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Frequently Asked Questions


I am currently taking tamoxifen. My daughter heard on TV that I should be eating soy to make it work more efficiently. Do you know of any studies verifying this information?

You are the very first person to write with this question!

Yes, I am familiar with two well-designed research studies that have shown increased benefit for rats with induced ER+ breast cancer when given both tamoxifen and soy together. That means fewer breast tumors, smaller tumors, even regression of breast tumors in one study for the rats fed both tamoxifen and a soy food called miso. I will give a short description of each of the studies below.

The first study by Gotoh et al., published in the Japanese Journal of Cancer Research 89(5):487 98, 1998, is a rat study in which all of the rats were given a chemical which induces ER+ breast cancer. The rats were then observed for their breast cancer (ER+) development (this is a "prevention" study) while receiving one of 4 different treatments in addition to their baseline diet:

(1) a control group which received no further treatments,
(2) a group which only received Tamoxifen,
(3) a group which only received miso (a soybean paste commonly eaten daily in Japan),
(4) and a group which received both Tamoxifen and miso.

The rats given Tamoxifen and miso together had a response that showed dramatically fewer numbers of tumors per rat and also decreased size of tumors compared to all of the other treatment groups. The reduction observed was actually synergistic (ie 1+1=3), meaning that combining the miso and tamoxifen together produced an even larger response than by adding the % reductions seen in the groups that received either tamoxifen and miso alone when comparing the results to the control rats which received no tamoxifen or miso.

Reported in this same research paper by Gotoh, a different experimental rat study observed rats that already had ER+ breast cancer. These rats were given tamoxifen or tamoxifen + miso together. These rats then were compared to rats given nothing by measuring the growth of their breast cancer tumors after a defined period of time. This experiment showed that the rats who received the combination of Tamoxifen + miso were the only treatment group that actually had their breast tumors decrease in size instead of continuing to grow as did the tumors in the other 2 groups. (This study can be found on PubMed website by typing in the search box miso + tamoxifen.)

The second study entitled "Consumption of Soy Products May Enhance the Breast Cancer Preventive Effects of Tamoxifen" by Andreas I. Constantinou. et al,. at the University of Illinois, Chicago, IL. was recently presented at The American Association for Cancer Research. The abstract can be found at the AACR website.

Four groups of 20 female rats were treated with an agent (DMBA) that produces ER+ breast cancer plus the following test agents fed into their usual diet:

1) no treatment (DMBA control);
2) tamoxifen
3) soy protein isolate (SPI)
4) tamoxifen + SPI.
5) A fifth group of 10 female rats was fed the basal diet without being exposed to DMBA.

Tamoxifen was effective in reducing mammary tumors per rat by 29%. A 37% reduction in tumors/rat was evident in the group that was fed SPI, and a 62% reduction in the group that was fed tamoxifen + SPI (this shows an additive effect - ie 1+1=2). The length of time it took for tumors to develop was significantly increased only in the tamoxifen + SPI group. None of these treatments showed any effect in the rate of animal growth or caused any toxicity.

These two studies clearly showed increased benefit to the rats given tamoxifen plus a soy food (miso or isolated soy protein) over the rats given tamoxifen or soy alone when compared to the control rats. Both studies showed clear preventive advantages to this combination in rats and the Gotoh study showed benefit to rats in treatment for breast cancer as well.

Please note that extrapolating conclusions from rat experiments to humans is not perfect science. It is not known at this time if women with breast cancer who are taking Tamoxifen will have this same response when consuming soy with Tamoxifen, as rat science is not always translatable to humans. It is also not known if women taking Tamoxifen as a preventive agent should be consuming soyfoods to enhance that effect expected from Tamoxifen alone. A recommendation cannot be made until this combination is tested in humans. However, these results looks promising. I consumed 1-3 soy foods per day the entire 5 years that I was on Tamoxifen after my 1995 breast cancer (Oct 95-Oct 00) without a recurrence of my advanced and aggressive ER+ breast cancer during that time.

Please consult with your personal health care professionals before making any decisions about your choices of therapy and nutritional intake. I recommend that you ask your physician and dietitian at your own cancer center for their recommendations. Feel free to share these references. Also ask for additional references to research that may be even more current than these as you gather information in order to make as informed decision as possible in this world in which all research data is not yet available for cancer survivors.

faq posted 5/01, updated 1/05


Back to Main QandA Page

Breast Cancer

Breast Cancer Is soy safe to consume for women with ER+ breast cancer? posted 4/01, updated 1/05
Breast Cancer Is there any new information for breast cancer patients who have ER+ tumors regarding the safety of eating soy foods? posted 2/02
Breast Cancer I am currently taking Tamoxifen. My daughter heard on TV that I should be eating soy to make it work more efficiently. Do you know of any studies verifying this information? posted 5/01, updated 1/05
Breast Cancer I have read conflicting reports on the benefits vs. the dangers of Tamoxifen for breast cancer treatment. Have you found better nutritional alternatives?
Breast Cancer Should women with ER+ breast cancer avoid flaxseeds and flax oil that contain the phytoestrogens called lignans?
Breast Cancer Is weight gain common during breast cancer treatments? posted 8/00
Breast Cancer What are some good breast cancer web sites? posted 4/04
Breast Cancer Are there any foods I should avoid while taking Tamoxifen? posted 6/04
Breast Cancer Does moderate alcohol consumption have a harmful effect on persons who have had breast cancer? posted 11/04
Breast Cancer Should I be avoiding all caffeine to help fight my breast cancer?
posted 11/04


These questions and answers are intended to be of a general informative nature. Please consult with the Registered Dietitian in your cancer center or your health care provider for nutritional advice that can be individualized to your specific medical condition.

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