Five grams of dried spirulina, the upper dose printed on most labels, provides 14.5 kcal, 2.87 g of protein and 1.43 mg of iron. Measured against the reference values in Annex 10 of the Swiss ordinance on food information, exactly one nutrient clears the legal threshold for a “significant amount” of 15 per cent at that dose: copper, at 30.5 per cent. Iron reaches 10.2 per cent, riboflavin 13.1 per cent, calcium 0.8 per cent.

That gap explains almost every misunderstanding about these powders. Composition tables circulate per 100 grams, where spirulina genuinely shows 57.5 g of protein and 28.5 mg of iron. Nobody eats 100 grams. At the dose actually recommended, you divide those figures by twenty.

None of which makes microalgae useless. It just moves the question. Instead of asking whether they are rich in something, ask what they deliver at the dose you take, what else they might deliver alongside it, and what the label is legally allowed to promise you.

What spirulina and chlorella actually are

Spirulina is not an alga. It is a filamentous cyanobacterium of the genus Arthrospira, now often reclassified as Limnospira. It grows naturally in warm fresh water around 25 °C, alkaline, with a pH of 8 to 11.5, rich in carbonates and bicarbonates: the alkaline lakes of Chad, Ethiopia, Mexico, Peru, India and Thailand. Its spiral shape gave it its commercial name, but it belongs to the same broad family as the cyanobacteria that bloom in some Swiss lakes each summer. That relationship is not trivia. It explains most of the safety section below.

Chlorella is a genuine single-celled green freshwater alga. Its cell wall resists digestion, which forces producers to break it mechanically before drying. Chlorella powder with an untreated wall passes through the gut releasing only part of its contents, which makes “broken cell wall” a more useful phrase on a label than most decorative certifications.

Both are grown in open ponds or closed photobioreactors. The cultivation method is not marketing filler: an open pond exchanges with its environment, and that is the route by which the contamination described below arrives.

In Switzerland neither counts as a novel food. The ordinance on novel foods lists products that may be placed on the market without authorisation because they are new but assessed; spirulina and chlorella do not appear on it, because their food use predates 15 May 1997 and they are therefore not considered novel at all. They are sold as ordinary foods or as supplements, with no prior procedure at the Federal Food Safety Office.

What a real dose delivers

Here is the composition of dried spirulina as published in the USDA FoodData Central database (SR Legacy 170495), then what that means at 3 g and 5 g against the Swiss reference values for adults.

Nutrient	Per 100 g	At 3 g	At 5 g	% of reference at 5 g
Energy	290 kcal	8.7 kcal	14.5 kcal	not applicable
Protein	57.5 g	1.72 g	2.87 g	5.7% of the 50 g reference
Iron	28.5 mg	0.86 mg	1.43 mg	10.2%
Copper	6.1 mg	0.18 mg	0.31 mg	30.5%
Riboflavin	3.67 mg	0.11 mg	0.18 mg	13.1%
Thiamine	2.38 mg	0.07 mg	0.12 mg	10.8%
Magnesium	195 mg	5.9 mg	9.8 mg	2.6%
Calcium	120 mg	3.6 mg	6.0 mg	0.8%
Zinc	2.0 mg	0.06 mg	0.10 mg	1.0%
Sodium	1048 mg	31 mg	52 mg	0.13 g of salt
Vitamin B12	0 µg	0	0	0%

Two readings coexist here, and they tell different stories. Per 100 g, spirulina clears the “high in” threshold of Annex 13 for ten nutrients, and its protein supplies 79 per cent of its energy value, far above the 20 per cent that permits a “high in protein” claim. Those figures are accurate. They are also practically meaningless, because the portion consumed is twenty times smaller.

For scale: you would need 28 g of spirulina to match the 16.1 g of protein in a 120 g portion of tofu under the Swiss dietary recommendations, and 10.1 g to match its 2.88 mg of iron. A plate of 200 g of cooked lentils supplies 18 g of protein and 2.88 mg of iron. No powder taken at 5 g a day competes with that, and none needs to in order to be worth having.

