile of isoflavones in foods or supplements (Hsieh et al, 1998), and individual metabolism and genetics (Setchell et al, 2002).…”

The role of early life genistein exposures in modifying breast cancer risk

“…Human studies suggest that there indeed is a modest inverse association between high soy food consumption and reduced risk (Trock et al, 2006;Wu et al, 2008). However, several questions remain regarding a causative role of soy consumption in reducing breast cancer risk; the possibility that the association simply reflects other factors/ 'healthy' lifestyle affecting breast cancer risk cannot be excluded at the present time.…”

“…One possible explanation for the discrepancy between the protective role of soy found in two meta-analyses (Trock et al, 2006;Wu et al, 2008) and the lack of effect on biomarkers of breast cancer risk or the results generated in animal studies during adult exposure to soy/genistein is that, to be protective, this bioactive food component may need to be consumed during early life. In support of this argument, epidemiological studies indicate that childhood/adolescence exposure to soy provides protection against breast cancer later in life (Shu et al, 2001;Wu et al, 2002).…”