4 juil. 2025

Flax, chia or hemp seeds: which one should you choose?

For alpha-linolenic acid, flax wins by a wide margin: 20 g of ground seed delivers 4.6 g of ALA, against 3.6 g for the same weight of chia and 1.7 g for hulled hemp. For protein the ranking flips, since 20 g of hemp gives 6.3 g of protein, roughly twice what flax or chia provide. And for fibre, hulled hemp is not really in the same competition, with 0.8 g per portion against 5.5 g for flax and 6.9 g for chia.

Three things change how these seeds should be used day to day. Raw ground flax has a specific published ceiling: the German Federal Institute for Risk Assessment puts it at 15 to 20 g a day for adults, because of cyanogenic glycosides and cadmium. Chia is an authorised novel food in the European Union, and packaged seeds must tell the buyer not to exceed 15 g a day. And the ALA in all three raises blood EPA but not DHA, which means they cannot stand in for a direct source of long-chain omega-3.

What these seeds actually contain

The figures below come from the USDA SR Legacy database, per 100 g of dry seed. The portion figures use 20 g, roughly two level tablespoons of ground flax, which matches how these seeds are really used.

Per 100 g	Flax (169414)	Chia (170554)	Hulled hemp (170148)
Energy (kcal)	534	486	553
Protein (g)	18.3	16.5	31.6
Fat (g)	42.2	30.7	48.8
ALA, omega-3 (g)	22.8	17.8	8.68
Linoleic acid, omega-6 (g)	5.90	5.84	27.4
Fibre (g)	27.3	34.4	4.0
Calcium (mg)	255	631	70
Magnesium (mg)	392	335	700
Iron (mg)	5.73	7.72	7.95
Zinc (mg)	4.34	4.58	9.9
Potassium (mg)	813	407	1200

Scaled to a 20 g portion, that gives 107 kcal and 4.6 g of ALA for flax, 97 kcal and 3.6 g for chia, 111 kcal and 1.7 g for hemp. None of the three is a light food: two tablespoons carry as much energy as a small slice of bread with oil on it.

Two of those numbers deserve flagging because they circulate wrong. Hulled hemp contains 4 g of fibre per 100 g, not 6 to 10 g, since the hull that carried the fibre is exactly what has been removed. And it contains 48.8 g of fat, noticeably more than most comparison tables claim. Hulled hemp is a fatty, protein-dense food rather than a fibre food.

The ceiling on raw ground flax, and why it exists

Flax contains cyanogenic glycosides, mainly linustatin and neolinustatin. While the seed stays whole, those compounds sit apart from the enzymes that break them down. As soon as you chew, grind or mill the seed, a beta-glucosidase reaches them and hydrogen cyanide is released.

The European Food Safety Authority set an acute reference dose of 0.02 mg of cyanide per kilogram of body weight. On that basis, the German Federal Institute for Risk Assessment concludes that an adult can eat up to 15 to 20 g of raw ground flax a day, two to three level tablespoons, with no expected effect. A tablespoon holds roughly 6 g of ground flax.

The arithmetic is spelled out. With the legal maximum of 150 mg of cyanide per kilogram that applies to linseed placed on the market for the final consumer, and a factor of 3 accounting for the lower bioavailability of cyanide from flax, a 70 kg adult would reach the reference dose at around 28 g of seed. The 15 to 20 g recommendation therefore leaves headroom.

That lower bioavailability has been measured in people. In the study EFSA relies on, twelve volunteers ate foods carrying the same bound cyanide dose of 6.8 mg. Mean peak blood levels reached 16.95 micromoles per litre with unprocessed cassava and 15.46 with apricot kernels, against only 6.40 with 30.9 g of linseed. Flax releases its cyanide more slowly and less completely. The relationship is not linear, though: in one volunteer, 60 g and then 100 g of the same linseed produced peaks of 19.8 and 42.3 micromoles per litre, a more than proportional rise.

Heat changes the picture. Hydrogen cyanide boils at around 26 °C and evaporates readily, and cooking inactivates the enzymes that release it. Ground flax worked into bread dough or baked crackers is not the same proposition as a spoonful of raw ground flax stirred into yoghurt. Some packs carry the warning ‘Only to be used for cooking and baking. Do not consume raw!’ provided for in European law, and that instruction should be followed.

Cadmium sets a second ceiling. Flax accumulates more of it than most seeds, to the point that the Swiss maximum level is 0.5 mg per kilogram, five times the general oilseed value. The kidney is the organ most sensitive to this metal, which builds up there over years. That is the second reason the adult recommendation stops at 20 g a day.

