Flaxseed - Overview of Health Benefits, Recommended Intake and Caution

For a brief overview, take a look at our Flaxseed - A Summary article. 

Flaxseed continues to be an increasingly popular ingredient, frequently added to meals and snacks due to its pleasant nutty taste, versatility, convenience and health-beneficial properties^[1-8].

The cultivation and consumption of flaxseed by humans can be traced as far back as 3000 BC, to the ancient kingdom of Babylon in modern-day Iraq, where the seeds were a valued food source and the fibres were used for linen production^[9]. In the 8th century it is documented that King Charlemagne believed so strongly in the health attributes of flaxseed that he passed a law to command his staff to consume flaxseed daily^[10]. Current research has provided evidence to credit the health benefits of past beliefs^[3-8]. Through to present day, flaxseed has seen a large increase in use and demand due to the evidence of health-protecting properties. Flaxseed particularly has been attributed to reduction of risk of diabetes mellitus, obesity, some cancers and heart disease^[11].

Flaxseed boasts an impressive nutrient profile (see Appendix 1), being a rich source of protein, fibre, essential fatty acids and vitamins. Flaxseed is a great source of the essential omega-3 fatty acid alpha linolenic acid (ALA), which can be converted to EPA and DPA by the body^[12]. Read here for more information.

Flax is grown predominantly in moderate climates such as Russia, Belgium, Canada, China and Kazakhstan^[13]. The fibres of the flax plant are used for the production of linen, a process leaving minimal waste^[14]. When fully grown, the flax plant resembles that of long meadow grasses growing to approx. 1.2m in height and bears a five-petalled blue flower where the dry round fruit ripens, containing the flaxseed. The seeds can be eaten whole or milled, or they can be pressed to extract the oil for use in cooking, medicine and health care^[15]. The grinding and milling processes prevent the seed from being damaged by oxidation; milling also increases the bioavailability of many nutrients as it breaks down the outer shell^{[16, 17]}. Read here for how the flaxseed in Huel is produced.

Brown and golden flaxseed

The two crop varieties of flaxseed are brown and golden (yellow) flax. Canada predominantly grows brown flaxseed and areas such as South Dakota golden flaxseed^[18]. Visually the varieties are easily distinguishable, and in taste comparisons, brown is often the more favourable choice due to its subtle nutty tones. However, the nutritional profiles of both are similar, with brown flaxseed having a slightly higher ALA content^[19].

The health benefits of flaxseed have been supported extensively through research. Despite this there have been concerns raised.

Phytoestrogens

Phytoestrogens (also known as polyphenol phytoestrogens) are bioactive molecules. As the name indicates, phytoestrogens are derived from plants (’phyto’ being Greek for plant) and have a chemical structure that is similar to but not the same as that of human oestrogen. Phytoestrogens are naturally present in many common foods such as vegetables, seeds, berries, wine and tea^[20]. There are a variety of structurally different compounds such as isoflavones (mainly found in soya), lignans (flaxseed and grains) and stilbenes (in grape skin)^[20]. Due to their structure, phytoestrogens have the potential to bind to the oestrogen receptor in humans and can act either like weak oestrogen promoters or inhibitors^[21].

Evidence credits the ingestion of phytoestrogens with protective qualities against a range of health problems including breast, bowel and prostate cancers, cardiovascular disease, cognitive deterioration and osteoporosis^[22].They have also been shown to reduce menopausal symptoms and side effects of polycystic ovary syndrome (PCOS)^{[22, 23]}.These benefits largely stem from lignans which have a high antioxidant content that protect cells from free-radical damage^[8].

Flaxseed and lignans

Flaxseed has the highest concentration of lignans, containing up to 800 times more than other plant species^[23]. Lignans play an important role in plant growth and act as antioxidants in human metabolism. The lignans from flaxseed are converted in the colon into the mammalian lignans enterodiol and enterolactone via gut bacteria. Enterodiol and enterolactone function as antioxidants preventing cell damage^[5]. Lignans from flaxseed have been shown to decrease the risk of some cancerous development, have protective effects against chemotherapy to healthy cells^[8] and can lower low-density lipoprotein (LDL–‘bad’ cholesterol) levels^[24].

