Flaxseed

Linum usitatissimum L., also called common flax, belongs to Linaceae, the flax family, in the order Malpighiales (Britannica). It is a herbaceous annual. Britannica describes small, lance-shaped leaves alternating along the stalk and flowers borne on stems from the branch tips, each with five petals, usually blue but sometimes white or pink. According to the Saskatchewan oilseed growers’ guide, new flowers open early each morning and usually shed their petals by midday, so each bloom lasts a single day, while a field keeps flowering for 15 to 25 days. All the flower parts come in fives, and the anthers may be blue or yellow. Flax is highly self-pollinating: the flowers make nectar and attract insects, but insects are not needed for seed set. The plant has a short, branched taproot that can reach more than a meter deep, though its root system is limited compared with other prairie crops (SK Oilseeds).

The fruit is a dry capsule, called a boll, divided into five segments that each produce two seeds. A fully set boll holds ten seeds, though six to eight is usual (SK Oilseeds), and North Dakota State University (NDSU) puts the average nearer six. Capsules that split open to scatter seed are a wild trait that was selected against, so ripe bolls of Canadian varieties gape only slightly and rarely spill their seeds. The seeds are flat, oval, and pointed at one end, and a thousand of them weigh only about 5 to 7 g. They range from light brown to dark reddish brown, and yellow or golden varieties are also grown. A coating of mucilage gives the seed its shine and turns sticky when wet, which is why wet flax is notoriously hard to thresh (NDSU). Farmers grow two types: fiber flax, densely sown so that its slender stalks reach 0.9 to 1.2 m, and seed flax, or linseed, which is shorter and more branched (Britannica). North Dakota is the leading US producer, and Britannica names China, Russia, and Canada among the countries where flax remains economically significant.

The seed’s chemistry explains both its uses and its cautions. EMA describes linseed as nearly 25% bulk material, including 3–6% mucilage and 4–7% dietary fiber. It is also rich in oil: according to the NIH Office of Dietary Supplements (ODS), a tablespoon of whole seed supplies about 2.35 g of ALA and a tablespoon of flaxseed oil about 7.26 g, against an adequate intake of 1.6 g a day for men and 1.1 g for women. Britannica notes that linseed oil has the highest ALA content of any vegetable oil and goes rancid quickly. Flaxseed’s best-known lignan is secoisolariciresinol diglucoside, or SDG (PubChem CID 9917980, C32H46O16), and gut bacteria convert plant lignans into the enterolignans enterodiol and enterolactone (Kuijsten). The seeds also contain cyanogenic glycosides, chiefly linustatin and neolinustatin, which release hydrogen cyanide when the seed is chewed or ground, and flax can accumulate the heavy metal cadmium from the soil (BfR).

Flax may be the oldest plant fiber people have used. Kvavadze and colleagues (Science 2009) reported wild flax fibers from Upper Paleolithic layers of Dzudzuana Cave, in the foothills of the Caucasus in Georgia, with the oldest layers dated to 32,000 to 26,000 years ago. Many of the fibers were spun, dyed, or knotted, suggesting that hunter-gatherers made cords, baskets, or garments from them. Cultivated flax came much later. Fu (Scientific Reports 2025) describes it as one of the founder crops domesticated in the Near-Eastern Fertile Crescent for oil and fiber, from a wild ancestor called pale flax (Linum bienne). Genetic work by Allaby and colleagues (Theoretical and Applied Genetics 2005) suggests that flax was first domesticated for its oily seed rather than its fiber, and Fu and colleagues (Ecology and Evolution 2012) call it the earliest oil and fiber crop.