Copper deserves its own note, because it is the only nutrient the usual dose supplies in a genuinely significant amount. It is also a nutrient rarely short in an ordinary diet, which is precisely why nobody talks about it. The result is mildly ironic: the nutrient these powders deliver most efficiently is almost never the one printed on the front of the tub.

Sodium deserves a brief note too, since it never appears in promotional tables. Dried spirulina contains 1048 mg of sodium per 100 g, which works out to roughly 52 mg at 5 g, about 0.13 g of salt. Negligible over a day. It does explain the faintly salty aftertaste that trips up a lot of first-time users, and it is one more illustration of how much the numbers shift depending on whether you read them per 100 g or per serving.

One last point on the table: a fibre content of 3.6 g per 100 g means 0.18 g at 5 g. Anyone counting green powders towards fibre intake is doing arithmetic that does not survive contact with reality. A single apple supplies twenty times as much.

The vitamin B12 problem

This is where the distance between marketing copy and evidence is widest, and it matters directly to vegetarians and vegans.

Spirulina does contain compounds that behave like vitamin B12 in microbiological assays. The chemical characterisation published by Watanabe and colleagues in 1999 showed that in spirulina tablets, 83 per cent of the dominant cobamide is pseudovitamin B12, an analogue that does not bind human intrinsic factor and is therefore inactive; only 17 per cent was actual vitamin B12. Microbiological assays returned values six to nine times higher than intrinsic-factor assays, which is how flattering numbers survive in some product literature.

ANSES drew an unambiguous conclusion: spirulina is not a reliable source of vitamin B12 for people excluding animal products. A supplementation trial in deficient children had failed to correct their macrocytic anaemia.

Chlorella is a subtler case, and that is exactly what makes it a trap. The analysis of nineteen chlorella supplements published in 2016 found contents ranging from under 0.1 µg to roughly 415 µg per 100 g of dry weight, a spread of more than four thousandfold between batches. In four tablet types a real share was present as active coenzyme forms, 5’-deoxyadenosylcobalamin and methylcobalamin. Other batches contained inactive corrinoids and a cobalt-free cobamide. So some chlorella batches contain usable B12 and some do not, and the label will not tell you which one you are holding, because nothing requires a producer to distinguish them.

The practical upshot fits in one sentence: if you need vitamin B12, take a dosed cyanocobalamin supplement and check your status with a blood test. Our page on vitamin B12 needs and reliable sources sets out the benchmarks. A microalga is not a substitute for that decision.

Safety: what can genuinely go wrong

Most buying guides handle this with a vague instruction to pick a reputable brand. There are actual figures and actual rules.

Heavy metals

Microalgae absorb what their growing medium contains, including the parts you would rather they left behind. The Swiss contaminants ordinance, as of 1 January 2026, sets maximum levels for food supplements of 3 mg/kg for lead, 1 mg/kg for cadmium, 1 mg/kg for inorganic arsenic and 0.1 mg/kg for mercury. Supplements made up of at least 80 per cent dried seaweed get a raised limit of 3 mg/kg for cadmium and inorganic arsenic, which shows the legislator identified algae as a category of its own.

At the legal limit and a dose of 5 g a day, a compliant product would supply 15 µg of lead, 5 µg of cadmium and 5 µg of inorganic arsenic daily. Small intakes, but they add to whatever the rest of the diet contributes, and the arithmetic is worth doing before you casually double the dose.

ANSES reported lead, mercury and arsenic above European limits in commercial spirulina samples. Conversely, an analysis of 52 commercial products published in 2025, 23 of them organic, found every sample compliant with EU limits for toxic metals. The dominant trace elements were aluminium, manganese, strontium and zinc. The same study did detect pharmaceutical residues in the supplements, caffeine being the most frequently found compound, which points to recent contamination linked to wastewater discharge.

Cyanotoxins

This risk is specific to cyanobacteria, and it justifies a distinction almost no consumer guide bothers to make.

ANSES reviewed the available contamination studies. Among products containing spirulina alone, most batches show no concerning level, with microcystins occasionally detected around 0.1 µg/g. By contrast, supplements containing Aphanizomenon flos-aquae, sold as Klamath algae, show markedly higher contamination, up to 18 µg/g. A profiling study published in 2025 confirmed the asymmetry: several cyanotoxins were detected in a Klamath-based supplement, with high concentrations of microcystin-RR.

