Do soy products cause breast cancer?

I have been asked many times to give my opinion on soy products and their relationship to breast cancer. Soy and other legumes contain one of three classes of phyto or plant-type oestrogen. These phyto-oestrogens can mimic the behaviour of the oestrogen produced by our bodies. The three classes of oestrogen are isoflavones found in soy and other peas and beans; lignans found in seeds, whole grains, berries, fruits, vegetables and nuts and coumestans found in broccoli and sprouts.

Although the evidence indicates that oestrogen, produced by our bodies or through hormonal therapy for menopause, is a cause of breast cancer, the role of phyto-oestrogens is still quite inconclusive.

In Eastern countries, the incidence of breast cancer is said to be a third of that of western countries. Studies also indicate that the consumption of phyto-oestrogen in these countries can be as high as 50mg per day.

One study shows the consumption in the US caucasian population to be less than 1mg per day and 1-3mg per day in the French population. The main phyto-oestrogen consumed in Eastern countries is isoflavones from soy while in the western countries it is lignans.

Some protective effects

A great deal of commercial spotlight has been put on isoflavones, not just on its food-based products but also on its purified protein isolate forms. Some studies show protective effect of isoflavones in the Asian population who consume diets high in soy as a part of a regular culturally accepted diet. Studies also looking at the Asian-American population showed a protective effect of soy intake during childhood and adulthood. Some studies indicate that the protective effect of dietary soy is greater in childhood and early adulthood.

The studies looking at breast cancer risk in non- Asian populations have found mixed results and may be related to the amount of isoflavones consumed as a part of the traditional dietary intake. The evidence does not support the increase of isoflavones intake in the Western diet to levels present in Eastern countries and does not support the intake of isoflavones supplements to bolster current intake.

Studies have shown different effects of the phyto-oestrogen daidzein to slightly increase tumour cell growth in the absence of oestrogen, while genistein, also found in soy, taken in high doses, resulted in cell death. These two soy isoflavones have been implicated in affecting the efficacy of the breast-cancer drug tamoxifen.

Take-home message

Consume phyto-oestrogen in its naturally occurring form as part of a healthy diet in sources such as isoflavones in peas and beans, lignans in fruits and vegetables and coumestans in sprouts and broccoli. Take advantage of the synergistic effects of these foods as all the components work in tandem with each other for your benefit. Isolating and consuming large doses of these phytonutrients can be harmful instead of beneficial.

Rosalee M. Brown is a registered dietitian/nutritionist who operates Integrated Nutrition and Health Services; email: yourhealth@gleanerjm.com.