For children the gap is stark. In a 15 kg child the reference dose is already used up by 6 g of seed. Toddlers should not be given raw ground flax at all, and from four years old the amount considered safe is 4 g a day, about a level teaspoon. None of this applies to flax baked into bread.

Chia is a novel food with a labelled limit

Chia had no significant history of consumption in Europe before 1997, which makes it a novel food in the regulatory sense. It appears on the Union list of authorised novel foods with precise conditions of use.

For pre-packaged seeds sold as such, the limit is 15 g a day, and the label must inform the consumer of it. The same list caps the seed content by product category: 5 % whole or ground chia in bread products, 10 % in baked products, breakfast cereals and fruit, nut and seed mixes, and 1.3 g per 100 g of yoghurt.

Fifteen grams of chia is roughly one generous tablespoon. That is well below the four tablespoons for two servings you find in many pudding recipes, which works out at two per person. A chia pudding is not something to eat without counting.

Hemp, THC and one mycotoxin

Food-grade hemp seeds come from low-cannabinoid varieties, but they are not free of tetrahydrocannabinol. The contamination mostly comes from contact with other parts of the plant during harvest and threshing.

The Swiss contaminants ordinance sets a maximum of 3 mg of delta-9-THC equivalents per kilogram for hemp seeds and products derived from them, and 7.5 mg per kilogram for hemp seed oil. A compliant product is nowhere near a psychoactive effect, but the value is not zero and poorly controlled batches can exceed it.

The same ordinance sets a maximum ochratoxin A level of 5 micrograms per kilogram for hemp seeds. That mycotoxin is tied to storage conditions, which is a practical argument for buying sensible quantities from a shop with fast turnover rather than keeping a large bag for a year.

Omega-3: what ALA does, and what it does not

All three seeds supply alpha-linolenic acid, a short-chain omega-3 the body cannot make. The body can elongate it into eicosapentaenoic acid and then into docosahexaenoic acid, but the yield at each step is very different.

Reviews of the human literature agree. Raising ALA intake over weeks to months increases the proportion of EPA in plasma, red cells, platelets and breast milk. DHA, by contrast, does not rise, and it can even fall in some pools at high ALA intakes. Stable isotope tracer studies show conversion to EPA is limited in men and onward conversion to DHA is very low. The fractional conversion is higher in women, an effect attributed to oestrogen.

The practical conclusion is blunt. Eating flax or chia improves ALA and EPA status. It does not replace a direct DHA source. For a vegetarian or vegan who wants to move DHA specifically, the documented route is preformed intake, which is what algae-based omega-3 supplements provide. The Swiss reference for adults is 250 mg of EPA plus DHA a day, a figure carried in the annex on nutrition claims.

Hemp has a quirk that comparisons usually mishandle. Its omega-6 to omega-3 ratio is about 3.2 to 1, genuinely better than most common vegetable oils. But that favourable ratio applies to a modest ALA total: 8.68 g per 100 g, roughly a third of what flax carries. A good ratio on a small amount does not replace a large amount.

Hulled hemp contains no DHA, contrary to what some tables claim. No land-grown oilseed produces it; plant DHA comes from microalgae.

What regulatory claims actually allow

The Swiss ordinance on food information sets precise thresholds, and applying them to these three seeds is revealing.

For omega-3, a food is a source if it contains at least 0.3 g of ALA per 100 g and per 100 kcal, and high in it from 0.6 g. Expressed per unit of energy, flax reaches 4.27 g of ALA per 100 kcal, chia 3.66 and hemp 1.57. All three clear the ‘high’ threshold, hemp included.

For fibre, the source threshold is 3 g per 100 g and the high threshold 6 g. Flax and chia clear both comfortably. Hulled hemp, at 4 g, is a source of fibre but not high in fibre, and you would need 150 g of it to get 6 g, which makes no sense in energy terms.

For protein, the source threshold is 12 % of energy. Hemp reaches 22.9 %, flax 13.7 % and chia 13.6 %. All three clear it, but the gap is real: hemp delivers twice as much per gram.

These thresholds describe composition, not effect. A food can be ‘high in omega-3’ and still supply a trivial fraction of daily needs if the portion eaten is tiny. That is the trap of thresholds expressed per 100 kcal.

Is hemp a complete protein?

The phrase ‘complete protein’ comes up every time hemp is discussed. It deserves testing against numbers rather than repeating.

The adult amino acid reference pattern published by WHO, FAO and UNU in 2007 requires 45 mg of lysine per gram of protein. USDA data give 1.28 g of lysine within 31.6 g of protein in hulled hemp, which works out at 40.5 mg per gram of protein: a score of 0.90, so below the pattern. Flax reaches 47.1 mg per gram, score 1.05, and chia 58.8 mg, score 1.31.

The result is counterintuitive. Hemp contains far more protein, but its protein is slightly poorer in lysine than chia’s or flax’s. As with most seeds and cereals, lysine is its limiting amino acid, which fits the mechanism set out in our article on essential amino acids.

This disqualifies nobody. A score of 0.90 on one amino acid, inside a diet that also contains lysine-rich pulses, causes no problem at all. But presenting hemp as the one seed with complete protein is wrong, and on this particular point the argument actually runs against it.

Fibre: how much, and how fast

British guidance puts fibre intake at 30 g a day, while adults average around 20 g. On that front flax and chia are efficient: a 20 g portion delivers 5.5 g of fibre for flax and 6.9 g for chia, close to a quarter of the daily target from two tablespoons.

That efficiency is also why people get uncomfortable. Going from 20 to 27 g of fibre overnight, with highly water-binding soluble fibre on top, routinely produces bloating and wind. The increase should be spread over two to three weeks, with enough fluid alongside. Flax and chia mucilage soaks up a lot of water; without liquid it firms up intestinal contents instead of softening them.

Soluble fibre also slows the absorption of some medicines taken at the same moment. Spacing doses by two hours covers most cases, but the question is genuine for drugs with a narrow therapeutic margin, and it belongs with a doctor or pharmacist rather than an article.

Whole or ground: the real difference

Whole flax often passes through the gut unopened. The hull resists chewing and the contents stay largely out of reach. For ALA, lignans and minerals, grinding is necessary.

Chia does not have that problem. Its hull is thin and it releases mucilage on contact with water, so it works whole as long as it is soaked. Dry chia swallowed straight down, especially a dry spoonful followed by a glass of water, swells into a gel and can obstruct the oesophagus. Soak it first, or mix it into something already wet.

Hulled hemp is eaten as it comes, with no grinding and no soaking. That is what makes it the easiest of the three to use.

Once ground, these seeds oxidise quickly: many double bonds, a hugely increased surface, and rancidity sets in within weeks at room temperature. Grinding small amounts and keeping the rest sealed in the fridge solves it. A smell of paint or linseed oil is a stop signal, not a flaw to mask.

What the trials actually show

Systematic reviews of these seeds exist, and they are more careful than the summaries built on them.

For flax and blood pressure, a meta-analysis of 33 trials totalling 2427 participants finds a fall in systolic pressure of 3.19 mmHg (95 % confidence interval: -4.15 to -2.24) and in diastolic pressure of 2.61 mmHg. Heterogeneity is very high, with I² above 92 %, meaning the trials are not all measuring the same thing. Effects are larger beyond 20 weeks and from 30 g of flax a day. That dose sits above the 15 to 20 g ceiling for raw ground flax, which is a reminder that trials often use flax baked into products, or oil, rather than spoonfuls of raw seed.

For chia, a meta-analysis of eight trials finds a fall in systolic pressure of 7.19 mmHg but no significant effect on body weight, body fat percentage, waist circumference, body mass index, fasting glucose or glycated haemoglobin. The blood pressure effect shows up in participants whose baseline pressure was below 140 mmHg and does not depend on the dose given, which weakens any causal reading. The authors themselves flag the small number of studies.

On blood lipids, a review of ten chia trials reports favourable trends where none excludes no effect: total cholesterol -2.98 mg/dl (95 % CI: -9.98 to 4.02), triglycerides -14.09 mg/dl (-33.46 to 5.28), LDL +2.07 mg/dl (-5.05 to 9.19).

On flax lignans and hormonal balance, a review of nine trials in menopausal women reports an effect on hot flush frequency and intensity that does not reach statistical significance, and insufficient data to say anything about hormone-dependent cancers.

So: signals, mostly on blood pressure, but nothing that justifies presenting a spoonful of seeds as a treatment. Someone on antihypertensive medication does not change their care because of a meta-analysis on seeds.

Choosing by what you need

To maximise ALA for the fewest calories, ground flax wins: 4.27 g of ALA per 100 kcal, and 4.6 g in a 20 g portion. Chia follows closely.

To add protein to a dish without changing its texture, hulled hemp is the obvious pick: 6.3 g of protein in 20 g, a mild nutty taste, no grinding and no soaking. It sits well alongside other plant protein sources.