The WHO set a tolerable daily intake of 0.04 µg per kilo per day for microcystins, which works out to 2.4 µg a day for a 60 kg adult. A spirulina batch contaminated at 0.1 µg/g would supply 0.5 µg at a 5 g dose, comfortably below. A product sitting at the 0.5 mg/kg maximum retained in the USP monograph, however, would supply 2.5 µg a day, above the tolerable intake, which is why ANSES considered the applicable thresholds in need of reassessment. Two caveats remain, which the agency states plainly: the data do not rule out that Arthrospira itself may produce microcystins, and contamination by other toxin-producing cyanobacteria remains possible in both artisanal and industrial production.

Beta-carotene, a question of dose

Here is the item missing from nearly every buying page. Spirulina contains 1.4 to 1.7 mg of beta-carotene per gram. A 5 g dose therefore supplies 7 to 8.5 mg of beta-carotene daily, while the daily intake limit from food supplements has been estimated at 7 mg. The maximum dose printed on many labels reaches or exceeds that limit on its own. At 3 g, intake falls to 4.2 to 5.1 mg, which leaves headroom.

That is a concrete reason to stay at the bottom of the range rather than follow the maximum on the tub.

Contraindications and adverse effects

ANSES explicitly advises against spirulina supplements in four situations: phenylketonuria, allergic predisposition, muscular vulnerability, hepatic vulnerability.

Phenylketonuria follows from the phenylalanine content, around 2.75 g per 100 g of dry powder, roughly 138 mg at a 5 g dose. The muscular and hepatic warnings rest on reported cases of muscle or liver injury after consumption, without any dose-response or duration relationship being established.

On allergy, a 2024 review screened 128 records and found only four case studies covering five patients, with reactions from grade 2 to grade 4 on the World Allergy Organization scale. Rare, then, but when it happens it is not trivial.

Contamination risk aside, ANSES concludes that spirulina does not appear to pose a health risk at low doses, up to several grams a day in adults. Clinical trials have gone as high as 19 g a day with nothing beyond digestive upset and headaches, though with sample sizes too small to detect rare reactions.

What this means in practice

Those four subsections compress into a simple ranking. The largest real risk is not the microalga itself but what travels with it: heavy metals, cyanotoxins, and in recent work pharmaceutical residues. Those risks depend almost entirely on origin and quality control, factors that could be on the packaging and rarely are. The second is dose, specifically beta-carotene at 5 g. The third concerns a small group of clearly named people and can be ruled out by reading the contraindications.

None of this makes spirulina or chlorella a dangerous product. Taken together, though, it explains why a certificate of analysis is worth more than any marketing promise, and why the advice to stay in the lower dose range is not excessive caution but simply the consequence of an arithmetic.

What the trials show, effect sizes included

The benefits attributed to these powders have been measured. The results are real and modest, an unusual combination in this aisle.

On the cardiometabolic side, a 2025 meta-analysis pooled 23 studies and 1,035 participants with overweight or obesity. Spirulina alone reduced body weight (g = -0.30, 95% CI -0.53 to -0.08), total cholesterol (g = -0.79), triglycerides (g = -0.64), LDL cholesterol (g = -0.71) and diastolic blood pressure (g = -0.73, 95% CI -1.43 to -0.03). Statistically significant, with confidence intervals wide enough in places to betray heterogeneity between trials.

For chlorella, a 2026 meta-analysis covering 18 studies and 717 participants, at doses scattered from 135 to 15,000 mg a day, reported a 0.72 percentage point drop in body fat, 0.35 kg/m² in BMI, 1.41 kg in weight, 5.73 mg/dl in LDL and 6.62 mg/dl in total cholesterol.

These populations carry excess weight. Nothing licenses transferring the results to someone of normal weight, and it would be honest to say so more often.