Plant lignans are converted into relatively weak oestrogenic compounds in the intestines via the gut flora^[25].These compete for binding sites with the much stronger endogenous and exogenous oestrogens^[26]. This process has shown to help reduce excessive hormonal stimulation on oestrogen-sensitive tissues, particularly offering a protective effect against breast and prostate cancers, reducing the requirement for hormone replacement therapy (HRT) in menopausal women and suppressing the overproduction of oestrogen in PCOS^[26].

Phytoestrogens and testosterone

Testosterone is the male sex hormone, mainly produced in men primarily in the testes, but also in the adrenal cortex. Testosterone is responsible for the development of male characteristics and sexual development and stimulates sperm production, sex drive, fertility, muscle mass and muscle development^[27]. Although it is thought of as a male hormone, testosterone is also produced in females – albeit in relativity low quantities (approx. 10 times lower than in males^[28]) – where it is produced in the ovaries and the adrenal cortex and is essential for the maintenance of health^{[28, 29, 30]}.

Concern has been raised that the ingestion of phytoestrogens could alter or inhibit the uptake of testosterone or exhibit pro-oestrogenic effects, particularly in males. However, these claims are unfounded and merely based on poorly designed studies on rodents that were administered large quantities of extracted phytoestrogens^[31]. Phytoestrogens can indeed bind to both alpha- and beta-oestrogen receptors, but methylation and glycosidation of phytoestrogens limit and even inhibit any pro-oestrogenic activity, thus preventing any potential adverse effects on testosterone uptake, production or signalling in both males and females^{[32, 33]}. As testosterone is one of the key hormones that drives muscle protein synthesis, low testosterone levels may indeed inhibit muscular development^[34]. However, the development of muscle tissue requires multiple factors, including resistance training, appropriate nutritional intake and adequate rest, all of which factor into a lengthy time process which requires a high level of dedication and patience^[35]. There are articles that suggest that the consumption of phytoestrogens might lower or limit the production of testosterone. However, the evidence points to the contrary: the fact that flaxseed is broken down by the body into lignans can actually lower the effectiveness of oestrogen in the body having the reverse effect^[36]. Moreover, research investigating testosterone levels in vegetarian and vegan men have presented higher levels of androgens, compared to that of male meat eaters, reducing concerns that phytoestrogens lower testosterone^[37].

It is also worth mentioning that there is evidence that suggests excessive alcohol consumption and a high body fat percentage present pro-oestrogenic effects on the body^{[38, 39]}.

The consumption of phytoestrogen from flaxseed does not appear to affect sex hormone metabolism in males or females, principally due to the phytoestrogens from flaxseed being lignans rather than isoflavones^{[40, 41]}.

Cyanide compounds in foods

Cyanide is present in the environment: it’s in air, drinking water, some soil and some foods. Cyanide comes from organic sources such as cyanide-producing bacteria, algae and plants, and chemical sources such as industry and pesticides ^[42]

Plants that are able to liberate significant amounts of hydrogen cyanide (HCN) from the environment, through respiration, photosynthesis and through the soil, are referred to as ‘cyanogenic’^[42]. Nearly 3,000 plant species have been reported to be cyanogenic: the most commonly consumed include almonds, cashews, cassava root, lima beans, spinach, millet, bamboo shoots, soya and flaxseed^{[42, 43]}. Cyanide compounds in plant-based foods occur naturally and consist of alpha-hydroxynitriles and cyanogenic glucosides, called cyanohydrins^[44]. The HCN and cyanide glycosides contained in plants and seeds support metabolic processes in the plant, aid the production of plant-based proteins and enzymes and are also believed to be produced to cause a bitter taste to ward off herbivores^{[42, 44]}.

You’re more likely to ingest these trace amounts of cyanide when such foods are consumed raw and dry, as heat and water breaks down the compounds. When flaxseed is eaten raw, the body has a natural capacity to break down a substantial amount of the cyanide compounds into thiocyanate which is expelled through urine and carbon dioxide when we breathe out^{[43, 44]}. The low exposure from naturally occurring plant compounds will leave the body with 12-18 hours without causing any harm to health^[43]. See Appendix 2 for the amount of HCN in certain foods.