Linen carried flax across the ancient world. Britannica records evidence of flax in the prehistoric lake dwellings of Switzerland and fine linen fabrics, showing a high degree of skill, in ancient Egyptian tombs; Phoenician traders apparently brought linen from the Mediterranean to Gaul and Britain, and the Romans spread linen manufacture throughout their empire. The seed had uses of its own. NCCIH notes that it was cooked into stews in Ethiopia and used as a laxative in ancient Greece. By the seventeenth century the German states and Russia were major sources of raw flax, and linen industries were established in the Netherlands, Ireland, England, and Scotland (Britannica). In North America, European settlers used flaxseed for poultices on cuts and burns and pressed its oil for paints (NCCIH), and flax was among the first crops planted as farmers moved west across the northern United States and Canada in the 1800s (NDSU).

Cotton eventually displaced flax as America’s main plant fiber. NDSU notes that fiber flax production fell dramatically in the nineteenth century and that almost none is grown commercially in the United States today, though some is still grown in Canada. Seed flax took its place. Crushed seed yields linseed oil and linseed meal, and the oil became a staple of oil-based paints, printing inks, varnish, oilcloth, and linoleum flooring (Britannica; NDSU), while lower grades of fiber went into canvas, twine, fire hoses, and fish nets. More recently the seed has found a second life as a health food, valued for its fiber, omega-3 fat, and lignans, and Britannica observes that this popularity has kept flax economically important even as synthetic fibers eroded the linen trade. MSKCC notes that flaxseed now turns up in crackers, waffles, and oatmeal, and NDSU reports that whole or, more often, ground seed is eaten mostly in bakery products.

Flaxseed’s laxative effect is largely mechanical. EMA explains that the mucilage binds water and swells into a soft gel in the intestine, holding water in the stool and making it softer. The larger volume of gut contents stretches the bowel wall, which speeds transit, and the slippery gel lubricates the passage of stool. Part of the fiber is passed unchanged; the rest is fermented by colon bacteria into short-chain fatty acids, which are mainly absorbed, and this fermentation is also why flaxseed causes gas. Linseed usually acts within 12 to 24 hours, but the maximum effect may take two or three days. EMA also points out a subtlety: broken seeds may start to swell in the stomach and so do not always produce the stretch stimulus, while whole or bruised seeds swell later. As a gel, the mucilage probably also coats and protects the lining of the gut, the basis for EMA’s traditional-use listing for mild digestive discomfort.

For flaxseed’s other components, grinding matters. Kuijsten and colleagues (Journal of Nutrition 2005) fed 12 healthy volunteers whole, crushed, and ground flaxseed in turn; compared with ground seed, the relative bioavailability of enterolignans was only 28% from whole seed and 43% from crushed seed. Austria and colleagues (Journal of the American College of Nutrition 2008) baked 30 g of whole seed, 30 g of milled seed, or flaxseed oil containing 6 g of ALA into muffins: plasma ALA rose significantly with milled seed and with oil, more so with oil, but not with whole seed. Once absorbed, ALA is only partly converted into EPA and DHA, the long-chain omega-3s found in fish. ODS reports conversion rates below 15% and says that consuming EPA and DHA directly is the only practical way to raise their levels, and in the Austria trial they did not rise significantly. Flaxseed is a source of ALA, not a replacement for fish oil.

Lignans are the reason flaxseed is grouped with phytoestrogens. MSKCC notes that flaxseed lengthened the luteal phase of the menstrual cycle in premenopausal women and shifted estrogen metabolism in postmenopausal women, raising the ratio of 2-hydroxyestrogen to 16α-hydroxyestrone in a dose-dependent way, and EMA concludes from studies in healthy women that long-term use may have an estrogenic effect. Whether that is helpful or harmful in a given tissue is not settled, which is why the literature contains both hopes of cancer protection and cautions about hormone-dependent tumors. For blood pressure, Caligiuri and colleagues (Hypertension 2014) analyzed blood from the FlaxPAD trial and found that flaxseed lowered eight plasma oxylipins, six of them made by soluble epoxide hydrolase, an enzyme targeted in blood pressure drug research; ALA inhibited the enzyme in a laboratory assay. In the original trial, plasma ALA tracked blood pressure and lignan levels tracked diastolic pressure (Rodriguez-Leyva). These are plausible mechanisms, not proof.