On the sports side, a 2026 meta-analysis gathered 22 randomised trials and 822 participants. The picture is mixed: time to exhaustion improved (SMD 1.06, 95% CI 0.16 to 1.96), VO2max only suggestively since its interval touches zero (SMD 0.88, 95% CI 0.00 to 1.75), maximal power output weakly (SMD 0.29), and time-trial performance not at all (SMD -0.27, 95% CI -0.74 to 0.21). Creatine kinase, a marker of muscle damage, fell clearly (SMD -0.78).

So the best-documented effect in athletes concerns recovery and endurance capacity, not timed performance. Roughly the opposite of what the packaging imagery suggests.

Detoxification: what the word covers

Chlorella has been sold on this promise for decades. Two things need separating.

On the regulatory side, the Swiss position is clear. The ordinance on food information provides that health claims are permitted only if they appear in Annex 14 or have been individually authorised by the Federal Food Safety Office. No claim relating to detoxification or heavy metal elimination appears in that annex. Article 35 adds that claims must not be inaccurate, ambiguous or misleading, and must not suggest that a balanced and varied diet fails to supply nutrients in appropriate amounts. A tub promising to “purify the body” sits in territory Swiss food law does not cover.

On the evidence side, most work on metal chelation by chlorella or phycocyanin sits in cell and animal models. There is a plausible mechanism. There is no clinical trial establishing that a few grams daily changes metal elimination in a healthy person.

There is also a logical tension the marketing handles poorly. These microalgae are prized precisely for their biosorption capacity, their ability to bind metals present in their medium. That same property is what makes them vulnerable to contamination and what led the legislator to set them specific maximum levels. A product sold to remove heavy metals is also, by construction, a product you need to check is not adding any.

Protein quality: the number rather than the adjective

“Complete protein” says little without a score. DIAAS, recommended by the FAO, measures true ileal digestibility of indispensable amino acids. According to the review published in Life in 2025, spirulina scores 85 per cent, against 56.6 per cent for chickpeas and 57.7 per cent for mung beans, 97 per cent for raw beef and 80 per cent for grilled or baked beef. Spirulina therefore sits above the pulses tested and above cooked beef, which is worth noting.

The limiting amino acids are methionine and cysteine, the sulphur-containing ones. Drying method matters: hot drum drying cuts methionine content by around 30 per cent compared with spray drying. That is the sort of detail that never appears on packaging.

The high score remains a quality per gram of protein. At 5 g of powder you are talking about 2.87 g of good-quality protein, which is a top-up rather than a source. To build intake properly, our complete guide to plant proteins and the page on essential amino acids are more use.

Worth a mention too is PER, the protein efficiency ratio, which serves as the official method in some countries including Canada. It measures weight gain in growing rats per gram of protein consumed. Spirulina has been estimated at 1.8 to 2.6, against 2.5 for casein, 1.23 for maize and 1.15 for wheat. That measure also places spirulina towards the top of the plant protein range, though a figure obtained in rats transfers to humans only loosely.

Iron, and why bioavailability changes the verdict

Iron is the argument most often deployed for these powders among vegetarians, and it is where the evidence has recently moved in a favourable direction.

At 5 g, intake is 1.43 mg. Against the NHS benchmarks of 8.7 mg a day for adult men and 14.8 mg for women aged 19 to 49, that is 16.4 per cent and 9.6 per cent respectively. A measurable top-up, no more.

But quantity is only half the story. A 2026 study measured mineral bioaccessibility and bioavailability across nine microalgae, using standardised in vitro digestion followed by a Caco-2 cell model. Iron bioaccessibility varied enormously by species, from 0.5 per cent in Tetraselmis chuii to 83.4 per cent in green Chlorella vulgaris. Iron bioavailability from Arthrospira platensis was the highest in the panel, comparable to ferrous sulphate, the pharmaceutical reference.

That is a genuinely favourable point worth stating: spirulina iron appears better absorbed than iron from most plant sources. It does not change the size of the intake, which stays small, but it does mean that gram for gram it counts for more. The usual levers still apply and are covered in our page on plant-based iron absorption.

How to choose in the shop

These are the points that genuinely separate two products, in descending order of importance.

