Flaxseeds and Breast Cancer
Should patients with Estrogen Receptor Positive (ER+) breast cancer avoid flaxseed due to the potential “phytoestrogenic” role of lignans?
To understand potential effects of flaxseed and how they may affect breast cancer, it helps to understand what flaxseeds are, and why some consider them a concern for women with a history of breast cancer.
Flaxseed and phytoestrogens
Flaxseed is the richest dietary source of lignans, a type of phytoestrogen. A phytoestrogen is a plant nutrient that is somewhat similar to the female hormone estrogen. Due to this similarity, lignans may have estrogenic and/or anti-estrogenic effects in the body. Lignans are the nutrients that are at the center of the controversy regarding whether it is safe for women with breast cancer to eat flaxseeds.
Phytoestrogens are found in a variety of foods, including soy, flaxseeds, other nuts and seeds, whole grains, and some vegetables and fruit. Most of the research regarding flaxseed and breast cancer focuses on the lignans found in flaxseeds, and their potential for weak estrogenic or anti-estrogenic effects in a woman’s body.
Phytoestrogens and breast cancer growth
Phytoestrogens are somewhat similar to human estrogen, and some health experts have speculated that phytoestrogens might even act like human estrogen in the body. This suggestion has raised concerns about whether phytoestrogens may not be safe for people with a history of hormone-linked cancers, such as prostate cancer, endometrial cancer, or ER positive breast cancer.
Lignans, which are the type of phytoestrogens in flaxseed, can change estrogen metabolism. In postmenopausal women, lignans can cause the body to produce less active forms of estrogen. This is believed to potentially reduce breast cancer risk. There is evidence that adding ground flaxseeds into the diet decreases cell growth in breast tissue as well. Again, this would be the type of change that would be expected to decrease breast cancer risk.
All cells have the ability to go through a process called apoptosis, or programmed cell death. It is believed that through this process, the body can prevent damaged cells from reproducing, and eventually developing into cancer. Researchers have shown that flaxseed sprouts can increase apoptosis (programmed cell death). Some cell and animal studies have shown that two specific phytoestrogens found in lignans, named enterolactone and enterodiol, may help suppress breast tumor growth.
Animal studies have shown that both flaxseed oil and lignans can reduce breast tumor growth and spread, even for ER- cancer cells. This result suggests that flaxseeds may have anti-cancer benefits that are unrelated to any type of effect on estrogen or estrogen metabolism.
Phytoestrogens and breast cancer treatment
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
One study in mice concluded that flaxseed inhibited the growth of human estrogen-dependent breast cancer, and strengthened the tumor-inhibitory effect of tamoxifen. Multiple other studies with mice have shown that dietary flaxseed works with tamoxifen to inhibit breast tumor growth.
Researchers don’t yet know if these results will apply to women with breast cancer, but this approach—adding flaxseeds to the diet—looks promising. And several studies in women have shown that higher intake of lignans, the key phytoestrogen in flaxseeds, is associated with reduced risk of breast cancer.
Further, lignans in the diet are associated with less aggressive tumor characteristics in women who have been diagnosed with breast cancer. In other words, women who have already been eating lignans at the time of diagnosis seem to have tumors that are less aggressive.
If you plan to add flaxseeds into your nutrition plan, please talk to your doctor or dietitian first, to make sure this is a good choice for you.
While research has shown some benefits with regards to ER+ breast cancer cell death and prevention of metastases within mice and cellular models, it is recommended that human intake should be through diet only, not supplementation. Only moderate amounts of ground flaxseeds, up to two to three tablespoons per day at most, should be eaten.
Always consult your health care team prior to making any changes to your diet or the dietary supplements you are using.
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 Amy DiCioccio, RD, CSO, CD-N on behalf of the ON-DPG
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