Spirulina: What It Actually Is, and What to Check Before Buying
Spirulina is sold as an algae supplement, is not an alga, and contains a form of vitamin B12 that does not work in humans. None of that makes it useless — but it does mean most of what you read about it needs correcting before the useful part starts.
- Botanical name
- Arthrospira platensis
- Part used
- Whole biomass
What spirulina actually is
Spirulina is Arthrospira platensis, a filamentous cyanobacterium — a photosynthetic bacterium, not an alga. The distinction is not pedantry: bacteria have no nucleus and no cellulose cell wall, which is why spirulina needs none of the wall-cracking processing that chlorella requires. Its contents are readily available.
It grows naturally in warm, highly alkaline lakes. Around Lake Chad it has been harvested and dried into cakes called dihé for centuries, and Spanish accounts of the Aztec capital describe a similar food gathered from Lake Texcoco.
Commercially it is grown in shallow alkaline ponds. The high pH is a genuine advantage: few other organisms tolerate it, so cultures stay relatively clean without heavy intervention.
Nutritional profile — and the B12 correction
Dried spirulina is roughly 55–70% protein by weight with a complete amino acid profile, plus iron, B vitamins, and the blue pigment-protein phycocyanin that gives it colour and is used as a natural food dye.
Spirulina is not a vitamin B12 source
It contains large amounts of pseudovitamin B12 — a chemically similar analogue that humans cannot use and which may actually interfere with true B12 metabolism. Anyone eating a vegan diet needs a genuine B12 source: a supplement or fortified food. Relying on spirulina for B12 is the single most consequential misconception about this ingredient.
The protein figure also needs perspective. It is high per 100 g, but a typical serving is a few grams — so the actual protein contribution to a day’s intake is small. Spirulina is a micronutrient supplement, not a protein source.
Contamination — the real safety issue
Spirulina itself does not produce toxins. Other cyanobacteria do, and open ponds can be contaminated by them — most importantly by microcystins, which are liver toxins.
Testing programmes have repeatedly found microcystins in some commercial spirulina products, particularly those harvested from natural water bodies rather than controlled ponds. This is the reason third-party testing matters more for spirulina than for most supplements.
- Look for products tested for microcystins specifically, not just for heavy metals.
- Prefer controlled-cultivation over wild-harvested.
- In Canada, an NPN indicates a Health Canada review; it is not a contamination test, but it is a baseline.
What research has investigated
Human trials exist across several areas, generally small and of mixed quality:
- Blood lipids, where meta-analyses have reported modest average changes
- Blood pressure, in a handful of small trials
- Exercise-related markers, with inconsistent results
- Allergic rhinitis, examined in a small number of studies
As with chlorella, there is essentially no vision-specific research. Spirulina appears in a marine-themed eye formula as a nutrient-dense component of the blend rather than on eye evidence.
Who should avoid it
- Phenylketonuria (PKU). Spirulina is high in phenylalanine — a hard contraindication.
- Autoimmune conditions. Spirulina has been studied for immune-stimulating effects; caution is generally advised, particularly with immunosuppressant medication.
- Anticoagulants. Vitamin K content and possible platelet effects; check with a pharmacist.
- Gout or high uric acid. High nucleic acid content in microbial biomass is a consideration at large intakes.
- Pregnancy and breastfeeding. Contamination risk makes this a poor category to guess in.
How to read this guide
This page describes a plant and what has been studied about it. It is not advice about whether you personally should take anything. