To Soy or Not to Soy for Breast Cancer Patients?

New soy research may be causing unnecessary worry among breast cancer patients – for example, even hesitating to eat tofu at their next meal.
It had seemed that questions of whether phytoestrogens — a plant chemical that can sometimes have estrogen-like effects — in soy might stimulate breast cancer growth had been resolved. Over the past few years, a growing consensus – drawn from five studies of both Asian and North American women with breast cancer – found that consuming soy after a breast cancer diagnosis actually reduced the risk of recurrence. But a new study stirred up new concern for those with breast cancer.
The issues raised: Is it safe to start eating soy after getting breast cancer? And does it benefit treatment? Or does soy only help prevent breast cancer and lower the risk of recurrence if it’s eaten routinely during childhood and adolescence?


With these questions in mind, the scientists designed a study in which lab rats were given a carcinogen that causes breast cancer, then fed genistein — a soy phytoestrogen. One key factor in the study? The rats were given a genistein dosage that was similar to an amount of what a woman would need to consume by eating 2 to 4 daily servings of soy products like tofu or tempeh. Although all the rats were given tamoxifen (a breast cancer drug that stops estrogen activity), the researchers tested three groups of rats: a group given genistein their entire lives, a group given it at specific points in their lives, and another that only got genistein after breast cancer developed.
The rats fed soy their whole lives were less likely to have tamoxifen-resistant tumors, and less likely to have recurrent tumors post-tamoxifen treatment than those rats given soy only after developing cancer. And the rats given soy before puberty or before developing cancer were also less likely to have recurrence rates. Based on this study, the indication seems to be that eating soy after developing breast cancer won’t have any benefit, and might even interfere with tamoxifen treatment.
But that’s not the whole story of soy and breast cancer – and rats are not breast cancer patients. As the authors of the study point out, we need to investigate whether the results hold up in humans.
One question worth asking: What benefits does soy have for people? Is soy simply a source of isoflavones – phytoestrogenic properties like genistein? Evidence suggests soy offers more than that. Soy foods can help lower risks of heart disease, which now kills more breast cancer patients than the cancer itself. Additionally, soy is a plant-based alternative for higher caloric and less healthy proteins, and may be worth eating in spite of the genistein content. If so, soy may on balance be healthful for breast cancer patients. Of course, this is not the case for those with soy sensitivities who should avoid these foods despite their possible benefits.
Another key issue to consider is dosage. As mentioned earlier, to consume the amount of genistein given to the rats in the study you would need to eat 2 to 4 servings of soy foods every day. That’s a lot soy for women in the US and definitely contradicts our advice. Our recommendation for woman with ER(+) tumors has always been to eat soy no more than 2 to 3 times per week, and as only one plant-based protein in varied menus. This would result in a much lower genistein intake than the rat study. Furthermore, processed soy foods – such as soy burgers – typically contain very low amounts of genistein, and other soy items such as miso and soy sauce contain even less. In fact, a whole bottle of soy sauce contains only about 6 or 7 milligrams of isoflavones, which would have no effect at all on breast cells — though it would be quite a lot of salt!
A very different issue is soy isoflavone supplements. Studies on isoflavone supplements given to cancer-free women have shown that they don’t increase breast density or breast cell proliferation – both of which are risk factors for actual breast cancer. Still, we feel follow-up evidence showing safety for breast cancer patients must be demonstrated before we can comfortably recommend these concentrated isoflavones – in pills and capsules — for breast cancer, or even with other hormone-driven malignancies such as ovarian.
So no need to pass on the tofu or tempeh – as long you’re eating soy foods in moderation.
If you have questions about the use of soy in your diet, or other vegetarian protein sources, a consultation at the Block Center may help you sort out the alternatives. Call us at 847-492-3040 for an appointment.

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