A high-soy diet may lead to the "overexpression" of breast cancer tumors.

Overexpression means the effects of the tumor and cancer manifest more strongly.

Some studies have prompted women, especially Asians, to increase their soy intake because their risk for breast cancer would be greatly reduced.

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Although previous studies could find a link between soy and breast cancer, they couldn't definitely prove a person could benefit from soy, said a new study led by Dr. Moshe Shike of Memorial Sloane Kettering Cancer Center in New York.

The team of researchers then took a look at how a high-soy diet affects those who already have breast cancer.

Soybeans contain phytoestrogens, which weakly mimic the effects of estrogen. Estrogen isn't good when one is diagnosed with breast cancer, because most breast cancers are estrogen-receptor positive, said V. Craig Jordan of the Georgetown University Lombardi Comprehensive Cancer Center in Washington, D.C

Being estrogen-receptor positive means tumors can possibly respond to estrogen signals in the body. So those with breast cancer are asked to avoid hormones and are usually given anti-estrogenic drugs.

Dr. Shike's study, supported by the Breast Cancer Research Foundation, observed how the molecular structure of breast cancer cells changed during the early stages: first from biopsies and next from when the tumor is removed.

Some 140 women participated in the study and were divided into two groups. One group took soy or milk protein supplements while the other group took water or juice.

What Dr. Shike and his team found was the tumors in some members of the milk group "overexpressed."

Shike said they still didn't know what it was about soy that caused this, however.

"All we can say is that in some of the women, the soy component drove proliferative genes, at the gene level. How it would translate into cancer outcome we can't say," he said.

"What we recommend is moderation, meaning we think that taking excessive amounts of soy may not be a good idea. We don't have clinical proof for that because this is only gene expression."