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Flax

Scientific Name(s): Linum usitatissimum L.
Common Name(s): Flax, Flaxseed, Linseed, Lint bells, Linum

Clinical Overview

Use

Flaxseed and flaxseed oil contain various essential fatty acids but are particularly rich in alpha linolenic acid (ALA). Flaxseed (but not flaxseed oil) also has a high fiber content that may have health benefits similar to those of other high-fiber products and phytoestrogens. Historically, linseed oil, derived from flaxseed, has been used as a topical demulcent and emollient, as a laxative, and as a treatment for coughs, colds, and urinary tract infections. Interest in flaxseed centers on atherosclerosis, cancer, diabetes, and to a lesser extent, menopause, attention deficit hyperactivity disorder, bipolar disorder, and systemic lupus erythematosus (SLE).

Dosing

Flaxseed (whole or ground) has been used in clinical trials at doses from 5 to 50 g/day or 4 to 60 mL flaxseed oil daily, and has been used in children at doses equivalent to 400 mg of ALA in divided doses.

Eight grams of ground flaxseed (or 2.5 g of flaxseed oil) per day provides a daily intake of 1.1 g of ALA for women and 1.6 g for men.

Contraindications

Contraindicated in patients with known hypersensitivity to flaxseed.

Pregnancy/Lactation

The use of flaxseed and flaxseed oil during pregnancy and lactation is not recommended.

Interactions

None well documented.

Adverse Reactions

Flaxseed and flaxseed oil appear to be well tolerated, with few adverse reactions reported usually related to flaxseed allergy. Flaxseed-induced anaphylaxis has also been reported.

Toxicology

The US Food and Drug Administration has not granted GRAS (Generally Recognized as Safe) status to flaxseed or flaxseed oil. The safety of ingested amounts greater than 50 g/day of flaxseed is not established.

Botany

The flax plant is a slender annual that grows to 0.3 to 0.9 m in height. It branches at the top and has small, pale green, alternating leaves. The dark yellow seed is oval and flat, about 5 mm long, with a hull, an endosperm, and 2 embryos. Flax was introduced to North America from Europe and now grows widely in Canada and the northwestern United States. Each branch is tipped with 1 or 2 delicate blue flowers that bloom from February through September. Additional members of the genus Linum are used throughout the world for their fiber and oil content.1, 2

History

Flax has probably been used for 12,000 years as a source of fiber for producing linen and was one of the earliest plants used for purposes other than food. The name usitatissimum is derived from Latin, meaning "very useful." Flax is processed from fibers in the plant's stem. Flaxseed or linseed oil, derived from the seed, has been used as a topical demulcent and emollient and as a laxative, particularly for animals. Flaxseed oil is also used in paints and varnishes, and as a waterproofing agent. Flaxseed cakes, the residue after pressing of oil, have been used as cattle feed. Traditional medicinal uses of the plant have been varied and at times unusual; one text notes use of the seed for removing foreign material from the eye. A moistened seed was placed under the closed eyelid for a few moments to allow the material to adhere to the seed, thereby facilitating removal. Other uses include the treatment of coughs and colds, constipation, and urinary tract infections. The related Linum catharticum yields a purgative decoction.2, 3, 4, 5

The German Commission E sanctions the use of flaxseed for the treatment of chronic constipation, colon damage from laxative abuse, irritable colon, and diverticulitis. It is also approved as a mucilage for gastritis and enteritis. When used externally, it is approved as a poultice for local inflammation.6

Chemistry

The medical properties of flax are primarily associated with the seed. Studies have attributed different medicinal properties to unsaturated fatty acids (mainly ALA and linoleic acid), lignans (predominantly secoisolarciresinol diglucoside), and nonstarch polysaccharides (ie, gum, fiber). Flax produces a seed that contains 38% to 45% oil. Also known as linseed oil, it is obtained by crushing and compressing the seeds. The oil is composed of approximately 70% polyunsaturated fatty acids, including linolenic, linoleic, and oleic acids; approximately 18% monounsaturated fatty acids; and approximately 9% saturated fatty acids. Flaxseed oil is among the best natural sources of ALA, an omega-3 fatty acid. Linoleic acid, a rich source of omega-6 fatty acid, and ALA are critical for the structural integrity of cell membranes. ALA is a precursor to the longer and more unsaturated omega-3 fatty acids eicosapentaenoic acid (EPA) and docosahexaenoic acid ; however, only a modest amount of ALA appears to be converted to these longer chain fatty acids in the human body.2, 4, 7, 8, 9

