There has been much speculation about phytoestrogens, particularly in soy products, and breast cancer risk. This is mainly based on the misconception that because these foods contain oestrogen-like compounds, they can stimulate breast tissue in the same way as our own body’s oestrogen or pharmaceutical oestrogen might.

In 2006 a study was published that examined all of the research on phytoestrogens and breast cancer between 1978 and 2004. (Trock et al – see References below). There were a number of important findings to come out of this analysis. Firstly, it was shown that there is a small reduction in the risk of breast cancer associated with phytoestrogen consumption. The risk of breast cancer was reduced more in premenopausal women who consumed soy compared to postmenopausal women. It has also been shown that a diet high in phytoestrogen in early life (before puberty) may be important for the anti-cancer effects of phytoestrogens in later life.

Secondly, there is conflicting data on the individual isoflavones found in soy, depending on the doses used. Some studies have shown a stimulation of human breast cells, others have shown a prevention of stimulation. It is important to consider that test tube studies of compounds isolated from foods do not fully represent what occurs in the body when a food containing differing amounts of complex compounds is eaten. It is generally agreed that, while test tube evaluation of compounds provides important information, there are more suitable methods of assessing safety when evaluating food products.

Thirdly, and most importantly, it is suggested that highly processed soy supplements such as soy protein isolate, isoflavone-rich soy extracts or isoflavone capsules act differently to foods made from soybeans or soy flour. The authors of this review concluded that it is not recommended for women at high risk of breast cancer or breast cancer survivors to take these highly processed soy supplements, such as soy protein isolate, isoflavone-rich soy extracts or isoflavone capsules.

There is no evidence to suggest that eating a diet of soy foods in amounts consistent with Asian diet is detrimental to breast health. This finding was again confirmed in a review published in Nutrition Journal in 2008, which stated that there is no evidence that isoflavone intake increases breast cell proliferation (growth) in pre- or postmenopausal women, with or without a history of breast cancer, and that eating phytoestrogens at levels consistent with historical Asian soy food does not result in adverse stimulation of breast tissue.

In summary, it is appears that eating a diet with moderate amounts of whole soy foods such as tofu, soy beans, and breads made from soy flour is healthy; but you should probably avoid the highly processed soy based supplements, as they work differently to the way soy exists in nature.

References

Hilakivi-Clarke, L. Andrade, JE. Helferich, W. 2010. ‘Is Soy Consumption Good or Bad for the Breast?’ J Nutr 140(12):2326S-2334S

Messina, M. 2010. ‘A Brief Historical Overview of the Past Two Decades of Soy and Isoflavones Research’. J Nutr 140(7):1350S-1354S

Trickey, Ruth 2011. ‘Women, Hormones & the Menstrual Cycle’. Trickey Enterprises, Fairfield Victoria Australia

Hartman J, Ström A, Gustafsson JA. 2009 ‘Estrogen receptor beta in breast cancer – diagnostic and therapeutic implications’ Steroids 74(8):635-641.

Messina M, Hilakivi-Clarke L. 2009 ‘Early intake appears to be the key to the proposed protective effects of soy intake against breast cancer’ Nutr Cancer 61(6):792-798.

Trock, BJ et al. 2006 ‘Meta-Analysis of Soy Intake and Breast Cancer Risk’ J Natl Cancer Inst 98(7):459-471.

Content updated June 21, 2011