I only encountered positive evidence about flaxseed when I was doing research for my book, and I would never recommend any foods I thought could be harmful, so I decided to look into why some are advising that it be avoided. Like most plant foods, flaxseed contains phytoestrogens, which have weak estrogen-like effects in the body. However, they do not raise the level of estrogen in the blood; in fact, they compete with estrogen for receptor sites on the cells and can reduce the levels of the most biologically active estrogen, estradiol(1). They also compete for those receptor sites with xenoestrogens, chemicals that have estrogen-like activity and are linked to increased incidence of reproductive dysfunction, cancer, and obesity(2). The pesticide DDT and Bisphenol A (BPA), a chemical found in plastic and epoxy products such as tin cans, are well-known xenoestrogens.
There are various types of phytoestrogens, including isoflavones, coumestans, and lignans. Soy contains isoflavones. There is both positive and negative evidence regarding the effects of soy consumption, and I think it's a complicated issue. Flaxseed contains lignans, which have both weak estrogenic effects and antiestrogenic properties. It's extremely low in net (digestible) carbs because the carbohydrates it contains are almost entirely fiber.
Concerns About Flaxseed
A Google search of "problems with flaxseed" led me to a site that cited research suggesting flaxseed intake was associated with increased risk of prostate cancer (3). However, this meta-analysis of several studies did not implicate flaxseed but rather high intake of the n-3 fatty acid alpha-lineolenic acid (ALA), which the authors concluded may have come from many different food sources. There are other studies which found that flax consumption may help prevent and serve as an adjunct to treating prostate cancer (4, 5).
Another concern was that flaxseed may have feminizing effects in men because of its high phytoestrogen content. I found no evidence for this, and it would seem unlikely given that lignans have only weak estrogen effects as well as being estrogen antagonists.
I've also read flaxseed quoted as being "banned for human consumption" in Europe. Interestingly, I only found this information on websites that sell chia seeds. One of the claims is that flaxseed contains "anti-nutritional factors"; however, these or similar factors are found in virtually all the plants we eat, including chia seeds. I consume both chia and flax, but at this point we have much more research about flax.
When I looked further into flaxseed bans in Europe, I found that this ban was only for:
1. flax oil (like all polyunsaturated oils, it may be harmful when heated to high temperatures)
2. GMO flaxseed or flaxseed contaminated by GMO varieties, including a case regarding flax imported from Canada.
I recommend using organic flaxseed, whenever possible, and currently all flax available for purchase is non-GMO even if it isn't organic.
A study on pregnant rats found that providing 5% or 10% of their diet as flaxseed had no effect on maternal health; however, their offspring had lower birth weights and later experienced early puberty and reproductive issues that did not occur in control rats(6). The authors of the study made the following conclusion: "Thus, caution is suggested when consuming flaxseed at high doses during the hormone-sensitive periods of pregnancy and lactation." (Note that "high doses" wasn't defined.) However, for healthy non-pregnant women and men without bleeding disorders (flax may make blood less likely to clot), it appears safe to consume and may provide several benefits.
Benefits of Flaxseed
Improvement in Breast Cancer Outcomes and Risk: There is a growing body of research suggesting that flaxseed is protective against breast cancer (7,8). A 2013 review of 10 studies, including 2 randomized controlled trials, found that flaxseed supplementation significantly improved symptoms and outcomes in women undergoing breast cancer treatment with Tamoxifen and reduced the risk of developing breast cancer by 30% (8).
Decreased Risk of Fibroids: Although concerns have been raised about the potential for phytoestrogens to fuel the growth of uterine fibroid tumors, high lignan intake is associated with lower risk of these growths (9).
Improvements in Cholesterol Levels and Obesity: When 25 young adult subjects with hyperlipidemia were given 30 grams of powdered flaxseed every day for 3 months, they experienced a significant lowering of LDL and VLDL cholesterol and an increase in HDL cholesterol and triglycerides, along with reductions in body weight and blood pressure, compared to the 25 controls (10). In a more recent, slightly larger study of 70 people with preexisting hyperlipidema, all biomarkers other than HDL improved in the group given the same 30-gram dosage daily for 40 days (11). A study of older adults with type 2 diabetes found reductions in waist circumference in the participants receiving a flax-lignan complex (12).
