Soy Health in Breast Cancer

Current views on soy and breast cancer stem from several comprehensive statistical analyses of epidemiologic studies. Epidemiology is the science of studying populations in order to determine the frequency and distribution of disease and measured risks. In the epidemiologic soy studies, investigators ascertained the quantity of soyfoods consumed by study participants to determine if high-soy consumers were more or less likely to have (in case-control studies), or to develop (in prospective studies), cancer.

In the most recent analysis, which was conducted by Anna Wu and colleagues from the University of Southern California, high-soy consumers were found to be about 30% less likely to report having breast cancer than Asian women who consumed relatively little soy.6 However, in a separate large study in the United Kingdom, there was no evidence that soy intake was protective.7 This second study purposely included a high percentage of vegetarians, who typically consume more soy than non-vegetarians.

What might account for the protective effect of soyfood intake in the Asian studies but not in the UK study? One explanation is that soyfoods are protective against breast cancer only when consumed early in life – during childhood and/or adolescence.8 In the UK study, it is likely the vegetarians adopted their dietary behavior as adults, and may well have consumed little soy prior. In contrast, because soyfoods are part of the traditional Asian diet, it is likely that Asians who consume soy as adults also did so as children.

The notion that early soy intake is protective against breast cancer has several lines of support. For example, studies show that when rats are exposed to isoflavones for just brief periods of time when young, and then given a mammary or breast carcinogen, tumor development is dramatically inhibited.9,10 More importantly, all four of the epidemiologic studies that investigated the relationship between early soy intake and later risk of developing breast cancer found protective effects,11-14 with risk reduction ranging from 28 to 60%. It is worth noting that in these studies, “high soy intake” was typically defined as only about 1 or 1½ servings of soyfoods per day.

Finally, the hypothesis that early soy intake is protective against breast cancer is consistent with studies showing that adult soy intake alone doesn’t affect markers or indicators of breast cancer risk.15 Because cancer is a difficult disease to study in humans, clinical studies generally examine the impact of an intervention, such as a dietary change, on indicators of cancer risk rather than cancer incidence per se. This is similar to the way in which we study the effects of diet on blood cholesterol levels, rather than on the incidence of heart attacks, as a means of assessing changes in heart disease risk. The fact that adult soy intake doesn’t affect markers of breast cancer risk strongly suggests that if soy is indeed protective against breast cancer, benefits are derived from consumption early in life.

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