For many people, flaxseed is a powerful “superfood.” But some experts have suggested that certain people, particularly women who have a history of certain cancers or are considered high risk for developing hormone-related tumors, should avoid them.
Flaxseeds contain phytoestrogens, plant chemicals that actually mimic estrogen in the body. However, not all phytoestrogens are the same. There are two major types of phytoestrogens: isoflavones and lignans. Lignans are the phytoestrogens found in the fiber portion of flaxseeds.
Because of the estrogen-like activity of lignans in flaxseeds, scientists aren’t sure whether they’re harmful or helpful for preventing such hormonal cancers as breast cancer. This is because phytoestrogens act like estrogen and may affect the production and/or breakdown of estrogen by the body, and they affect the levels of estrogen carried in the bloodstream.
On the contrary, some research has demonstrated that phytoestrogens don’t mimic estrogen at all. Rather they block estrogen and can even prevent the formation of blood vessels to tumors.
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So, do phytoestrogens cause cancer or not? The answer depends on the type of cancer you’re dealing with.
Many epidemiological studies have been performed to determine whether phytoestrogens raise or lower the risk of developing breast, ovarian, and uterine cancers, since these types of cancers are typically estrogen-driven. Overall results of these studies have been inconclusive. Some research found a decrease in risk with higher phytoestrogen intake, while other research suggests that for certain people, the risks of consuming these “fake” estrogens may outweigh the benefits.
Who Should Avoid Flaxseeds?
If you or your loved one has a history of any of the following conditions, then substitute fish oil in place of flax to get your daily dosage of healthy omega-3 fatty acids.
- Women who have a history of breast, ovarian, or uterine cancer, especially estrogen-receptive breast cancer. In addition to their ability to mimic estrogen, phytoestrogens have been shown to cause growth of breast tissue in animals and healthy women, so they are not recommended for breast cancer survivors.
- Women who have a BRCA1 or BRCA2 gene defect. Women with one of these defects have up to an 80 percent chance of getting breast cancer sometime during their life; therefore, these women should avoid phytoestrogens including flaxseeds and soy.
- Women with a history of endometriosis or polycystic ovarian syndrome. Again, these are hormonal-driven conditions, so phytoestrogens are not recommended.
- Women who are taking birth control pills or any type of hormone replacement therapy. Both birth control pills and HRT contain estrogen, so phytoestrogens might disrupt or amplify the effect of the estrogen.
- Women who are pregnant or breastfeeding. Pregnant or breastfeeding women should not consume substantial amounts of flaxseeds on a regular basis. In fact, in animal studies, the phytoestrogens in flaxseeds have been shown to cause developmental abnormalities and have been linked to an increase in susceptibility of cancer in offspring.
- Infants and young children. Phytoestrogens are not recommended for young children; use fish oil in place of flax.
- Adolescent girls and young women under 30. Adolescent girls and young women under 30 can consume flax on a limited basis (one to two times per week), but should not have flax daily.
Who Should Take Flaxseeds?
It is important to remember that the healthy omega-3 fat from flaxseeds is alpha linolenic acid (or ALA), a type of omega-3 that must be converted by the body to the highly beneficial fatty acids (EPA and DHA) found in salmon and other fatty cold-water fish.
Because the ALA conversion to EPA and (especially) DHA is very poor in most people, it is best not to rely solely on flax for your total omega-3 intake.
Aside from those “Who Should Avoid Flax” in the aforementioned groups, flaxseed consumption can be healthy, particularly for:
- Men who have a history of prostate disease, including prostate cancer. A study performed by researchers at the University of Texas M.D. Anderson Cancer Center found that men with prostate cancer who ate 3 tablespoons of milled or ground flaxseeds each day had decreased prostate cancer cell proliferation compared to similar men who did not eat flaxseeds. According to the American Cancer Society, men who supplement their diets with flaxseed have lower PSA levels and slower growth of benign as well as cancerous prostate cells.
- Postmenopausal women or women who are estrogen-deficient. Postmenopausal women (with no personal or familial history of hormonal cancers) can benefit from flaxseed consumption, particularly to help with menopausal symptoms and preventing or reversing osteoporosis.
- Men or women with a history of diabetes, high cholesterol, or obesity. As mentioned in the previous article, flaxseeds have been shown to help control blood sugar levels, decrease LDL cholesterol, and promote weight loss.
How Do I Take Flaxseeds or Oil?
- Flaxseeds: Use whole flaxseeds and grind them immediately prior to use with a coffee grinder or similar grinder. The whole flaxseeds ground up this way are recommended over liquid flaxseed oil because you get the fiber benefit that the liquid doesn’t provide. Whole flaxseeds are stable and have a reasonably long shelf life but ground flaxseed and flaxseed oils are very prone to rancidity and must be refrigerated. Flaxseeds are also available pre-ground in a special mylar package so that the components in the flaxseeds stay active and do not oxidize. Adults can take 1 tablespoon of flaxseeds two to three times daily or 2 to 4 tablespoons one time per day.
- Oils and capsules: Flaxseed oil contains approximately 7 grams of ALA per 15 mL. Adults can take 1 to 2 tablespoons daily or 1 to 2 capsules daily. Flaxseed oil and capsules should be refrigerated.
- Do not eat raw or unripe flaxseeds as they may be poisonous.
So, do the flaxseed benefits outweigh the risks for you? For many people they do, but now you’ll know exactly how to determine that for yourself.
 “Flaxseed Supplementation (Not Dietary Fat Restriction) Reduces Prostate Cancer Proliferation Rates in Men Presurgery.” Cancer, Epidemiology, Biomarkers & Prevention. December 2008 17; 3577.
 Cornell Univeristy.
 American Cancer Society.
 Duffy, Christine, et. al., “Implications of Phytoestrogen Intake for Breast Cancer,” CA: A Cancer Journal for Clinicians, September 2007 (57:260-77).
This article was originally published in 2012 and has been updated.