Whole or ground flaxseed is a rich source of lignans (often termed phytoestrogens), including secoisolariciresinol and matairesinol. These lignans are believed to exert antioxidant and phytoestrogenic effects. They are converted by bacteria in the colon to the active metabolites enterodiol and enterolactone. These metabolites are reported to have greater antioxidant activity than the parent lignan secoisolarciresinol diglucoside and also exert either weak estrogenic or antiestrogenic effect, depending on biological levels of estradiol.4, 10, 11, 12, 13, 14

Whole or ground flaxseed is a source of soluble fiber mucilage that is also found in other members of the genus Linum. This gum-like material contains polysaccharides containing D-xylose, L-rhamnose, L-galactose, arabinose, D-galacturonic and mannuronic acids, fucose, and glucuronic acid. Flaxseed may also contain phenylpropanoids (eg, p-coumaric acid, o-coumaric acid, linusitamarin), niacin, folic acid, tocopherol, potassium, and phosphorous, and provides about 28% of total dietary fiber per 100 g dry weight.2, 3

The nutrient composition of the 3 forms of flaxseed differs. Unlike whole and ground flaxseed, flaxseed oil is devoid of fiber and lignans. Flax leaves and seed chaff contain the cyanogenic glycosides (including linamarin, linustatin, and neolinustatin); however, heating appears to destroy these constituents.2, 3, 4, 8, 15, 16

Uses and Pharmacology

Antioxidant effects

The lignan secoisolarciresinol diglucoside has been shown to possess antioxidant properties.7, 14 The clinical relevance of the effects of lignan alone or flaxseed have not been established.14, 42, 48

Antiplatelet effects

Secoisolarciresinol diglucoside, which is associated with anti-inflammatory activity, has been shown to act as an antagonist of platelet-activating factor.14, 25, 26 While theoretical mechanisms of action and epidemiological data point to a potential platelet inhibitory effect of ALA, human studies have not shown a consistent antiplatelet effect.8, 27

Appetite suppression

Flaxseed dietary fiber may contribute to a sensation of satiety and fullness.49

Attention deficit hyperactivity disorder (ADHD)

ADHD is commonly associated with decreased blood omega-3 fatty acid levels.50 A pilot study evaluated the effect of ALA-rich supplementation with flax oil and antioxidant emulsion on blood fatty acid composition and behavior in children with ADHD. Postsupplementation levels of red blood cell membrane fatty acids were higher than pretreatment and control levels. An improvement in symptoms was reflected by reduction in total hyperactivity scores of ADHD children.51

Bipolar disorder

Despite theoretical support and evidence suggesting that omega-3 oil is beneficial,52 the effects of flaxseed oil supplementation did not differ from those of placebo in either bipolar depression or mania in a clinical study.53

Constipation

Effects of flaxseed oil and olive oil compared with mineral oil were evaluated in 50 adult hemodialysis patients with constipation. Initial doses of oils were 4 mL/day and adjusted as needed over the 4-week study period; 82% of patients on flaxseed oil required adjustments (average final dose, 6.9 ± 2.7 mL/day) compared with 69% and 53% for olive and mineral oils, respectively. Rome III scores improved significantly with each of the 3 oil treatments (P < 0.01 for each). Flaxseed oil yielded improvements in the frequency and consistency of stools, whereas mineral and olive oils improved scores in 5 of the 6 total constipation symptoms. Diabetic nephropathy, hypertensive nephrosclerosis, glomerulonephritis, and polycystic kidney disease were the main etiologies that led to chronic kidney disease.88

Dermatology

Oral supplementation with flaxseed oil may improve skin hydration and reduce sensitivity.54, 55

Dyslipidemia

Animal studies

Numerous animal studies have been conducted, demonstrating reduced atherosclerotic lesions, decreased platelet aggregation, decreased total and low-density lipoprotein - cholesterol (LDL-C), and prolongation of the QT interval. Studies investigating the role of flaxseed in stroke prevention in animals are lacking.7, 14, 17, 18 However, the availability of clinical trial and epidemiological data makes evidence from animal studies less relevant.

