A meta-analysis found that soy intake may be associated with a small reduction in the risk of breast cancer.

However, because of limitations and inconsistencies in the studies examined within the meta-analysis, the authors caution that evidence is not sufficient to recommend soy as a means of preventing breast cancer.

Women in Asian countries have much lower breast cancer rates ( 39 per 100,000 ) than Western women ( 133 per 100,000 ), and when Asian women move to the United States their rates of breast cancer increase. Some studies have suggested the difference may be due to dietary habits, noting that Asian women consume higher quantities of soy products than Western women.
Soybeans contain high quantities of compounds called isoflavones, molecules that affect biological pathways that could alter breast cancer risk. Because of these potential associations with lower breast cancer risk, women are increasingly taking high dose soy or isoflavone supplements.

Bruce J. Trock, of the Johns Hopkins School of Medicine, and colleagues conducted a meta-analysis of 18 epidemiologic studies published between 1978 and 2004 that examined the association between soy intake and breast cancer risk. Studies included women in Western countries and women in Asian countries.

The results of the meta-analysis indicated a small association ( 14% relative reduction ) between soy intake and a reduced risk of breast cancer.
Risk appeared to be reduced to a somewhat greater extent in premenopausal than postmenopausal women, however only 10 of 18 studies evaluated the effect separately by menopausal status.
Because of limitations in the studies examined - such as difficulty measuring soy intake accurately, or the possibility that soy intake may serve as a surrogate for other healthy behaviors - the authors feel that the data are not adequate to provide a clear answer about the effect of soy foods on breast cancer risk.

The authors write, " Because of the many gaps in knowledge and inconsistencies revealed by this analysis and review of relevant studies, we cannot recommend widespread use of high dose isoflavone supplements by women at high risk for breast cancer or by breast cancer survivors. […] Now that soy food consumption is increasing in Western societies, epidemiologic studies should evaluate exposure at different periods of life, and focus on better quantitation of soy foods and soy additives to food."

In an accompanying editorial, María Elena Martínez, of the Arizona Cancer Center in Tucson, and colleagues write, " We commend the authors for tackling this complex and controversial, yet highly important issue." The editorialists describe limitations authors of the study faced and endorse the conclusion that future investigations are needed before a recommendation for soy consumption can be made for breast cancer reduction.

Source: Journal of the National Cancer Institute, 2006


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