Flaxseed: Examining the Research
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, in France (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.
References
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
Great article. I had read mixed reviews of flaxseed, including some of the criticisms you cite, so this is great to have an expert review of the evidence. And not only an expert, but a low-carb one at that. Thanks!
Thanks so much for the nice feedback, Wendy! I’m glad you found the post helpful.
I just ordered your book and can’t wait to dive in! I have a follow-up question about flax, and hypothyroid. This may be beyond your research. After a year of low-carbing, I’ve been diagnosed with hypothyroid. I’m wondering if there could be any relationship; I see suggestions both pro and con. In researching it, I see that Mayo Clinic and others recommend using flax “with caution” for those with hypothyroid. Did you happen to review any research on this? Thanks in advance.
Hi Wendy,
Thanks so much for ordering my book!
I’m sorry to hear you’ve been diagnosed with hypothyroidism. There’s no research that I’m aware of that moderate amounts of flaxseed (i.e., 2 Tbsp daily) can cause or exacerbate hypothyroidism. Flax does contain a small amount of goitrogens, which can interfere with thyroid production if consumed in excess, but many other plant foods do as well, including nuts, strawberries, spinach and other greens, cauliflower, and others. Getting enough iodine from seafood is important to counteract the goitrogens in these foods.I developed hypothyroidism prior to going low carb, and I include all of the foods listed above in my diet.
I wish you the best of luck!
– Franziska
Flaxseed banned in Europe? I have never heard such a thing. Flaxseed is widely available in Germany and flaxseed oil can also be easily purchased.
Hi Anna,
Actually, I think it was specifically France that has a ban on flaxseed oil, and only GMO flaxseed is banned in Europe. I will edit the post to clarify that the ban on flaxseed oil only pertains to France. Thanks!
You very briefly mention bleeding disorders, but the problem is not only for people with preexisting issues. I notice a marked increase in bruising and nose bleeds when I eat flaxseeds regularly. I do not have any bleeding disorder (when I do not eat flaxseeds).
Also, sorry for TMI, but eating ground flaxseeds regularly leads to recurrent poop issues. No diarrhea, no constipation, only monster poops that clog the toilet. Like, every day. (I hear chia seeds can do the same, but I never tried.)
No more flaxseeds for me.
Hi Valerie,
Thanks for your comments. I eat flax regularly and don’t experience either of those problems. I do limit intake to ~2 Tbsp per day.
As I said in my article, if flax doesn’t agree with you, it’s best to avoid it. Based on your experience, I’m glad you’ve decided that this is the way to go for you 🙂 I wish you the best of health!
Thanks for shedding light on another complex issue. When first going LC, I used flaxseed meal regularly. Then, gave it up after reading warnings. Appreciate your thorough research!
Thank you so much for your comments and feedback, Gerri!
“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.”
A problem I had around six years ago while on the drug Plavix after heart stents and eating home made flax seed bread, was severe bruising looking like bleeding under the skin at the slightest knock, very often not even realising I had bumped into anything at the time. I was told that flaxseed helps thin the blood and can help against clotting. I dumped the Plavix immediately, preferring the natural real food remedy.
Plavix is free in the UK provided by the state NHS system. I believe Plavix is an expensive drug in the US, maybe some may want to consider a much cheaper option, if money is tight. Especially as some are on the drug for years, some for a lifetime.
I am not a Luddite but strongly believe in a whole food alternative to drugs when appropriate. From personal experience the correct diet has transformed my life and done far more for me than drugs ever could.
I have looked into flax big time and as a diabetic I see it as a true wonder food.The net carb content is very low, some even say it can reduce BG, but only by a small amount. Possibly the high fibre content.
Thanks for another great article.
Kind regards Eddie
Thanks so much for your comments and for sharing your story, Eddie! I’m so glad you enjoy flax as part of your whole foods, low carb diet. I agree that it’s very beneficial for people with diabetes.
Thanks again for sharing, and for your continued support. I wish you all the best!
Franziska
What about cyanide? Aren’t flax seeds relatively high in cyanide? Does cooking vaporize the cyanide in flaxseed?
Flaxseed contains thiocyanates, a form of cyanide; however, many other healthy foods contain thiocyanates, including spinach and cruciferous vegetables like broccoli and cauliflower. I’m not sure whether cooking/heating reduces the thiocyantes as it does with goitrogens. I’m not concerned about the small amount in 2 Tbsp of flaxseed daily.
Finally someone with half a brain. That is the problem with smany people. They read something on a website that is there to sell there product and slam the good product and they repeat what they heard over and over etc. For the people who do research they don’t even know what there reading or they read the two bad studies on rats but don’t read the 10 good ones on humans etc. Thank you for your info.
Thanks for your comment, Angelo! Appreciate your feedback and the points you raised.
Here’s my personal experience. 4 months ago, started taking flax oil (Udo’s oil) at 1 tbsp per day. Noticed that there was a lot more belly fat. It was the only thing I changed diet wise. So for men, it is not a good idea at all.
Thanks for your comment. I’m recommending ground flaxseed, not flaxseed oil. Also, if you added 1 Tbsp of oil to your diet every day for four months and didn’t change anything else, you should expect to gain weight from the additional calories.
“I’m recommending ground flaxseed, not flaxseed oil.”
Do you recommend against flaxseed oil? If so, why?
As a highly unsaturated fatty acid, flaxseed oil is prone to oxidation and rancidity, so I don’t recommend it in that form. Whole flaxseed that is ground shortly before consumption doesn’t have this issue, and it also provides the beneficial lignans discussed in the post.