Vitamin B12 is an essential micronutrient. A supplemental form of B12 is cyanocobalamin which is the active component bound to a cyanide molecule. Extensive trials have concluded that because the cyanide is bound within the structure of cyanocobalamin, the cyanide cannot harm human or animal health^[45] and this form is highly stable^[46]. Diets rich in protein have been shown to be protective and to aid clearance of cyanide from the body from both naturally occurring and chemical exposure^{[47, 48]}.

Inorganic cyanide was a main constituent of many strong pesticides, and their use has been banned or restricted in most countries. However, it is still used in industrial processes in Germany, Japan, Netherlands and the USA and has been found in some unregulated weed killer treatments; although leaching from such compounds is very low^[49]. Similarly, cyanide can be found in water; exposure through drinking water (tap and bottled) is prevented by regular testing to ensure the levels of HCN do not exceed 0.02ppm^[50]. To limit exposure, global law dictates that all chemical spillages must be reported and foods that have been treated with pesticides are subjected to strict food and contaminant testing laws with large penalties for those found to be in violation^{[49, 50]}. Tobacco smoke is the most common cause of cyanide entering the body.^[46].

Flaxseed cyanide content has surfaced recently from the health warning from the Swedish ‘National Food Authority’ (NFA) who highlighted what is claimed to be a potential risk of cyanide poisoning from flaxseed if consumption exceeds two teaspoons per day^[51]. The warning was released to the public to make consumers aware that flaxseed can produce HCN. However, the report also states (translated):

‘is very unlikely that you get in such a dose over crushed flaxseed. Acute poisoning symptoms can include headache, nausea, dizziness, confusion and numbness. Serious hydrogen cyanide poisoning can affect breathing. There are no published reports of acute hydrogen cyanide poisoning caused by crushed flaxseed’^[51].

While it's essential to be aware of potential food toxicity, in this instance, the warning is overly cautious and unnecessary. Sweden is the only country globally that has raised a concern regarding HCN levels in flaxseed. To support this evaluation, studies in 2008 and 2009 concluded that 40g of flaxseed per day saw the participants achieve the maximum health benefits without any adverse effects^[52]. Also studies in 1994 and 1995 showed observations in improved health status in all participants who consumed 50g of raw flaxseed per day^[6]; improvements were particularly noted in cardiovascular and respiratory health, improved blood glucose and cholesterol ratios, improved digestive health and weight compared with those who did not consume flaxseed. Furthermore, there was no reported incidence of toxicity^{[6, 53]}.

In mid-2019 the British media picked up on an EFSA report and suggested that a third of a teaspoon could lead to poisoning in children. The EFSA later addressed these concerns stating that the results had been misinterpreted and that there has yet to be a case of flaxseed poisoning^[53].

The flaxseed in Huel is cold-pressed, milled, packaged and stored under special conditions and subjected to regular ‘critical control points’ (CCP) to ensure that all the valuable nutrition and health attributes are preserved. For those who regularly consume Huel there need not be any concern about HCN levels, as the quantity of flaxseed is well within the lower range of that which is deemed safe for human consumption. Based on research findings, cyanide compounds in flaxseed leave the body via the urine and are expelled along with carbon dioxide^[43].

Flaxseed and health

The health benefits of flaxseed have been examined and demonstrated to overwhelmingly aid and protect human health. The publications highlighting possible adverse effects from phytoestrogens and cyanide have been based on assumptions or on rodent trials, rather than on how the compounds are broken down or taken up in the human body. As described above, the body has mechanisms to break down and eliminate phytoestrogens and HCN. While it is important to be aware of and to take health warnings seriously, the evidence does not support the notion that flaxseed is deleterious to human health; in fact, evidence supports the overwhelming benefits of consuming flaxseed regularly.

Appendix 1

Table 1: Nutritional value of brown flaxseed per 100g

Nutritional value per 100g

Table 2: Micronutrient profile of brown flaxseed per 100g

Appendix 2

Table 3: Hydrogen cyanide (HCN) content mg/kg in commonly consumed foods^{[42, 43]}

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