Women have been using soy to ease their menopausal symptoms for years. Evidence suggests it can also help protect against many types of cancer, including breast cancer.
However, a new study shows a potential downside of soy supplements. Prolonged use of soy tablets may increase the risk of endometrial hyperplasia, the thickening of the uterus lining. Endometrial hyperplasia is linked with endometrial and uterine cancer.
Italian researchers conducted a study of the effects of soy on the uterus. Nearly 400 healthy, postmenopausal women took either tablets containing 150 mg of soy isoflavones or placebos daily for a period of five years. (Only 298 of the 376 women who began the study completed the process. No explanation was given for why 78 women did not finish the study.) The researchers, led by Dr. Vittorio Unfer at the University of Perugia, analyzed uterus biopsies taken at the beginning of the study, midway through, and at the end.
Six women who took soy supplements during the study had hyperplasia in their final biopsy, while no women who took placebos had hyperplasia at the end of the study. The positive hyperplasia rate of 3.37% is considered "statistically significant." Although hyperplasia is a risk factor for cancer, none of the biopsies showed any cancer cells in the women’s uteruses.
Soy is a popular menopausal supplement. It is a source of phytoestrogens, which act similarly to estrogen in the body. Soy reduces symptoms such as hot flashes and loss of bone density. It is also believed to reduce the risk of estrogen-related cancers.
The dose of isoflavones prescribed in the study was nearly twice the recommended dosage necessary to increase menopausal health. A typical dose is 80 mg or less per day.
Previous studies have reached conflicting conclusions regarding the effect of isoflavones on the endometrium. Some have stated that phytoestrogen in soy may lead to a thickening of the uterine walls or that it will not protect a person from hyperplasia, while others have said that there is no discernable link. Other studies have found that soy extracts can prevent hyperplasia.
The results of the five-year study were published in the July issue of Fertility and Sterility.