Eating soy foods has been proposed to lower breast cancer risk. This theory arose from the fact that Asian women have a much lower risk of breast cancer than American women. But when Asian women move to the US, their risk for breast cancer jumps as much as 80% within one generation, and their American-born daughters’ risk is close to that of American women. This rapid rise in risk may be linked to dietary differences, particularly intake of foods made from soybeans. The Asian diet is higher in soy foods (tofu, tempeh, miso, etc.) than the American diet. In addition, the isoflavones in soy have shown anti-cancer effects in test tubes, leading to the sale of isoflavone supplements for cancer prevention. Further study, though, has shown that isoflavones may be harmful, as they also have pro-cancerous effects.

To examine the link between soy and breast cancer, researchers combined data from 18 epidemiologic studies looking at this association. Their findings, in the April 5, 2006 Journal of the National Cancer Institute , reveal that soy foods have a small protective effect. However, due to flaws in the 18 studies, researchers caution against the use of isoflavone supplements.

About the Study

By combining data from 18 studies, researchers from Johns Hopkins and Georgetown University examined soy intake and breast cancer risk in more than 25,000 women. The women were divided into two groups—high and low intake of soy—and breast cancer risk was compared between these groups.

The high soy group had a small, but significant, reduction in breast cancer risk, compared with the low soy group. Risk was lowered more in premenopausal than postmenopausal women. Researchers then adjusted these findings to remove the effects of BMI, as having a high BMI (being overweight) increases the risk of breast cancer. With BMI out of the picture, there was less of a drop in breast cancer risk in the high soy group; this implies that women who ate more soy were also less likely to be overweight, and some of the risk reduction seen was due to a lower BMI rather than a high soy intake.

These findings are limited by the many inconsistencies among the 18 studies. For example, to measure soy intake, some studies looked at diet, while others analyzed urine samples. In addition, many studies did not account for the effects of BMI or other factors that influence breast cancer risk.

How Does This Affect You?

Do not depend on soy to reduce your risk of breast cancer. While this study reports a reduction in breast cancer risk among women eating the most soy, there are too many unknowns and inconsistencies to put much weight behind this finding. One thing is clear, though: isoflavone supplements are not recommended. There is little evidence these supplements will protect, and there are hints they may actually promote breast cancer. Women at elevated risk for breast cancer could be especially vulnerable to adverse effects of isoflavone supplements.

While soy is not a miracle food ( nothing is), it does have a place in a healthful diet. Soy protein may help lower cholesterol and the risk of heart disease. Replacing red meat and cheese with soy products is a good way to reduce saturated fat, cholesterol, and calories, while boosting fiber and unsaturated fat. To reduce your risk of breast cancer, be screened regularly and take a cue from Asian women by adopting a healthier lifestyle:

  • Be physically active on a daily basis
  • Consume alcohol in moderation (one drink/day for women)
  • Strive for a more Asian-style diet:
    • Rich in vegetables and grains
    • Small amounts of red meat
    • Moderate amounts of protein from fish and soy