Clinical studies

Multiple studies have been conducted among healthy volunteers but may be of questionable relevance; baseline lipid levels and comorbidity may affect measured outcomes. Clinical data are also often limited to measuring secondary end points, with epidemiological data on ALA in general, and not flaxseed in particular, providing a basis for recommendations for use of flaxseed in preventing primary end points such as myocardial infarction, stroke, or death from atherosclerotic disease.17

Reviews of the effect of flaxseed on serum cholesterol7, 8, 14, 17, 19 and a meta-analysis18 have been published. Studies are heterogeneous in participant inclusion, dosage regimens, and flaxseed products used.14

A meta-analysis of 28 clinical trials up to 2008 found that whole flaxseed and lignan alone (but not flaxseed oil) modestly reduced total cholesterol by −0.10 mmol/L (−3.9 mg/dL) (95% confidence interval [95% CI], −0.2 to 0 mmol/L [−7.7 to 0 mg/dL]) and LDL-C by 0.08 mmol/L (−3.1 mg/dL) (95% CI, −0.16 to 0 mmol/L [−6.2 to 0 mg/dL]). No changes in high-density lipoprotein - cholesterol (HDL-C) or triglycerides were found. Gender and initial lipid profiles influenced efficacy.18

Clinical studies published after the meta-analysis in patients with metabolic syndrome or elevated cholesterol have produced similar findings on LDL levels.20, 21, 22, 23 No effect on markers of inflammation was found in these or other studies.19, 20, 21, 24, 80 However, a cross-sectional and follow-up dietary intervention study in hemodialysis patients (n = 30) documented significantly improved blood pressure, total cholesterol, triglyceride/HDL-C ratio, serum triglycerides, glucose tolerance, serum fatty acid composition, and inflammatory markers (ie, interleukin [IL]-6, tumor necrosis factor [TNF]-alpha, high-sensitivity C-reactive protein [hs-CRP]) after 12 weeks of dietary supplementation with 30 g/day of a milled flax-sesame-pumpkin seed (3:1:1) mixture added to 200 mL of fat-free milk before dinner. Pruritis also improved in all patients.85 A smaller double-blind, randomized, controlled study in 38 hemodialysis patients observed a significant reduction from baseline (P<0.05) and controls (P<0.01) only in serum triglycerides (−42 mg/dL) after 8 weeks of 6 g/day flaxseed oil. Total cholesterol, LDL, HDLC, and lipoprotein(a) were not significantly affected in either group.93 A double-blind, randomized, placebo-controlled trial from 2015 studied the effect of flaxseed in 110 patients with peripheral artery disease, of which 74% were on concomitant cholesterol lowering medication (mostly statins). A significant reduction (8.5%) in LDL cholesterol was observed compared with baseline in patients taking antihyperlipidemics plus supplemental foods containing 30 g of milled flaxseed over a 12-month period. Milled flaxseed lowered total and LDL cholesterol compared to placebo when used in combination with cholesterol-lowering medication. In contrast, no differences were found between flax and control groups despite changes over time in percent and rate of platelet aggregation.87 A nonblind, randomized, controlled trial reported significant benefit in weight, body mass index (BMI), and lipid values in hyperlipidemic Iranian adults who supplemented their diet with 30 g/day of flaxseed for 40 days. Mean differences within the flaxseed and control groups in total cholesterol, LDL-C, and triglylcerides were −11.5 and +16.4, −3.1 and +9.5, and −49.5 and +17.7, respectively; HDL-C was not significantly affected. A mean reduction in weight of 1 kg and in BMI of 0.5 was also reported as significant.90 A study in healthy adults found statistically significant reductions in total (−5%) and LDL-C (−6.7%) levels.80 A pediatric study in patients with elevated LDL-C levels found no significant reduction in total or LDL-C levels, but HDL-C levels were significantly reduced (−15%), and triglyceride levels were significantly elevated (26%).81

The effects of ALA on cardiovascular risk factors, including small dense LDL-C, were evaluated in a randomized, double-blind, crossover study in 15 healthy Japanese men. Flaxseed oil 10 g (ALA 5.49 g/day) or corn oil (ALA 0.9 g/day) were administered for 12 weeks, each separated by an 8-week washout period. ALA levels were significantly higher while linoleic acid and alpha-tocopherol were significantly lower during flaxseed administration compared with corn oil. Additionally, eicosapentaenoic acid (EPA) was significantly higher during the flaxseed oil period and the ratio of omega-6 to omega-3 fatty acid levels was significantly lower. Lipid parameters were significantly improved with supplementation of flaxseed oil compared to corn oil, including total cholesterol, LDL, HDL, non-HDL, apolipoprotein A1 and B, as well as cholesterol ester transfer protein activity. Small dense LDL concentrations were significantly lower with flaxseed oil at 4 weeks but not at 12 weeks. No symptoms or side effects of the test diets were reported by subjects.89

