"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 diet alone), or 240 milligrams (levels obtained through supplements). Estrogen doses were designed to mimic either a low or high-estrogen environment found in postmenopausal women. Estrogen levels in postmenopausal women can vary depending on their amounts of body fat, which produces estrogen, and whether they are taking hormone therapy.
The researchers measured how the diets affected markers for breast cancer risk, including breast cell proliferation. In the low-estrogen environment, no evidence of increased proliferation was seen at any level of isoflavone exposure, even at doses almost several times higher than in a typical Asian diet.
In the high-estrogen environment, there was higher breast cell proliferation both when isoflavones weren't in the diet and when they were present in lower doses. However, the addition of high levels of dietary soy isoflavones tended to block estrogen effects in breast tissue. This finding suggests that postmenopausal women with higher levels of estrogen may derive the greatest benefit from soy.
"For women at increased risk of breast cancer due to higher estrogen levels, a diet rich in soy isoflavones may offer a modest breast-protective effect," said Wood. However, he said the study may not apply to premenopausal women, who have higher and more dynamic hormone levels, or to women taking combined hormone therapy with an estrogen and a progestin.
The senior investigator of the study was J. Mark Cline, D.V.M., Ph.D. Other researchers involved in the study were Thomas C. Register, Ph.D., and Mary S. Anthony, Ph.D., both from Wake Forest Baptist, and Adrian A. Franke, Ph.D., from the Cancer Center of Hawaii.