Flaxseed consists of the seeds of the flax plant (Linum usitatissimum). Flaxseed oil is made by grinding and pressing flaxseed. This webpage includes information on the influence of flaxseed and flaxseed oil on breast cancer risk and survival, as well as the effects of consuming flaxseed or its oil during various types of breast cancer treatment.
Breast cancer-related effects of consuming flaxseed
Flaxseed components
Flaxseed is an excellent dietary source of alpha-linolenic acid (ALA), campesterol, ferulic acid, fiber, magnesium, manganese, phosphorus, and thiamin (vitamin B1). In addition, flaxseed is a very good source of zinc and can also incorporate significant levels of other metals and minerals such as cadmium, calcium, copper, iron and selenium, depending on the soil in which it is grown. Flaxseed's calcium content may help it prevent bone loss. Flaxseed also incorporates some folate and the B vitamins niacin (B3), pantothenic acid (B5), riboflavin (B2), and vitamin B6.
Flaxseed is the richest known source of plant lignans (mainly secoisolariciresinol), which are converted by intestinal microbiota to enterodiol and enterolactone. While enterolactone is a phytoestrogen, it has been shown to have antiestrogenic activity by binding competitively to the estrogen receptor, thereby reducing the breast cancer-promoting effects of estrogen.
Flaxseed cell studies
Flaxseed and flaxseed extracts have been shown to reduce breast cancer cell growth and promote breast cancer cell apoptosis (programmed cell death). Flaxseed can reduce the growth and proliferation of both ER- and ER+ breast cancer cells. Flaxseed has also been demonstrated to counteract the breast cancer promoting effects of the soy isoflavone genistein on ER+ breast cancer in mice. The chemopreventive effects of flaxseed appear to be derived primarily from its lignan and ALA content.
Flaxseed population studies
A meta-analysis combining the results of seven studies found that plant lignan consumption was associated with reduced risk of breast cancer in postmenopausal women. A major Swedish prospective study reported a reduction in the risk of breast cancer in postmenopausal women for those in the highest quartile of plant lignan intake, especially among hormone replacement therapy (HRT) users. The reduction in risk did not vary by the ER or PR status of the tumors. Another study reported that high circulating levels of enterolactone reduce the risk of breast cancer recurrence and death among postmenopausal women, especially those with ER- breast cancer.
Flaxseed can have relatively high levels of cadmium and copper
Cadmium uptake by flaxseed plants can be significant
Flaxseed can incorporate unacceptable levels of cadmium, a toxic heavy metal with estrogenic properties that has been reported to increase the risk of breast cancer. Mouse studies have demonstrated that significant levels of cadmium can be absorbed from dietary flaxseed. In addition, the female pups of pregnant or lactating rats fed flaxseed have increased susceptibility to developing mammary tumors.
High cadmium content of flaxseed is not as unlikely a problem as it may first appear. In addition to the U.S. and Canada, China and India are major flaxseed producers. Both China and India are known for heavy metal pollution in some regions. Various flax plant cultivars are grown for their fiber (to produce linen), and for seeds to produce inedible linseed oil for industrial applications (for example, as a pigment binder in paints), as well as to produce edible flaxseed and flaxseed oil (which is referred to as linseed oil in some countries). Farmers do not have to be concerned about heavy metal soil contamination when growing flax for linen or inedible linseed oil production. However, the market for flaxseed meant for human consumption has been increasing and is more lucrative. Therefore, it is possible that flaxseed from highly polluted areas may be reaching U.S. consumers.
In addition, some areas of Canada, which is the world's largest producer of flaxseed, have naturally high levels of cadmium in the soil. North and South Dakota, the two largest producing U.S. states, also are known to have soil containing cadmium. The organic label does not necessarily mean a low-cadmium product. Purchasing organic low-cadmium flaxseed makes sense.
Flaxseed consumption can contribute to excess copper intake
Flaxseed contains approximately 0.1 mg to 0.3 mg of copper per ounce. One 2024 study found that relatively high levels of copper in the urine of girls were associated with increased breast density, a breast cancer risk factor, two years after menarche. While not all studies agree that copper increases breast cancer risk, it does appear to increase the risk of recurrence. Copper has been shown to promote angiogenesis and metastasis, especially in aggressive forms of breast cancer such as inflammatory (IBC), triple negative (ER-/PR-/HER2-), and HER2 overexpressing (HER2+) breast cancer.