Cancer

Phytoestrogens and ALA have been purported to have cancer protective effects; however, the mechanism by which these compounds exert their chemoprotective effect has not been completely explained.4, 28

Animal data

Animal studies have been conducted on flaxseed and constituent lignans in colon cancer, melanoma, and the mammary gland with positive results. Flaxseed oil did not appear to exert a protective effect in the colon cancer study.7, 14 Observational studies and in vitro investigations on cancer cell cultures have also been reported.14, 29, 30

Clinical data

Trials are lacking for the specific use of flaxseed in breast cancer,27 and the relevance of omega-3 fatty acids or ALA in risk of breast cancer is debated.31 In a small study in newly diagnosed postmenopausal breast cancer patients, dietary flaxseed showed positive effects on tumor markers compared with those of placebo, including a reduction in Ki-67 labeling index (a measure of the rate of tumor cell proliferation), a reduction in c-erbB2 score, and an increase in apoptosis index.32

Data are conflicting for prostate cancer. Previous epidemiologic evidence suggested that ALA increases the risk for prostate cancer33, 34, 35; however, small studies have shown benefit for flaxseed.33, 36, 37 Until unequivocal data are available, flaxseed consumption by men with prostate cancer should be under the guidance of a medical practitioner.7

The antioxidant properties and the antiangiogenic effects of lignans cannot be excluded as mechanisms by which flaxseed may exert its beneficial effect on prostate cancer.33 Flaxseed lignans were effective in preventing symptoms associated with benign prostatic hyperplasia in 1 clinical study.38

Diabetes/Metabolic syndrome

Animal data

Animal studies have been conducted both to ascertain the effects of flaxseed on metabolic syndrome and diabetes models as well as to elucidate mechanisms of action. Soluble fiber and other components of flaxseed fractions could potentially affect insulin secretion and its mechanisms of action in maintaining plasma glucose homeostasis, possibly decreasing postprandial glucose response and improving glucose tolerance.8, 39

Clinical data

Improved glycemic control due to flax fiber and the lignan secoisolarciresinol diglucoside has been demonstrated in healthy volunteers,40 postmenopausal women, and hyperlipidemic men.41, 42, 43 The lignan secoisolarciresinol diglucoside and ground flaxseed, but not flaxseed oil,44 reduced insulin resistance and improved glycemic control and lipid profiles in patients with type 2 diabetes,14, 45 and in glucose-intolerant obese people.46 In a randomized trial (n = 25), daily ground flaxseed supplementation for 12 weeks improved glucose, insulin, and insulin sensitivity in prediabetic overweight or obese men and postmenopausal women; significant results were seen with 13 g (2 tablespoonsful) but not 26 g/day.76 A study in Chinese adults with risk factors of metabolic syndrome (n = 173) reported that flaxseed (30 g/day contained in bread) provided beneficial changes in some cardiometabolic biomarkers but overall provided no additional benefit over lifestyle counseling; the form of flaxseed used (ground or whole) is uncertain due to ambiguity throughout the text.77 However, neither ground flaxseed nor flaxseed oil enhanced glycemic control in individuals with well-controlled type 2 diabetes in another study.47

The addition of flaxseed gum to glucose solutions or liquid and gelled dairy products was associated with minimal effects on postprandial blood glucose or insulin levels in healthy males in a randomized, double-blind, crossover, postprandial study (n = 12). Although glucose area under the curve and GI values were significantly lower with flaxseed gum- (2.5 g) and soy-soluble polysaccharide-fortified (2.25 g) products than the glucose reference, the significant inverse correlation to product viscosity (which was greatest with flaxseed gum) pointed to this as the more relevant factor. Fortification of food products, especially dairy, with low-viscosity soluble fibers does not appear to be a useful means of attenuating postprandial blood glucose or insulin response.79

As a component of medical nutrition therapy for patients with type 1 or type 2 diabetes, the American Diabetes Association Standards of Care (2014) recommend an increase in foods containing alpha-linolenic acid based on beneficial effects observed on lipoprotein profiles, heart disease prevention, and overall positive health in patients with diabetes (moderate-quality evidence).84

Gout

The American College of Rheumatology guidelines on the management of gout (2012) suggested that the use of various oral complementary agents, including flax, was inappropriate for the treatment of an acute attack of gout.74

