Long-term consumption of genistein, an isoflavone produced from soybeans, has been found to have a protective effect against breast cancer, and now we know why. The research is the latest in a long debate about the benefits and risks of a high-soy diet for cancer.
Soy and fava beans contain genistin, which our digestive system converts to genistein—whose molecular structure resembles estrogen. Studies have shown that genistein can stimulate the growth of estrogen-positive breast cancer cells, leading doctors to tell women with breast cancer to stay away from soy foods.
On the other hand, some studies have found women with high-soy diets have lower breast cancer recurrence. Professor Leena Hilakivi-Clarke of Georgetown University found that rats with a long-term high-genistein diet showed a stronger response to anti-estrogen treatment than controls with the same genetics but a different diet. Moreover, they were less likely to experience a cancer relapse.
Since soy is so popular as a meat substitute, the topic has become highly politicized, with supporters and opponents of vegetarian diets brandishing contrasting research.
In an effort to explain this apparent contradiction, Hilakivi-Clarke investigated the immune response of rats fed genistein. She found that the T-cells of the genistein-eating rats were already primed to attack the tumor before they were given tamoxifen, the drug given for estrogen-positive breast cancer. The T-cells responded more quickly and were able to get around the mechanisms the cancer used to evade attacks.
Presenting on Sunday at the American Association for Cancer Research annual conference, Hilakivi-Clarke said, "Our results suggest that genistein's ability to activate anti-tumor immune responses and reduce expression of immunosuppressive mechanisms may explain why lifetime genistein intake reduces risk of breast cancer recurrence."
However, she thinks that it is not as simple as oncologists reversing their advice for breast cancer patients to avoid soy. “It is critical that genistein is consumed well before a tumor develops to program the tumor to exhibit good immune responses," said Xiyuan Zhang, a doctoral student working in Hilakivi-Clarke's lab and lead author of the presentation.
Humans and rodents metabolize soy differently, making application to humans difficult. However, epidemiological studies suggest a protective effect from 10 milligrams of isoflavones a day, which is equivalent to a third of a cup of soy milk. There is evidence that genistein also has a protective effect against other cancers, including brain, colon, and for men prostate.
The same resemblence to the estrogen is thought to be the reason studies have found high-soy intake reduces the symptoms of menopause and premenstrual syndrome, although the latter in particular remains debated.