Although copper is a vital nutrient, women with breast cancer who are not anemic probably should not exceed the RDA (recommended dietary allowance) of approximately 0.9 mg. High copper foods such as calf's liver and beef liver should be avoided. Foods with moderate copper content, such as shellfish, textured soy protein, chocolate, most tree nuts, and sunflower seeds also should be limited or avoided. Copper consumption should be reserved for beneficial foods such as flaxseed and walnuts (which contain approximately 0.45 mg copper per ounce, but which have exceptional anti-breast cancer properties).
The potentially high cadmium and copper content of flaxseed are the reason we recommend consuming flaxseed in moderation.
Breast cancer-related effects of consuming flaxseed oil
Flaxseed oil does not have the significant B vitamin, fiber, or lignan content of flaxseed. Instead, it incorporates concentrated levels of the omega-3 fatty acid ALA found in flaxseed. Refined cold pressed flaxseed oil has almost no heavy metal content.
Alpha-linolenic acid (ALA)
Consumption of ALA is associated with reduced breast cancer risk. Flaxseed oil is among the best sources of this plant-derived omega-3 fatty acid. ALA is partially converted during metabolism into eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) (found in abundance in fatty fish), which have been reported to have chemopreventive effects. For example, these compounds have been shown to reduce lung metastasis in a mouse model of breast cancer and to inhibit the early stages of HER2+ breast cancer.
However, note that the conversion of ALA to EPA and DHA can be blocked in the presence of vegetable oils with high omega-6 fatty acid content (e.g., corn oil, safflower oil, soybean oil, and sunflower oil). Therefore, flaxseed and flaxseed oil should be consumed separately from such oils.
Flaxseed can enhance some breast cancer treatments
Enterolactone has been found to increase the sensitivity of breast cancer cells to radiation, thereby potentially enhancing the treatment effects of radiotherapy.
Flaxseed, flaxseed oil, and the lignan secoisolariciresinol each have been shown to reduce tumor size in combination with tamoxifen more than tamoxifen alone in animal models of breast cancer.
Flaxseed oil extract has been shown to increase the cytotoxic treatment effects of Adriamycin (doxorubicin) in ER+/PR+ and triple negative breast cancer cells without harming normal breast cells. Combining a flaxseed extract consisting of secoisolariciresinol and enterolactone with Adriamycin or Taxotere (docetaxel) has been demonstrated to enhanced the ability of both chemotherapy drugs to cause cytotoxicity in HER2+ and triple negative breast cancer cells.
Flaxseed has also been shown to enhance the effectiveness of Herceptin in HER2+ breast cancer cells and to reduce the potential heart damaging effects of Herceptin and Adriamycin.
Additional comments
Because of the high nutrient content of flaxseed, it is subject to spoilage and must be stored and handled properly. Make sure the market where you purchase flaxseed has frequent flaxseed product turnover. Check for moisture before buying flaxseed and discard any flaxseed stored at home if there is any evidence of moisture or a rancid smell. Whole flaxseed can safely be stored in an airtight container in a dry, dark, and cool place for several months. Ground flaxseed is also available; the nutrients in ground seeds are more easily absorbed. However, ground flaxseed is more subject to oxidation and spoilage than whole flaxseed and should be kept refrigerated.
Flaxseed oil should be stored refrigerated in an opaque container. It is best used at room temperature rather than in frying. In other words. flaxseed oil should be added to foods after they have been heated, if desired, or used in salad dressings or cold sauces prepared without heating. Flaxseed oil is used in stir-frying in parts of Asia, but this is not optimal. One study that examined the effects of using flaxseed oil in stir-frying found that pan-heating caused a significant loss of important nutrients, as well as increases in undesirable oxidation products.
Sources of information provided in this webpage
The information above, which is updated continually as new research becomes available, has been developed based solely on the results of academic studies. Clicking on any of the underlined terms will take you to its tag or webpage, which contain more extensive information.
Below are links to 20 recent studies concerning this food. For a more complete list, including less recent studies, please click on flaxseed.