Constipation is flaxseed’s best-supported medical use and the one with formal recognition. EMA’s 2015 monograph accepts linseed as a well-established herbal medicine for habitual constipation and for conditions in which easy bowel movements with soft stool are desirable; its public summary says the committee considered a number of clinical studies showing that linseed reduced habitual constipation. Recent trials are encouraging but small. Sun and colleagues (Asia Pacific Journal of Clinical Nutrition 2020) randomized 90 Chinese adults with functional constipation to 50 g a day of flaxseed flour with meals or 15 ml of lactulose solution for four weeks. Median weekly bowel movements rose from 2 to 7 with flaxseed and from 2 to 6 with lactulose, and the median Wexner constipation score fell from 14 to 6.5 and from 15 to 9 respectively; the authors judged flaxseed somewhat more effective than lactulose at increasing bowel movements. The study was not blinded and had no placebo group, and 50 g is a large daily amount.

Soltanian and Janghorbani (Clinical Nutrition ESPEN 2019) randomized 77 constipated adults with type 2 diabetes in Iran to cookies containing 10 g of flaxseed or psyllium, or placebo cookies, twice a day for 12 weeks. Both fibers improved constipation symptoms and stool consistency more than placebo, along with weight, fasting glucose, and cholesterol, and flaxseed appeared somewhat better than psyllium; the trial was single-blinded. For irritable bowel syndrome (IBS) the evidence is thinner. NICE guidance in England suggests that people with IBS who have wind and bloating may find oats and linseeds, up to one tablespoon a day, helpful. But in an open pilot trial by Cockerell and colleagues (Journal of Human Nutrition and Dietetics 2012), 40 people with IBS took two tablespoons of whole or ground linseeds a day or none for four weeks; 31 finished, improvements in symptom severity did not differ significantly between the groups, and stool frequency and consistency did not change.

Blood pressure is the most striking cardiovascular finding. In FlaxPAD, Rodriguez-Leyva and colleagues (Hypertension 2013) randomized 110 patients with peripheral artery disease, a narrowing of the leg arteries, to foods containing 30 g of milled flaxseed or a placebo every day for six months in a double-blind design. Systolic pressure ended about 10 mmHg and diastolic about 7 mmHg lower than with placebo, and in patients who started with systolic pressure of 140 mmHg or more, systolic pressure fell by 15 mmHg. The authors called it one of the most potent antihypertensive effects achieved by a dietary intervention. Pooled evidence is far more modest. Khalesi and colleagues (Journal of Nutrition 2015) combined 11 studies with 14 trials and found falls of 1.77 mmHg systolic and 1.58 mmHg diastolic, describing the effect as slight. Ursoniu and colleagues (Clinical Nutrition 2016) pooled 15 trials with 1,302 participants and found reductions of 2.85 and 2.39 mmHg, larger in trials lasting 12 weeks or more; ground flaxseed lowered systolic pressure, while lignan extracts had no significant effect.

Cholesterol and blood sugar: Pan and colleagues (American Journal of Clinical Nutrition 2009) pooled 28 trials and found that flaxseed products lowered total and LDL cholesterol by about 0.10 and 0.08 mmol/L (roughly 4 and 3 mg/dL), small changes driven by whole flaxseed and lignan supplements; flaxseed oil had no significant effect, and HDL cholesterol and triglycerides did not change. Effects were clearer in women, particularly after menopause, and in people with high cholesterol. In a year-long trial, Dodin and colleagues (Journal of Clinical Endocrinology and Metabolism 2005) gave 199 menopausal women 40 g of flaxseed or wheat germ daily; total cholesterol fell slightly but so did protective HDL cholesterol, an effect the authors called favorable but not clinically significant. Villarreal-Renteria and colleagues (Complementary Therapies in Medicine 2022) pooled seven trials in prediabetes and type 2 diabetes and found modest improvements in fasting glucose, HbA1c, and insulin resistance, using a fixed-effect model that can overstate certainty. For weight, NCCIH cites one trial suggesting that a flaxseed mucilage powder might help, while lignan extracts and oil do not seem to.