The exact species, in Latin. Arthrospira platensis or Limnospira platensis for spirulina, Chlorella vulgaris or Chlorella sorokiniana for chlorella. A label content with “blue microalgae” or “green superfood” denies you the one piece of information that tells you what you are buying. Check in particular that it is not Aphanizomenon flos-aquae, Klamath algae, whose microcystin contamination profile is markedly worse.

An accessible certificate of analysis. The parameters that matter are lead, cadmium, arsenic, mercury and microcystins. A producer publishing batch results gives you more than one displaying three certifications with no numbers. You can compare those values against the maximum levels in the contaminants ordinance cited above.

Broken cell wall, for chlorella. Without that treatment, part of the cell contents is never released.

Cultivation method and location. Open pond or closed photobioreactor, with geographic origin. An open pond is not disqualifying, but it justifies insisting on analyses.

Country of production. ANSES recommends favouring supply chains best monitored by public authorities, with regulatory compliance, traceability and an identified manufacturer. In substance, that is advice about jurisdiction rather than about brand.

Price per kilo of powder is a secondary criterion. The stated phycocyanin content, often promoted as a quality marker for spirulina, corresponds to a minimum requirement of 5 per cent in the USP monograph; below that, the product falls outside specification. Our pages on reading a food label and on Swiss labels cover the broader ground.

Doses and how to take them

Doses documented in the literature run from 0.25 to 5 g a day for commercial products, with 5 to 10 g a day in supplementation studies addressing malnutrition, and up to 19 g a day in some clinical trials. In traditional consumption in Chad, intakes can exceed 50 g a week in dry weight.

For everyday use in adults, 2 to 3 g a day is a sensible compromise: copper intake stays significant, beta-carotene stays under the 7 mg limit, and the cost stays contained. Starting lower, at 1 g, lets you spot digestive intolerance before it becomes a nuisance.

On timing, there is no serious data. Taking it on an empty stomach in the morning for a “detox effect” is a belief, not a protocol. Taking the powder with a meal containing some fat has a rationale, since carotenoids are fat-soluble. Pairing it with a source of vitamin C in the same meal has a rationale for non-haem iron.

These microalgae replace no portion of anything. The Swiss recommendations call for one daily portion of a protein-rich food, defined as 60 g of dry pulses, 120 g of tofu, or 100 to 120 g of meat or fish. Three grams of powder does not substitute for that; it sits alongside it.

Cooking without wrecking the taste

Spirulina has a pronounced marine, faintly salty flavour that degrades with heat: phycocyanin, its blue pigment, is heat-sensitive and the whole thing turns bitter easily. So it goes in at the end, off the heat.

A few things that work:

In a vinaigrette, half a teaspoon to two tablespoons of oil, with lemon. Acidity and fat balance the bitterness better than anything else.

In hummus or an avocado cream, a teaspoon per 250 g of preparation. The fatty texture carries the flavour and the colour turns properly green, which looks rather good.

In a smoothie, over a tart base: pineapple, mango, berries. Mild dairy-style bases do not mask the taste, they underline it.

In soup, only after the heat is off and the temperature has dropped, or the colour turns an unappetising brown-green.

Chlorella is milder and more vegetal, with a background bitterness. It handles sweet preparations better, but it colours them even more intensely.

Common mistakes

Reading the per-100 g composition as if it were a serving. This is the parent error from which most of the others descend. Divide by twenty as a reflex.

Counting spirulina as a B12 source. It is not, ANSES says so explicitly, and chlorella only is so unpredictably from batch to batch.

Taking 5 g because that is the stated maximum. At that dose beta-carotene intake reaches or exceeds the estimated limit for supplements.

Treating spirulina and Klamath algae as equivalent. The microcystin contamination levels are not in the same range.

Expecting a detoxification effect. No such claim is authorised in Switzerland and no clinical trial supports it in a healthy person.

Stacking several green powders without checking doses. Spirulina, chlorella, barley grass, Klamath: the beta-carotene, copper and potential contaminants all add up.

Ignoring the contraindications. Phenylketonuria, allergic predisposition, muscular or hepatic vulnerability: these are explicit recommendations from a food safety agency, not legal boilerplate.

By profile

Vegetarian or vegan trying to cover B12. These powders are unsuitable for that goal. A dosed cyanocobalamin supplement, and a blood test.