Hypertension

A randomized, double-blind, placebo-controlled trial enrolling 110 adults with peripheral artery disease evaluated the effects of milled flaxseed (30 g/day provided in a variety of food products) on blood pressure. After 6 months, systolic (SBP) and diastolic blood pressure (DBP) were significantly lower in the flax group (approximately 10 mm Hg and 7 mm Hg reductions, respectively); greater improvement (15 mm Hg reduction in SBP) was noted in hypertensive patients with SBP 140 mm Hg or higher at baseline. The effect was maintained at a 1-year follow up and was demonstrated in the presence of antihypertensive medications, indicating an effective nonpharmacological addition to current pharmacological regimens.78 The mechanism of action of blood pressure reduction with flaxseed was assessed in a follow-up study using the plasma of the participants in the FlaxPAD study. Although blood pressure was inversely associated with alpha-linolenic acid (ALA), the mechanism of action was still unclear. Oxylipins, polyunsaturated fats that regulate vascular tone, were evaluated from 76 participants. Of the 9 plasma oxylipins that had significantly different changes from baseline between flaxseed and control groups, 6 were significantly lower than controls (P < 0.05) and were products of soluble epoxide hydrolase. A strong inverse association was seen between ALA and percent of initial activity of soluble epoxide hydrolase (P = 0.0048), a pharmacological target for treatment.83

A meta-analysis of randomized, controlled trials (11 studies, N = 1,004) published until July 2014 that studied flaxseed (whole, ground, or extracts) in adults identified a significant reduction in the mean difference in systolic blood pressure by −1.77 mm Hg (P = 0.04) and diastolic blood pressure by −1.16 mm Hg (P < 0.003) with no heterogeneity. Whole or ground seed provided greater reduction in systolic blood pressure when compared to oil or lignin. Subgroup analysis revealed parallel study designs to be a factor for showing significant reductions in blood pressure. Additionally, supplement duration longer than 12 weeks, consumption of whole flaxseed, and a baseline blood pressure less than 130 mm Hg to each be factors for reporting significant reductions in diastolic blood pressure. Doses varied from 30 to 50 g/day for whole flaxseed, 1.2 to 28 g/day of oil, and 360 to 600 mg /day of lignin. Mild cases of constipation and other GI complaints were reported in 2 studies; no major side effects were noted.86

Lupus nephritis

Glomerulonephritis is a common and serious complication of SLE, and flaxseed has shown potential in helping patients who develop lupus nephritis.56 Preliminary evidence derived from a mouse model of lupus indicates that diets supplemented with 15% flaxseed for 14 weeks delayed the onset of proteinuria and reduced overall mortality compared with controls.26 Flaxseed contains high levels of the lignan precursor secoisolarciresinol diglucoside, which is associated with anti-inflammatory activity (eg, formation of less inflammatory classes of renal prostanoids), thus offering a renoprotective effect. Nine patients with SLE taking part in a single-blind, randomized clinical trial who received flaxseed 30 g/day and prednisone for 1 year, followed by crossover to a year without flaxseed supplementation, experienced improvements in renal function.58

Menopausal symptoms

The photoestrogenic content of flaxseed may benefit women suffering from menopausal symptoms.59 Clinical studies, however, are equivocal in their results for either flaxseed or lignans alone in reducing menopausal symptoms (hot flashes).41, 59, 60, 61, 62 Doses of up to 410 mg lignan were not more effective than placebo in reducing hot flashes in a study of 188 women.61 The Society of Obstetricians and Gynaecologists of Canada revised clinical practice guidelines on managing menopause (2014) do not recommend flaxseed for reducing menopausal symptoms based on a lack of evidence to support clinical benefit.91 Likewise, the Endocrine Society clinical practice guidelines for the treatment of symptoms of menopause (2015) recommend counseling patients on the lack of consistent evidence for benefit of complementary medicine therapies, including flaxseed, as an alternative nonhormonal therapy for vasomotor symptoms (weak recommendation; low quality evidence).92

Dosing

The US Food and Drug Administration has not granted GRAS status to flaxseed or flaxseed oil, although the agency does allow up to 12% flaxseed in food by weight.7 However, based on available data, it appears that flaxseed and its oil are safe in doses of up to 50 g/day.2, 3, 7, 86

Flaxseed oil is rich in ALA and is suitable for purposes attributed to the fatty acid; however, it does not contain fiber or lignans.3

Eight grams of ground flaxseed or 2.5 g of flaxseed oil per day provide the Institute of Medicine’s daily adequate intake of 1.1 g of ALA for women and 1.6 g for men.3