For fibre and gelling power, chia is the most effective, with 6.9 g of fibre per portion and enough water-holding capacity to set a pudding without cooking.

For magnesium, hulled hemp dominates: 140 mg in 20 g, against 78 mg for flax and 67 mg for chia. For calcium it is chia, at 126 mg per portion, a real contribution but a secondary one next to the sources covered in our article on calcium and magnesium without dairy.

For iron the three are broadly similar per portion, between 1.1 and 1.6 mg, but this non-haem iron is poorly absorbed and its availability depends on the rest of the meal, as explained in our article on plant-based iron absorption.

Using them in the kitchen

Flax gel replaces an egg in leavened mixtures and patties: one tablespoon of ground flax to three tablespoons of water, ten minutes to stand. Since the mixture will be cooked, the raw flax question disappears. Chia gel works the same way, with a slightly grainier texture.

Ground flax goes into bread, pancake batter and crackers, where baking settles both the bioavailability and the cyanide question at once. Count on 30 to 50 g of ground flax per 500 g of flour; beyond that the crumb turns dense.

Soaked chia makes a pudding at roughly one tablespoon per 150 ml of plant milk, two hours in the fridge or overnight. It also thickens a compote or a cold sauce without cooking, though it leaves a beaded texture that does not suit everything.

Hulled hemp gets scattered over a salad, a soup or a plated dish at the last moment. It works well in make-ahead breakfast bowls and rounds out a meal salad. Blended with water it gives a plant drink that needs no straining, since no coarse fibre is left to remove.

These three combine well with pumpkin, sunflower and sesame seeds, whose profile is different: more vitamin E and zinc, far less ALA.

Varieties: brown or golden flax, black or white chia

Flax comes in two common shades, brown and golden. The difference lies in hull pigment rather than in a radically different nutritional profile: both supply ALA, fibre and lignans in comparable orders of magnitude. Golden flax tastes slightly milder and less bitter, which is why bakers tend to prefer it, while brown flax, more widely available and often cheaper, works perfectly well in bread or crackers. The intake ceilings and label instructions apply identically to both.

Chia is sold as black seeds, white seeds, or a mix. Here too the colour reflects hull pigment rather than any useful compositional difference; the same plant produces both shades, sometimes on the same stem. The only reason to choose is visual, when you want the seeds to show or to disappear in a pale preparation.

Hemp is sold in two forms that genuinely are not equivalent. Whole seeds keep their hull, and therefore their fibre, and have a crunch; they suit toasting and sprouting. Hulled seeds, sometimes sold as hemp hearts, have lost the hull and with it most of the fibre, but they concentrate protein and fat and are eaten as they come. The figures in the table above refer to hulled seeds; a comparison that mixes the two forms produces incoherent results, which explains part of the contradictory numbers in circulation.

Reading the label

Two statements on the pack matter more than any claim on the front.

For flax it is the warning ‘Only to be used for cooking and baking. Do not consume raw!’. It is not decorative: it marks stock that does not have to meet the ordinary maximum level for raw consumption, and stirring it raw into muesli bypasses precisely the safety margin that warning protects. Where the warning is absent, the maximum level applies and raw consumption within the 15 to 20 g a day range is what the rules anticipate.

For chia it is the statement about a 15 g daily intake. It is required on pre-packaged seeds, and its absence suggests the pack was not put together for the European market.

Loose or bulk sales are a special case. The information has to be displayed on or next to the container, but it disappears the moment the seeds go into your own jar. If you buy unpackaged regularly, it is worth writing the purchase date and the form on the jar yourself, if only because ground stock has such a short life.

Buying and storing them

Whole flax, chia and hulled hemp usually sit on the same shelf, often in 200 to 500 g bags. With flax it is worth checking whether the pack says whole or ground, because the two forms keep for completely different lengths of time and carry different label instructions.

Whole flax is forgiving. The hull protects the fat, and a sealed jar in a cool cupboard lasts months. Ground flax is the opposite. Match the pack size to what you will actually use and move the remainder to the fridge once opened. A small hand mill or a mortar handles a few days’ worth in a minute.

Chia causes few storage problems, since the seed stays whole and the hull keeps the fat enclosed. What matters here is quantity: if you take the 15 g a day label instruction seriously, a 500 g bag lasts more than a month, which argues against bulk packs.

Hulled hemp is the most fragile of the three, because the hull is gone and the fat content is the highest at 48.8 g per 100 g. It belongs in the fridge once opened and is best used within a few weeks. A bitter or sharp aftertaste means it has gone rancid.

