It is the fashionable superfood recommended by wellness gurus as a perfect way to start the day when it’s sprinkled on porridge.
But scientists are warning that eating too much flaxseed could cause cyanide poisoning.
Also known as linseed, it is rich in fibre, omega-3 fatty acids and micronutrients, and in the current trend is to add it to breakfast cereal or blend it into smoothies.
But the seeds also contain a naturally occurring compound called amygdalin, a type of "cyanogenic glycoside" that can produce cyanide gas as it degrades.
More cyanide is released if the flaxseed has been ground – a form in which it is commonly sold, as the seeds themselves are quite hard.
Now scientists at the European Food Safety Authority (EFSA) have published a report warning that eating as little as a third of a teaspoon of ground flaxseed can be dangerous for a small child.
Adults could end up ill if they consume just three teaspoons of it at one sitting. Signs of cyanide poisoning include headache, confusion, agitation, irregular heart beat and trouble breathing. In severe cases, it can be lethal.
Long-term damage including neurological problems can result from repeated exposure.
The EFSA’s report concludes that eating 1.3g of ground flaxseed – a third of a teaspoon – could result in the amount of cyanide in a toddler’s body reaching dangerous levels.
All its figures are based on a "worst case scenario" which assumes the flaxseed contains a high concentration of cyanogenic glycoside. The report warns: "Taking into account all uncertainties, a risk for younger age groups cannot be excluded if ground linseed (eg when put in a blender) is consumed."
The 1.3g figure is for a toddler who is small for their age. By comparison, an average-sized adult could develop cyanide poisoning if they consumed 10.9g of ground flaxseed – just less than three teaspoons – in a meal. Many people might eat more than that. Health food shop Holland & Barrett, which sells Linwoods Milled Organic Flaxseed, says on its website: "We recommend each person takes 25 to 30g = two heaped spoonfuls daily."
It enthuses: "Flaxseed has been called one of the most powerful plant foods on the market and is rich in calcium, which contributes towards strong teeth and healthy bones."
The EFSA report, which looks at potential cyanide content in a variety of foods, names flaxseed/linseed alongside bitter almonds as one of "the foods with the highest occurrence values (of cyanide)".
Swedish authorities have already advised people against eating ground flaxseed. A government website warns: "The National Food Agency totally discourages eating crushed flaxseed."
The Swedish agency explains: "When eating whole flax seeds, only a small proportion of flax seeds break when chewed on. The vast majority are swallowed whole and therefore the risk of consuming harmful amounts of hydrogen cyanide is less compared to eating crushed flax seeds."
But it goes on: "When flax seeds are crushed or ground, the content of flax seeds becomes more accessible to the body. This increases the risk of getting harmful amounts of hydrogen cyanide."
The UK Food Standards Agency said it was aware of the EFSA’s report and the matter was being "kept under review".
A spokesperson said it should be noted that the EFSA opinion makes clear it is "conservative in nature", adding: "The assessment is therefore more likely to overestimate risk than underestimate it."
Linwoods said it had been selling milled flaxseed for 15 years and was unaware of any adverse effects from consuming it. "All our products are produced to the highest standards," it added. Holland & Barrett said it had "no reason to believe" the ground flaxseed contains the concentration of cyanide that would present a risk.