Indeed, the study suggests that highly purified soy foods and soy supplements could in fact stimulate the growth of pre-existing oestrogen-dependent breast tumours - an altogether more worrying possibility.
William G. Helferich, professor of food science and human nutrition at the University of Illinois at Urbana-Champaign in the US, suggested that the healthy properties of the soy used widely in Asian cuisine - and on which the burgeoning popularity of the soy-based health food industry is founded - were largely destroyed by the processing techniques used in the west.
"Soy has been correlated with low rates of breast cancer in Asian populations, but soy foods in Asia are made from minimally processed soybeans or defatted, toasted soy flour, which is quite different from soy products consumed in the US," he said in the study, published online this week in advance of regular publication by the journal Carcinogenesis.
"Isoflavone-containing products consumed in the US may have lost many of the biologically active components in soy, and these partially purified isoflavone-containing products may not have the same health benefits as whole soy foods," he added.
Soy isoflavone products are marketed as dietary oestrogens to women over the age of 50 as a natural alternative to hormone replacement therapy (HRT), but since this is also the age group in which most breast cancers occur, these new findings - if supported - could have widespread implications.
Breast cancer is the second leading cause of cancer deaths in women (after lung cancer) and is the most common cancer among women, excluding nonmelanoma skin cancers. According to the World Health Organisation, more than 1.2 million people will be diagnosed with breast cancer this year worldwide.
According to Helferich. 75 per cent of breast cancer cases are diagnosed in women over 50, and the majority of these cases are oestrogen-dependent. For these women, consumption of highly processed isoflavone products could pose a risk, he suggested.
Helferich used an animal model that has been used extensively to evaluate breast cancer therapies such as tamoxifen. "The results of this preclinical investigation are especially relevant to post-menopausal women with oestrogen-responsive breast cancers who are looking for alternatives to HRT," he said.
In the study, mice were fed equal concentrations of the soy isoflavone genistein, allowing Helferich to determine the influences that various bioactive soy compounds had on genistein's ability to stimulate oestrogen-dependent breast tumour growth. "As bioactive compounds were removed, we observed an increase in oestrogen-dependent tumour growth," he said.
If genistein had been the only biologically active compound, all diets would have resulted in similar tumour growth, but that was not the case, he said.
A soy flour and mixed isoflavones diet and a mixed isoflavone diet each contained equal amounts of genistein, but differed in the amount of other bioactive components originally present in the soy flour. Tumours neither grew nor regressed in animals fed these diets.
"The minimally processed soy flour used in these diets is more like the soy foods in the Asian diet," Helferich said. "Dietary soy products that contained isoflavones in more purified forms were associated with greater tumour growth. These products are similar to the materials used in isoflavone-containing dietary supplements, which is how many Americans consume these compounds," he added.
The US research is supported by earlier findings from a team of Dutch scientists which also highlighted the differences between soy isoflavone effectiveness in Europe and Asia - although that did not go as far as suggesting that processed soy products could be a risk factor for tumour growth.