Flaxseed (whole or ground) has been used in clinical trials at doses from 5 to 50 g/day or 60 mL flaxseed oil daily,3 and has been used in children (combined with vitamin C 25 mg) at doses equivalent to 400 mg of ALA in divided doses.51 Twenty-five grams of unground seed did not appear to be adequate as a therapeutic dose to induce changes in plasma lipids and several biomarkers of oxidative stress in a small study.63 A meta-analysis investigating the effect of flaxseed consumption on blood pressure summarized the daily dose of flaxseed from 11 studies as follows: 30 to 50 g/day (whole flaxseed), 1.2 to 28 g/day (flaxseed oil), and 360 to 600 mg/day (flaxseed lignan).86

Pregnancy / Lactation

The use of flaxseed and flaxseed oil during pregnancy and lactation is not recommended. Animal studies show possible harmful effects,64, 65 and human data are insufficient.66 Flaxseed may increase serum luteinizing hormone or testosterone and could be detrimental in pregnancy.7, 14, 67, 68 Flaxseed oil may increase the risk of premature birth if consumed during the second and third trimesters.82

Interactions

Flaxseed, but not flaxseed oil, contains mucilage; thus, absorption of coadministered drugs may be affected.7 Oral drugs should be taken 1 hour before or 2 hours after flaxseed to prevent decreased absorption.7

Theoretically, flaxseed and flaxseed oil may potentiate the effects of anticoagulants, antiplatelet agents, nonsteroidal anti-inflammatory drugs, antihyperlipidemic agents, antihypertensive agents, and mood stabilizer agents; however, no clinical cases have been reported.7 Flaxseed, but not flaxseed oil, by virtue of its fiber content may increase or enhance the effects of laxatives.69 Because of its phytoestrogen content, it may exhibit weak estrogenic or antiestrogenic effects and has the potential to interact with oral contraceptives or hormone-replacement therapy.7 Linatine inhibits the absorption of pyridoxine (vitamin B6), and phytic acid reduces calcium, copper, iron, magnesium, and zinc absorption.2, 3 At dosages of 45 g flaxseed daily, no change in serum pyridoxine was observed after 5 weeks, and there is no evidence that mineral absorption is clinically affected.3

Ibrutinib: Flaxseed oil may enhance the antiplatelet effect of ibrutinib. Monitor therapy. The ibrutinib Canadian product monograph recommends avoiding this combination.95, 96

Adverse Reactions

Flaxseed and flaxseed oil appear to be well tolerated, with few adverse reactions reported.7 Many of the reported reactions have been associated with allergy to flaxseed7 and include palmar pruritus, generalized urticaria, nausea/vomiting, intestinal/abdominal pain, diarrhea, sneezing, nasal obstruction, and intense general malaise.7, 70, 71 Rarely, anaphylaxis, including facial angioedema, oral pruritus, dyspnea, urticarial, and persistent abdominal cramps, has been reported.75, 94 Because type 1 hypersensitivity to flaxseed oil has been confirmed, it is contraindicated in patients with known hypersensitivity.7, 70, 71 Approximately 50% of workers exposed to flax demonstrated immunologically positive antigen tests in 1 survey.72

Flaxseed, but not flaxseed oil, can cause digestive symptoms similar to those of other dietary fibers, including bloating, flatulence, abdominal pain, diarrhea, dyspepsia, and nausea.59 Anecdotally, intestinal obstruction may occur from mass effect when large amounts of flaxseed, but not flaxseed oil, are ingested or taken with an inadequate amount of water.7

Toxicology

The US Food and Drug Administration has not granted GRAS status to flaxseed or flaxseed oil, although the agency allows up to 12% flaxseed in food by weight.7 However, based on available data, it appears that flaxseed and its oil are safe in doses of up to 50 g/day.2, 3, 7

The cyanogenic properties of some flax constituents theoretically suggest that ingestion of large amounts of the plant may be harmful; heating, however, appears to destroy the cyanogenic glycosides.3

Anecdotally, an overdose of flaxseed or its oil may result in weakness, unstable gait, paralysis, or seizures.70 The physiologic balance between ALA and linoleic acid may be relevant, and ALA deficiency may cause neurologic abnormalities.73 Rats fed high-dose secoisolarciresinol diglucoside lignan showed no adverse effects on growth, nor hepatotoxicity.14 Flaxseed contains small, probably insignificant, amounts of cadmium (0.2 to 0.6 mg/kg).19 Immature flaxseed seedpods may be poisonous.7

References

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