Soy could adversely affect patients with breast cancer, according to a recent study.

The impact of soy consumption on breast cancer prevention and treatment is not clear, however many women believe soy supplementation is beneficial based primarily on results from epidemiological studies.

For the recent study, researchers conducted a randomized placebo-controlled study of the effects of soy supplementation on gene expression and markers of breast cancer risk among women diagnosed with invasive breast cancer.

The study, which ran from 2003 to 2007 at Memorial Sloan-Kettering, enrolled a total of 140 patients who were randomized to either soy supplementation (soy protein) or placebo (milk protein), which lasted from the initial surgical consultation to the day before surgery (range=7-30 days). Researchers then analyzed Tumor tissues from the diagnostic biopsy (pre-treatment) and at the time of resection (post-treatment).

Researchers observed changes in several genes that promote cell cycle progression and cell proliferation among women in the soy group.

"These data raise concern that soy may exert a stimulating effect on breast cancer in a subset of women," researchers said in the study.

V. Craig Jordan from the Department of Oncology at the Georgetown University Lombardi Comprehensive Cancer Center said the study "illustrates the dangers of phytoestrogen consumption too soon, around menopause, but the biology of estrogen in estrogen-deprived conditions suggests that phytoestrogen could have benefit a decade after menopause."

Jordan cautions that appropriate doses of soy and timing of consumption are critical considerations.

The findings were recently published in the JNCI: Journal of the National Cancer Institute.