Women who begin eating soy as adults could risk developing breast cancer tumors that are more resistant to treatment, according to a new study.

The study, presented at a medical conference in Chicago, examined female rats that were fed soy isoflavone genistein at various points in their lifetime and exposed to a substance that triggered breast cancer tumors responded to tamoxifen, a popular treatment for breast cancer patients.

The findings show that rats that were fed the soy compound all their lives responded well to tamoxifen, whereas those that only began eating soy as adults, and after they developed breast cancer, were tumors that were resistant to treatment.

"Genistein intake in adult life which continues during tamoxifen treatment appears to make the tumors resistant to tamoxifen," said senior author Leena Hilakivi-Clarke, professor of oncology at Georgetown, in a statement released on Monday. "However, if animals were fed genistein during childhood, and intake continues before and after tumors develop, the tumors are highly sensitive to the tamoxifen," she explains.

Scientists from Georgetown University said that the latest findings may explain why tamoxifen sometimes stops working and allow tumors to regrow again in some women.

"These results suggest that Western women who started soy intake as adults, should stop if diagnosed with breast cancer," said Hilakivi-Clarke.

Soy contains isoflavones, an estrogen-like compound that is considered a healthy protein source that can be found in foods like tofu, miso, soy beans and soy milk, and its potential benefits against breast cancer have been associated to the lower rates of hormone receptor positive types of breast cancer seen in Asian women living in parts of the world where soy is often eaten.

Researchers explain that because tamoxifen is typically given to breast cancer patients with estrogen receptor and/or progesterone receptor positive types of tumors, the latest findings suggest that late-life adoption of soy consumption may have made the drug impotent.

The study was presented at the American Association for Cancer Research (AACR) Annual Meeting.