The study, by researchers from the Wake Forest University Baptist Medical Center in the US, looked at the impact of soy isoflavones, similar to the female hormone oestrogen, on monkeys with their ovaries removed, designed to bear resemblance to postmenopausal women.
Women are increasingly taking soy products as a natural alternative to traditional hormone therapy, as they are thought to reduce hot flushes and other symptoms and also benefit some of the postmenopausal conditions, such as decrease in bone mass.
While evidence of these benefits is not conclusive (a study published yesterday questions the benefits of soy in postmenopausal women), there has also been much debate about whether high levels of dietary soy are safe for postmenopausal women.
It is known that populations that typically consume diets high in soy have lower rates of breast cancer. On the other hand, some studies have shown that soy isoflavones can stimulate breast cancer cells grown in the laboratory.
"Evidence from observational studies in women indicates that soy intake may help prevent breast cancer," said Charles E. Wood, lead researcher. "But there has still been reluctance to conduct research studies in women because of concerns that isoflavones may stimulate breast cell growth and increase the risk of breast cancer."
Wood and colleagues measured how a diet high in soy isoflavones affected markers for breast and uterine cancer risk in postmenopausal monkeys. The monkeys ate one of three diets for three years: soy without isoflavones, soy with the isoflavones intact, or soy without isoflavones, but with added Premarin (oestrogen therapy).
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 breast density, numbers of dividing breast and uterine cells, and levels of the oestrogen produced by the body - all markers for cancer risk. Monkeys on the soy plus oestrogen diet had increased levels of all markers, while monkeys that ate soy with isoflavones did not.
In fact, the monkeys eating soy with isoflavones had lower levels of the oestrogen produced by the body. High levels of this oestrogen are considered an important predictor of breast cancer risk in postmenopausal women.
"These findings suggest that high dietary levels of soy isoflavones do not increase markers for breast and uterine cancer risk in postmenopausal monkeys and may contribute to an estrogen profile associated with reduced breast cancer risk," write the researchers in The Journal of Clinical Endocrinology & Metabolism, (vol 89, no 7, pp 3462-3468).
"The findings should be especially interesting to women at high risk for breast cancer who take soy products," said Wood but he added that it is important to note that the research addressed the effects of plant oestrogens on normal breast tissue, and not in breast cancer.
"A big unanswered question is whether it is safe for breast cancer survivors to turn to soy," he said.
Researchers are not certain how plant oestrogens and the oestrogen produced by the body, or given in pills, act together. One theory is that the plant oestrogens bind to cells that have oestrogen receptors, such as breast tissue, and block the effects of the other types of oestrogen. Isoflavones may also help reduce the amount of active oestrogen in the body.
To investigate these ideas, Wood and colleagues are currently looking at whether soy may block breast cell proliferation induced by oestrogen therapy.