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Soy protein can cause proliferation of breast cancer tumor cells, a new study has concluded.

Co-author Dr Jacqueline Bromberg told Daily Mail that consumption of soy should be moderated, given that women consume large amounts believing it offers protection against breast cancer.

'We're not talking about 20 times more soy. We're talking about something that a person could eat. If you currently have early-stage breast cancer, don't eat soy in large amounts. If you've had breast cancer, you can eat soy but in moderation," Bromberg said.

According to Fox News, 140 women with breast cancer who were part of the study were divided into two groups. One group was asked to take 51.6 grams of soy protein every day for nearly a month before undergoing surgery. Researchers compared the expression of genes associated with cell proliferation in both groups before and after surgery. They found that the observed genes in women who took soy showed greater expression.

Reuters reported that phytoestrogens in soy can mimic the action of female hormone estrogen which is associated with greater tumor growth in women with breast cancer.  Dr Moshe Shike lead author of the study said the findings are interesting given that most women think soy would prevent breast cancer.

"Some studies of Asian populations have found that as soy intake increased, breast cancer risk decreased, but those studies couldn't prove a beneficial effect of soy - they could only identify an association," Shike told Rueters.

The study also pointed out that women in Asian countries were more at risk as their diet is known to contain high levels of tofu and soy. While the study showed overexpression of genes that promote tumor growth, it did not conclude if soy actually causes tumor growth. Also, researchers did not say if the effects of soy can be reversed.