"Even at high doses, we found no evidence that the estrogen-like compounds in soy, called isoflavones, stimulate cell growth or other markers for cancer risk in breast tissue," said Charles E. Wood, D.V.M., Ph.D., lead researcher, from Wake Forest University Baptist Medical Center. "The study also suggests that women who have higher levels of estrogen may actually gain a protective effect from higher doses of soy isoflavones."
Wood said there has been much debate about whether higher levels of dietary soy are safe or beneficial for postmenopausal women. Some evidence has suggested that isoflavones may protect against the more powerful estrogen produced by the body, which is an important risk factor for breast cancer in postmenopausal women. For example, population studies show that women who consume diets high in soy generally have lower rates of breast cancer.
On the other hand, soy isoflavones have been shown to stimulate breast cancer cells in mice and in cells grown in the laboratory.
"Our study sought to make sense of these seemingly contradictory data," said Wood. "Our hypothesis was that estrogen levels in the body may influence the effects of soy isoflavones."
Wood and colleagues evaluated the effects of dietary isoflavones in the presence of different levels of estrogen by rotating 31 postmenopausal cynomolgus monkeys through eight different diets. Each diet contained one of four different isoflavone doses along with either a low or a high dose of estrogen.
Isoflavone doses were equivalent to the following human levels: no isoflavones, 60 milligrams (comparable to the typical Asian diet), 120 milligrams (the highest levels that can be consumed through di