Researchers reporting in the journal Cancer (Vol. 94, No. 4: 1166-1174) said soy has the potential to lower breast cancer risk.

Yet, the exact role of soy — either in the diet or in the form of supplements — in reducing the risk of breast cancer is still not well defined.

Studies have shown that people in China and Japan who consume large amounts of soy have lower rates of breast cancer. But people in these countries also eat less fat and higher amounts of fiber, so finding the true effect of soy has been difficult.

Researchers from the H. Lee Moffitt Cancer Center in Tampa, Fla., studied soy's effects in healthy women. Women in the study were divided into two groups. One group got daily soy protein in a supplement, and the other got a placebo (non-active substance) daily for 12 weeks.

The doctors then looked at the hormone levels in the blood and at the menstrual cycle lengths of these women.

Thirty-three women in each group completed the study. At the end of three months, the doctors found that women in the soy group had their menstrual cycle length increase by more than three days. The menstrual cycles in the placebo group stayed about the same.

The effects on hormone levels were not as pronounced. Levels of estrogens in the blood were slightly lower overall among the women taking soy.

Experts say some substances in soybeans can act like hormones in the body. High levels of estrogens, the main female hormones, have been linked to increased breast cancer risk.

Substances in soy called isoflavones, such as genistein, may be able to lower natural estrogen levels. This may, in turn, lower the risk of breast cancer. While it is an attractive theory, the evidence to support it remains mixed.

Another theory is that isoflavones somehow increase the time between menstrual periods. Normal breast cells would then be less exposed to estrogens over time, which might decrease the risk they would eventually become cancerous.

Soy’s subtle effects on hormone levels may turn out to have a variety of health benefits. Some studies have suggested that increasing consumption of soy isoflavones may reduce the symptoms of menopause, or even help lower cholesterol levels.

But, as with the present study, these effects are still far from well proven. Further studies, including those that determine the best dosage to take and any possible long-term effects, are needed before soy supplements can be recommended, the authors said.

Soy may even be harmful in certain groups of people. For example, in women with breast cancer or a history of breast cancer, isoflavones may actually act as weak estrogens, increasing the risk of the cancer growing or coming back.

While promising, these results must be viewed with caution, experts concluded. Soy is a good source of protein and a good option other than meat, but studies have yet to show a direct link between soy supplements and a reduced risk of breast cancer. In fact, no dietary supplements has conclusively been shown to reduce the risk of cancer.

For this reason, the American Cancer Society recommends that women consume only modest amounts of soy foods as part of a healthy plant-based diet, and not take high levels of soy products, either in the diet or as supplements.