Menopausal women who take a certain type of soy supplement long-term aren't at increased risk for breast cancer or any other ill effects, a new two-year study suggests.
But the researchers point out that the findings may not apply to all soy supplements. The ones used in the study contained a different combination of the estrogen-like compounds called isoflavones than preparations typically available over the counter.
Given their estrogen-like properties, soy isoflavones have been suggested as an alternative to hormone therapy for women experiencing menopausal symptoms, Dr. Francene M. Steinberg of the University of California, Davis, and her colleagues note in the American Journal of Clinical Nutrition. While there is little evidence from current research that the supplements can cause harm, the researchers add, they allow women to consume up to four times the amount of isoflavones contained in a typical Asian diet.
Very few studies have looked at the long-term safety of consuming so much soy, however.
To investigate, the researchers analyzed data from the Osteoporosis Prevention Using Soy (OPUS) Study. A previous analysis of the results found no effect of soy supplements on the density of breast tissue, one indicator of breast cancer risk. In the current study, the researchers looked at blood tests, health exams and adverse events to further examine the risks of soy.
A total of 403 women completed the study, including 134 women taking placebo, 135 women taking 80-milligram soy tablets, and 134 taking 120-milligram tablets.
While the researchers identified a few differences in blood tests between the women on placebo and those taking soy supplements, all but one were not statistically significant, meaning they could have been due to chance. The one measure that was significantly higher among the women taking soy, blood urea nitrogen (a measure of how well a person's kidneys are functioning), was still within normal range.
Well-woman exams revealed no differences in blood pressure, Pap smear results, thickness of the uterine lining and other health and screening tests. One woman in the 120-milligram soy group developed breast cancer after taking the supplement for 14 months, while another woman in the 80-milligram soy group developed uterine cancer. However, the researchers note, the rate of both cancers in the study was lower than would have been expected in the population of menopausal women as a whole.
The findings "support the safety of soy hypocotyl isoflavone supplementation over a 2-year period," the researchers conclude.
One limitation of the findings, the authors acknowledge, is that the soy supplements used in the study are different from many used in other studies or sold over the counter. Dr. Steinberg and her colleagues used supplements manufactured by Frutarom Netherlands BV and made from the hypocotyl portion of the soy bean. The proportions of isoflavones in the study supplements -- 54 percent daidzein, 34 percent glycitein and 12 percent genistein -- are significantly different from OTC supplements, which may contain up to 50 percent genistein as well as isoflavones from other plant sources than soy.
"It is, therefore, important to appreciate that a limitation of the current findings is that the clinical outcomes information derived from the OPUS Study cannot be necessarily used to establish the safety profile of the other available isoflavone preparations," the researchers warn.