Someone with borderline iron intake. A real 1.43 mg top-up at 5 g, with apparently good bioavailability, but no replacement for pulses or for treatment of a diagnosed deficiency.

Endurance athlete. The documented effect concerns time to exhaustion and muscle damage markers, not timed performance.

Someone with overweight and an unfavourable lipid profile. This is the population where measured effects are most consistent, with moderate effect sizes. It substitutes for no clinical management.

Pregnant or breastfeeding. Specific data are lacking, and medical advice is warranted before any supplement.

Anyone with phenylketonuria. An explicit contraindication.

Limits of this page

The composition values come from the American SR Legacy database, since spirulina and chlorella are absent from the ANSES Ciqual table. Real contents vary by strain, growing medium and drying process, sometimes considerably; the figures here are orders of magnitude, not guaranteed values for a specific product.

The ANSES opinion underpinning the safety section dates from 2017. The meta-analyses cited cover specific populations, mainly people with overweight or obesity, often with a few hundred participants and substantial heterogeneity between trials. The 85 per cent DIAAS comes from a review rather than an independent measurement on products sold in Switzerland.

Finally, the analysis that found pharmaceutical residues in 52 products does not say whether the detected quantities carry toxicological weight; it signals environmental contamination, which is already useful when choosing an origin.

Frequently asked questions

Spirulina or chlorella, which should I pick? It depends on the goal. For a top-up of good-quality protein and well-absorbed iron, spirulina has the better data. For lipid profile, both have favourable meta-analyses. For vitamin B12, neither is reliable.

Should I take courses or use it continuously? No data supports one schedule over another. Trials run from a few weeks to three months. Continuous low-dose use is more consistent with the fact that these effects, where they exist, are modest and gradual.

Is organic spirulina safer? The analysis of 52 products found no clear difference between the 29 conventional and 23 organic products for toxic metals, all of them compliant with EU limits. Organic certification covers the production method, not cyanotoxin screening. A batch certificate of analysis tells you more.

Can children take it? Trials in children in malnutrition settings used 5 g a day with no toxicity reported, but that is a medically supervised context. Outside it, the absence of specific safety data and the lower body weight argue for paediatric advice.

Is the blue colour a sign of quality? It comes from phycocyanin, for which the USP monograph sets a minimum of 5 per cent in spirulina. An intense colour is consistent with a decent content, but phycocyanin degrades with heat and light, so a faded powder mainly signals poor storage.

Is it environmentally worthwhile? Biomass yield per hectare is high and needs no quality arable land. That said, these powders are eaten by the gram: they do not replace a food crop at the scale of a diet, and the environmental argument bears more on industrial uses than on a tub of supplement.

How do I know whether my product contains microcystins? Only an analysis tells you. Ask the manufacturer for the batch report; if none exists, that is itself an answer.

Sources

• ANSES opinion on food supplements containing spirulina, request 2014-SA-0096
• Swiss ordinance on maximum levels for contaminants (SR 817.022.15)
• Swiss ordinance on food information (SR 817.022.16)
• Swiss ordinance on novel foods (SR 817.022.2)
• Swiss dietary recommendations for adults, Federal Food Safety Office
• USDA FoodData Central, dried spirulina (SR Legacy 170495)
• NHS, iron reference intakes
• Watanabe et al. 1999, pseudovitamin B12 in spirulina tablets
• Watanabe et al. 2016, vitamin B12 compounds in chlorella supplements
• Life 2025, Arthrospira platensis as a protein source, DIAAS
• 2025 meta-analysis, spirulina and cardiometabolic health
• 2026 meta-analysis, chlorella and cardiometabolic markers
• 2026 meta-analysis, algae and exercise performance
• 2024 review, spirulina and allergy
• 2025 analysis of 52 spirulina and chlorella supplements
• 2025 cyanotoxin profiling in spirulina and Klamath supplements
• 2026 study, bioaccessibility and bioavailability of microalgal minerals

For neighbouring topics: spirulina and chlorella, superfoods or marketing, edible seaweeds, nutrition and recipes, choosing algae-based omega-3 supplements and zinc supplements on a vegetarian diet.