For all three, the simplest check is to smell them. Fresh seeds smell mild and nutty. A smell of old oil, cardboard or paint means the fatty acids have already oxidised, and what is being lost is precisely the omega-3 you bought them for.

What these seeds will not do

They do not replace a meal. A 20 g portion delivers 97 to 111 kcal and 3.3 to 6.3 g of protein depending on the seed, a fraction of what a plate of pulses or tofu provides. They are useful as an ingredient, not as a base.

They supply no vitamin B12. On a fully plant-based diet that is the one nutrient which has to come from fortified foods or a supplement, as set out in detail in our article on vitamin B12.

They do not offset an otherwise poor diet. The measured blood pressure effect of flax is around 3 mmHg systolic, and salt intake, physical activity and body weight move that number considerably more.

And they do not call for a course of treatment. No body of evidence supports a short high-dose period; the effects studied come from regular moderate intake over weeks to months.

Common mistakes

Swallowing dry chia with a glass of water afterwards. The gel forms in the oesophagus rather than in the bowl. Soak it first.

Assuming whole flax will do. It comes out intact, with the ALA and lignans still inside.

Piling on portions because ‘it is healthy’. Raw ground flax has a published ceiling, chia carries a 15 g a day label instruction, and all three are energy dense.

Counting on these seeds for DHA. They contain none, and they do not generate useful amounts of it.

Buying pre-ground flax in a large pack and leaving it in a cupboard. It goes rancid. Whole seeds keep far better; grind as you go.

Serving raw ground flax to a young child. The reference dose is reached at 6 g in a 15 kg child. Flax baked into bread is a different matter.

Particular situations

Pregnant and breastfeeding women have a specific DHA need, with a reference of 200 mg a day on top of the 250 mg of EPA plus DHA recommended for adults. That intake cannot come from these seeds.

People on anticoagulants or antihypertensives should mention a large regular intake to their doctor, less out of fear of the seeds themselves than because the measured blood pressure effects, modest as they are, stack on top of treatment.

People with irritable bowel syndrome often tolerate a rapid soluble fibre load badly. A very gradual increase, or ground flax rather than whole chia, works better in practice.

Endurance athletes will find hemp a convenient protein addition, but the amount stays modest against the needs described in our article on vegetarian diets and endurance sports.

Sesame allergy is common; hemp and flax allergies are rare but documented, sometimes with cross-reactions. A skin or respiratory reaction after eating them warrants medical advice rather than a dose adjustment.

Frequently asked questions

How much ground flax a day at most? Fifteen to twenty grams of raw ground flax for an adult, two to three level tablespoons, according to the BfR assessment. That ceiling covers both the cyanide released and the cadmium accumulated in the seed.

Does cooked flax count towards that limit? Heat destroys the enzymes and drives off hydrogen cyanide, which settles the first problem. Cadmium is unaffected by cooking.

Can children have flax? No raw ground flax for toddlers. From four years old, around 4 g a day, a level teaspoon. Flax baked into bread is not covered by the cyanide part.

Does chia help with weight loss? The available meta-analyses find no significant effect on weight, body mass index, body fat percentage or waist circumference.

Can hemp trigger a drug test? Compliant seeds stay below 3 mg of THC equivalents per kilogram, far from any perceptible effect. Positive screening results have historically been reported with heavily contaminated products, which is precisely why that maximum level exists.

Should you pick just one of these seeds? No, but stacking all three every day serves no purpose. Flax for ALA, hemp for protein, chia for fibre and texture: one or two at a time is enough.

Does chia have to be organic? The organic argument changes neither the measured composition nor the 15 g a day labelling limit.

Are sprouted versions of these seeds worthwhile? Sprouting works for unhulled hemp and is awkward for flax and chia because of the mucilage. The topic is covered in detail in our article on sprouting sunflower seeds, whose microbiological precautions apply here too.

Limits of this article

The composition data come from the USDA SR Legacy database, last published in 2018 and no longer updated. Real contents vary with cultivar, soil and year, and the hulled hemp values in particular rest partly on industry-supplied data.

The clinical trials cited cover heterogeneous populations and doses, often in people who already have a metabolic disorder, which limits how far they transfer to a healthy person adding a spoonful of seeds to breakfast.

No Swiss-specific assessment of raw ground flax consumption is published; the numerical recommendation used here comes from the German assessment, built on the European reference dose and on maximum levels identical to those applying in Switzerland.