Soy isoflavones are naturally occurring oestrogen-like compounds, and supplements are currently marketed as a way of reducing symptoms of the menopause and offer an alternative to hormone replacement therapy.
Conflicting reports however have clouded the picture about the beneficial effects of soy isoflavones, with some studies indicating that breast cancer cells in mice were stimulated by the isoflavones. Population studies have shown that women with a high-soy diet generally have lower rates of breast cancer.
The research, led by Charles Wood from the Wake Forest University Baptist Medical Center, sought to make sense of the conflicting data. "Our hypothesis was that oestrogen levels in the body may influence the effects of soy isoflavones," said Wood.
The effects of dietary isoflavones were evaluated by rotating 31 postmenopausal cynomolgus monkeys through eight different diets. The diets contained either a high or low dose of oestrogen and one of four isoflavones levels: no isoflavones, 60 mg (comparable to a typical Asian diet), 120 mg, and 240 mg.
The researchers reported that isoflavone levels for the low oestrogen diet did not affect markers for breast cancer, such as breast cell proliferation. For the high oestrogen diet, no and low-level isoflavone intake were actually linked to increase in breast cell proliferation.
However, at higher levels of oestrogen and higher intake of isoflavones, the oestrogen effects on breast tissue were blocked by the isoflavones.
"Even at high doses, we found no evidence that the oestrogen-like compounds in soy stimulate cell growth or other markers for cancer risk in breast tissue," said Wood. "The study also suggests that women who have higher levels of oestrogen may actually gain a protective effect from higher doses of soy isoflavones."
"For women at increased risk of breast cancer due to higher oestrogen levels, a diet rich in soy isoflavones may offer a modest breast-protective effect," he said.
The mechanism by which isoflavones may protect against breast cell proliferation remains unclear, and may not be applicable to pre-menopausal women.
An earlier study, sponsored by Italian soy ingredients supplier Indena, also reported that soy extracts reduced tumour progression in an experimental animal model. The researchers of the Indena study suggested that soy isoflavones might induce an oestrogen effect on oestrogen-dependent cells, without promoting cellular growth or cell proliferation.
Over one million women worldwide are diagnosed with breast cancer every year, with about 400,000 new cases in Europe. China has the lowest incidence and mortality rate of the disease.
The Wake Forest study is published in the 15th January issue of the journal Cancer Research (Vol. 66, Issue 2).