Last year, researchers from Italy reported that six women who took soy tablets for up to five years had an increased occurrence of endometrial hyperplasia, a condition in which the lining of the uterus grows too much and may progress to cancer. The study involved 375 women half took the soy pills and half took an inactive placebo tablet.
"This observation from this study and its interpretation should be carefully considered," said Cline. "Both human observational studies and several short-term trials of soy isoflavones have not shown any proliferation-inducing effect of soy isoflavones on the uterus."
Cline said that preliminary results from a two-year study of women who consumed 58 milligrams of soy isoflavones a day show no relationship between the soy and endometrial proliferation, and that investigators at other institutions have made similar findings. Similarly, Cline said, monkeys who received soy isoflavones at dietary doses for the equivalent of 10 human years did not show increased risk of endometrial hyperplasia. The monkeys ate one of three diets: soy that didn't contain isoflavones, soy with the isoflavones intact, or soy without isoflavones, but with Premarin, or estrogen therapy, added.
The isoflavone group consumed the human equivalent of about 129 milligrams a day, more than most people would get in a soy-rich diet. The researchers measured numbers of dividing uterine cells and levels of the estrogen produced by the body all markers for cancer risk. Monkeys on the soy plus estrogen diet had increased levels of all markers, while monkeys that ate soy with isoflavones did not.
In a short-term study, researchers gave monkeys 10 times the levels of isoflavones normally consumed through diet and again found no effects on the uterus.
Cline said that the results of these studies apply to dietary soy and not to the high levels of isoflavones found in soy pills. He also noted that the
Contact: Karen Richardson
Wake Forest University Baptist Medical Center