Experts have cautioned that eating soy, in the form of tofu or milk, could speed up the rate at which breast cancer cells spread in women already diagnosed with the disease.
The warning came after scientists at the Sloan Kettering Cancer Centre in New York studied 140 women diagnosed with invasive breast cancer.
Between seven and 30 days before the women had surgery to remove their tumours, half of them were given soy protein powder containing genistein, while the other half received a placebo.
The researchers compared tumour tissues from before and after the operation. In the women who had taken the soy supplement, changes were found in the expressions of certain genes that are known to promote cell growth. These findings suggested that soy protein could potentially accelerate the progression of the disease.
In the words of the study, which was published in the September 4 edition of the Journal of the National Cancer Institute: “These data raise concern that soy may exert a stimulating effect on breast cancer in a sub set of women.”
Participants in the study had recently had breast biopsies, were diagnosed with stage one or two breast cancer, and were scheduled to have a mastectomy or lumpectomy in two to three weeks.
According to co-author of the study Jacqueline Bromberg: “Although the genes were being expressed, it is not clear that this will translate into actual tumour growth. But the concern is that there may be the potential.
“Only 20% of those patients who took the soy had really high levels of the genistein metabolite.”
Dr Bromberg said the reasons behind the disparity are unclear, adding that there is no way to predict who would have this reaction after consuming soy.
She went on to explain that of the women with high genistein levels, a few of them experienced changes in a specified set of genes that are known to affect breast cancer cell growth, death, or some aspect of breast cancer pathology.
Women who consumed around 51.6 grams of soy, the equivalent of about four cups of soy milk a day, exhibited the changes.
It was thought likely that those who eat soy regularly could “reasonably consume that amount” daily, particularly vegetarians and those who do not eat dairy products.
The scientists cautioned that women living in Asian countries could be especially at risk, due to the high levels of soy and tofu in traditional diets.
The research did not address the question of whether soy would have any effect on women who have not already been diagnosed with breast cancer.