Adding soy to your diet could speed up the rate at which breast cancer cells spread, scientists have warned. Researchers at the Memorial Sloan Kettering Cancer Centre in New York studied 140 women diagnosed with invasive breast cancer.
Half took soy protein powder containing genistein while the other half took a placebo for between seven and 30 days before they had surgery to remove their cancer.
The scientists compared tumour tissues from before and after the operation and found changes in the expressions of certain genes, that are known to promote cell growth, in those women taking the soy supplement. The findings led them to conclude the soy protein could potentially accelerate the progression of the disease.
The study states: 'These data raise concern that soy may exert a stimulating effect on breast cancer in a sub set of women.'
All those taking part in the study had recently had breast biopsies and were diagnosed with stage one or two breast cancer. They were all scheduled to have a mastectomy or lumpectomy two to three weeks later.
The researchers said it is not yet clear if the effects can be reversed.
Jacqueline Bromberg, co-author of the study, said: '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 per cent of those patients who took the soy had really high levels of the genistein metabolite.'
She said the reasons behind the disparity are unclear, adding that there is no way to predict who would have this reaction after consuming soy.
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, Dr Bromberg, said.
The changes were seen in women who consumed around 51.6 grams of soy - the equivalent of about four cups of soy milk a day.
The researchers concluded those who eat soy regularly could 'reasonably consume that amount' through the course of a day, particularly vegetarians and those who do not eat dairy products.
Posted by John J. Ray (M.A.; Ph.D.).