Breast cancer patients who consume large amounts of soy may make the disease worse, according to a study conducted by researchers from Georgetown University and the Memorial Sloane Kettering Cancer Center in New York, and published in the Journal of the National Cancer Institute on September 4.
“These data raise concern that soy may exert a stimulating effect on breast cancer in a subset of women,” the researchers wrote.
The study was conducted with support from the Breast Cancer Research Foundation.
Certain studies, conducted primarily on Asian populations, have suggested that soy intake might be associated with a decreased risk of breast cancer. Other studies, including many on Western populations, have found no such benefit.
The current study looked at a different question, however: Following a breast cancer diagnosis, what effect does soy have on the progression of the disease?
Soy associated with tumor growth
Soybeans are known to contain naturally occurring chemicals that mimic the effects of estrogen hormones in the body, known as phytoestrogens. Researchers have suspected that this may become problematic during breast cancer, because many such cancers (known as estrogen-receptor -positive) actually use estrogen to fuel their growth. For that reason, many women are advised to avoid extra hormones after a breast cancer diagnosis and may even be put on hormone-blocking drugs such as tamoxifen.
“Estrogen is no good at any time after breast cancer diagnosis,” said researcher V. Craig Jordan, who wrote an accompanying commentary on the study.
In the new study, the researchers assigned 140 women who had been diagnosed with estrogen-receptor-positive breast cancer to take two 25.8-gram packets of either soy protein or milk protein supplements daily. The supplements were mixed into water or juice. The researchers then compared breast cancer cells obtained from the initial biopsies (used for diagnosis, before the trial began) with cells extracted from tumor-removal surgery (after the intervention).
They found that, compared with women in the milk-protein group, women in the soy-protein group had tumors that over-expressed genes associated with cell proliferation. This suggests that those tumors were replicating more rapidly. In some women, tumors also had increased expression of a gene associated with cancer growth.
The researchers noted that many things remain unknown. For example, the study did not look directly at tumor growth, merely at gene expression. It is also unknown whether the effects seen on gene expression are reversible or not.
“We do not know what the consequences of taking soy for longer [periods] or taking higher amounts of soy, would have on the biology of a breast tumor,” researcher Dr. Jackie Bromberg said.
“All we can say is that in some of the women, the soy component drove proliferative genes, at the gene level,” Dr. Moshe Shike said.
“For populations of women with breast cancer, soy products aren’t good. Now doctors have a set practice and can tell patients that this isn’t a good thing,” said Jordan.
Be cautious: limit intake
While the findings may not be conclusive, they certainly suggest that breast cancer patients should limit their soy intake, according to Dr. Michael S. Cowher of Cleveland Clinic’s Breast Services, who was not involved in the study.
“I think the study provided preliminary evidence that soy supplementation can affect gene expression in breast tumors,” Dr. Cowher said.
“For the individual patient with breast cancer, soy intake is part of the discussion to have with your doctor regarding potential modifications,” he said. “I generally advise both patients with and those at higher risk for breast cancer to avoid excessive soy intake, but remind them that it is possible that future research will find that for some patients soy supplementation may be beneficial, while harmful to others.”
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