Soy Foods, Diet, and Tamoxifen
Are there any foods I should avoid while taking tamoxifen? What about soy foods and flaxseeds?
What is tamoxifen?
Tamoxifen is a medication known as a selective estrogen receptor modulator, or SERM. Tamoxifen often is prescribed as part of the treatment for ER+ breast cancer. Tamoxifen binds with estrogen receptors, without activating growth in breast cancer cells. In this way, tamoxifen prevents a women’s own estrogen from binding with these cells. As a result, breast cancer cell growth is blocked.
Why is there concern over food and tamoxifen?
It is smart to have concerns about things that you eat, or take, such as dietary supplements, that may lessen the effectiveness of your medications. Tamoxifen is no different. You certainly don’t want to take a medication to reduce breast cancer risk, only to eat or drink something that renders the tamoxifen less effective.
The foods of most concern for women taking tamoxifen are grapefruit and tangerines. Grapefruit is well-known to interfere with numerous medications. Many drug-interaction resources specifically advise that women taking tamoxifen avoid grapefruit. For this reason, you shouldn’t drink grapefruit juice or regularly eat whole grapefruit if you are taking tamoxifen or other medications to reduce your breast cancer risk.
Tangerines are less of a problem, though to be on the safe side, you probably should not eat large quantities of them regularly. Tangeretin, a nutrient found in abundance in tangerine peels, can lessen the effectiveness of tamoxifen. The flesh of the fruit contains much less tangeretin than the peels, so eating moderate amounts of the fruit is unlikely to cause a significant lessening of tamoxifen’s effectiveness.
Confusion about the health effects of soy foods arises from the term “phytoestrogens.” Some soy nutrients—the isoflavones—have chemical structures that look a bit like the estrogen found in a woman’s body. This is where the term phytoestrogen originated. However, phytoestrogens are not the same thing as female estrogens. Soy foods do not contain estrogen.
Because of the concern around the “phytoestrogens” in soy foods, some people have recommended that women taking tamoxifen should avoid soy foods, because these foods might “undo” the estrogen blocking effects of the medication. Interestingly, the research supports the opposite conclusion—soy foods appear to enhance or improve the breast cancer blocking actions of tamoxifen.
Cell and animal studies have shown that adding soy food nutrients to tamoxifen inhibits the growth of estrogen receptor positive (ER+) breast cancer cells. Even more encouraging are numerous studies showing that women who regularly eat soy foods have a lower risk of breast cancer and breast cancer recurrence. This is true for both Asian and American women, and for women who ate soy foods early in life and for women who added them into their diet as adults. One study from Korea found that even for women who had a BRCA 1 or 2 mutation, eating soy foods appears to reduce the risk of breast cancer.
One research review considered a total of 131 articles, including 40 randomized controlled trials, 11 uncontrolled trials, and 80 observational studies. This study supports soy’s safety for women with a history of breast cancer, noting “Soy consumption may be associated with reduced risk of breast cancer incidence, recurrence, and mortality. Soy does not have estrogenic effects in humans. Soy intake consistent with a traditional Japanese diet appears safe for breast cancer survivors.” In most of these studies, a significant proportion of the study participants were taking tamoxifen.
If you would like to eat soy foods, it appears that this is safe, and potentially beneficial. However, if you don't enjoy soy foods, there is no reason that you have to eat them. At this time, there isn’t enough information about soy supplements, such as concentrated isoflavones, to know if they are safe. You should not take concentrated soy supplements.
As with soy foods, many women have concerns about flaxseeds, because they too contain nutrients that are referred to as “phytoestrogens.” The “phytoestrogen” nutrients in flax are called lignans.
There is much less research on flaxseeds, lignans, and tamoxifen. What we do know suggests that flaxseeds are safe for women taking tamoxifen. Cell and animal studies support that flaxseed lignans can significantly decrease the growth of breast cancer cells.
There is one controlled trial of flaxseeds in women with breast cancer. Researchers looked at cancer cells from breast biopsies of women who were diagnosed with breast cancer. Half of these women were randomly selected to eat muffins containing flaxseeds, while the other half ate a “control” (no flaxseed) muffin. Researchers then were able to collect breast tumor cells again, from the mastectomies and lumpectomies that these women had as part of their treatment.
In this way, the researchers could look at how the cancer cells were “behaving” before and after the women ate flaxseeds. Compared with cells from the women eating the control muffins, the breast cancer cells from the women who ate flaxseeds had a lower rate of tumor cell proliferation (growth), a higher rate of apoptosis (cancer cell death), and lower expression of c-erbB2, which is a gene that is associated with more aggressive types of breast cancer. Estrogen and progesterone receptor levels on the cancer cells did not change in either group, meaning that the flaxseeds did not affect these receptor levels.
Finally, one research review considered 2 randomized controlled trials, 2 uncontrolled trials, 1 biomarker study, and 5 observational studies. This study supports the safety of flaxseeds for women with a history of breast cancer, noting, “Current evidence suggests that flax may be associated with decreased risk of breast cancer. Flax demonstrates antiproliferative effects in breast tissue of women at risk of breast cancer and may protect against primary breast cancer. Mortality risk may also be reduced among those living with breast cancer.”
As with soy foods, flaxseeds appear to be safe, and potentially beneficial for women with a history of breast cancer. If you would like to eat flaxseeds, you should feel good about doing so. However, if you don't enjoy flaxseeds, there is no reason that you have to eat them. At this time, there isn’t enough information about flaxseed supplements, such as concentrated lignans, to know if they are safe. You should not take concentrated flax supplements.
The original question and answer were generously donated by Diana Dyer, MS, RD a cancer survivor, registered dietitian, organic garlic farmer, and the author of "A Dietitian's Cancer Story: Information & Inspiration for Recovery & Healing from a 3-time Cancer Survivor.
Question and Answer updated by Suzanne Dixon, MPH, MS, RD on behalf of the ON DPG
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Page updated: January 2014