Reduced Inflammation: In a small study of men at risk for cardiovascular disease, the experimental group received a low-carbohydrate (32%) diet supplemented with 60 grams of flaxseed powder, and the control group followed a low-carbohydrate (35%) diet supplemented with 60 grams of raw rice powder. Although both groups experienced weight loss and improvements in blood pressure and cholesterol, only the flaxseed group had a decrease in the inflammatory markers c-reactive protein (CRP) and tumor necrosis factor (TNF), as well as a reduction in triglycerides (13).
Improved Skin Quality: People who consume flaxseed may experience younger-looking skin. According to one study, women who supplemented their diet with flaxseed for 12 weeks experienced smoother skin, decreased redness, and improved hydration (14).
These are just a few of many studies I found reporting favorable effects of flaxseed consumption on a variety of health issues. Flaxseed can also help improve your ratio of n3:n-6 fatty acids.
Is There an Upper Limit for Flaxseed Consumption?
Although there haven't been many studies on high flaxseed intake, at this point I would recommend limiting consumption to about 2 tablespoons per day, which provides ~45 mg lignans (300 mg lignans per 100 grams of ground flaxseed). This is just under the 50 mg/day amount suggested to improve breast cancer outcomes and risk (15). Only ground flaxseed can be absorbed; whole flaxseed will simply pass through your system. To maximize nutrition benefits, grind whole seeds in a food processor, blender, or coffee grinder, and make sure to store them in the refrigerator or freezer to prevent rancidity.
I'm not suggesting that you eat flaxseed if you truly don't like it or if it doesn't agree with you for whatever reason. But the evidence doesn't warrant avoiding it; indeed, the available research appears to support its use, in modest quantities, for therapeutic purposes, reduction of cancer risk, and overall health.
1. Brooks JD, et al. Supplementation with flaxseed alters estrogen metabolism in postmenopausal women to a greater extent than does supplementation with an equal amount of soy. Am J Clin Nutr. 2004 Feb;79(2);318-25
2. Patisaul HB, et al. The pros and cons of phytoestrogens. Front Neuroendocrinol. 2010;31(4):400-419
3. Brouwer IA, et al. Dietary alpha-linolenic acid is associated with reduced risk of fatal coronary heart disease, but increased prostate cancer risk: a meta-analysis. J Nutr. 2004 Apr;134(4):919-22
4. McCann MJ, et al. Role of mammalian lignans in the prevention and treatment of prostate cancer. Nutr Cancer. 2005;52(1):1-14
5. Demark-Wahnefried W, et al. Flaxseed supplementation (not dietary fat restriction) reduces prostate cancer proliferation rates in men presurgery. Cancer Epidemiol Biomarkers Prev. 2008;17(12):3577-3587
6. Tou JC, et al. Flaxseed and its lignan precursor, secoisolariciresinol diglycoside, affect pregnancy outcome and reproductive development in rats. J Nutr. 1998 Nov;128(11):1861-8.
7. Bilal I, et al. Phytoestrogens and prevention of breast cancer: The contentious debate. World J Clin Oncol. 2014;5(4):705-712
8. Flower G, et al. Flax and breast cancer: a systematic review. Integr Cancer Ther. 2013 Sep 8;13(3):181-192
9. Atkinson C, et al. Lignan and isoflavone excretion in relation to uterine fibroids: a case-control study of young to middle-aged women in the United States. Am J Clin Nutr. 2006 Sep;84(3);587-93
10. Saxena S, et al. Evaluation of flaxseed formulation as a potential therapeutic agent in mitigation of dyslipidemia. Biomed J. 2014 Nov-Dec;37(6):386-90
11. Torkan M, et al. Effect of Flaxseed on Blood Lipid Level in Hyperlipidemic Patients. Rev Recent Clin Trials. 2015 Jan 21. [Epub ahead of print]
12. Barre DE, et al. Flaxseed lignan complex administration in older human type 2 diabetics manages central obesity and pro thrombosis - an invitation to further investigation into polypharmacy reduction. J Nutr Metab. 2012; 2012: 585170.
13. Cassani RSL, et al. Impact of weight loss diet associated with flaxseed on inflammatory markers in men with cardiovascular risk factors: a clinical study. Nutr J. 2015;14:5
14. De Spirt S, et al. Intervention with flaxseed and borage oil supplements modulates skin condition in women. Br J Nutr. 2009 Feb;101(3):440-5
15. Mason JK, et al. Flaxseed and its lignan and oil components: can they play a role in reducing the risk of and improving the treatment of breast cancer? Appl Physiol Nutr Metab. 2014 Jun;39(6):663-78