Hot flashes and cancer: an uncontrolled pilot by Pruthi and colleagues (Journal of the Society for Integrative Oncology 2007) reported hot flash scores falling by more than half in 30 women taking 40 g of crushed flaxseed daily, but the placebo-controlled follow-up was negative. In the North Central Cancer Treatment Group trial N08C7 (Menopause 2012), 188 postmenopausal women with or without breast cancer ate a flaxseed bar providing 410 mg of lignans or a placebo bar daily for six weeks; hot flash scores fell by 4.9 and 3.5 respectively, a difference that was not significant, and the authors concluded that the results do not support flaxseed for hot flashes. Dodin’s year-long trial likewise found no difference in menopausal symptoms between flaxseed and wheat germ. On cancer, an Ontario case-control study of 2,999 women with breast cancer and 3,370 without linked eating flaxseed with 18% lower odds of breast cancer (Lowcock, Cancer Causes and Control 2013), and small presurgical trials found lower tumor cell proliferation with 25–30 g a day in breast and prostate cancer. Flower and colleagues (Integrative Cancer Therapies 2014) concluded that flax may be associated with decreased breast cancer risk; these are observational and short biomarker studies, not proof that flaxseed prevents cancer.

For constipation, EMA’s well-established-use dose for adults and adolescents over 12 is 10–15 g of whole or broken seeds two to three times a day, each dose taken with 150 ml of water, milk, fruit juice, or similar liquid, followed by adequate fluid through the day. It should be taken during the day rather than immediately before bedtime, and at least half an hour to an hour apart from other medicines. The effect starts after 12 to 24 hours, and EMA advises seeing a doctor or pharmacist if constipation has not resolved within three days. For mild digestive discomfort, EMA’s traditional preparation is a mucilage: 5–10 g of whole or broken seeds soaked in 250 ml of water until fully swollen, taken half an hour before eating up to three times a day, with or without the seeds; a doctor should be consulted if symptoms last longer than a week. Food-based trials used other amounts: 30 g of milled flaxseed a day in FlaxPAD, 10 g twice a day in cookies in the Iranian constipation trial, and 50 g of flour in the Chinese one.

Ground or milled seed is the usual choice for nutrition, because whole seeds can pass through the gut largely intact: whole seed did not raise blood ALA in the Austria trial and compliance with it was poor, while nobody withdrew from the milled-seed group. For a laxative effect, EMA allows whole or broken seeds. The cyanide question pulls the other way, because grinding raw seed lets its own enzymes release hydrogen cyanide. BfR advises adults to eat no more than 15–20 g of raw ground linseed a day, about two to three level tablespoons (a level tablespoon is about 6 g), partly also to limit cadmium. Young children should not be given raw ground linseed, and children aged four and over should have no more than about 4 g a day, roughly a level teaspoon. Baking or cooking reduces the risk, because hydrogen cyanide boils off at about 26 °C and heat inactivates the enzymes. EFSA expects intact seed that is not freshly ground to give much lower cyanide exposure, but noted that a risk for adolescents could not be excluded if ground linseed, for example from a blender, were eaten at EMA’s laxative dose.

Flaxseed oil is sold as a liquid and in capsules. It supplies concentrated ALA but none of the seed’s laxative fiber, and in Pan’s meta-analysis it did not lower cholesterol; Britannica notes that the oil is unstable and goes rancid quickly. Lignan extracts standardized to SDG and flaxseed mucilage products are also sold, and results from one form should not be assumed to apply to another. In the Austria trial, digestive discomfort was common in the first weeks but faded in the milled-seed group, so starting with a smaller amount and building up is a sensible approach. Drink a full glass of liquid with each dose, keep drinking through the day, and keep flaxseed at least half an hour to an hour away from medicines. Both brown and golden seeds are available (NDSU). People who want flaxseed mainly as a food can add it to baking, which also reduces the raw-seed cyanide concern.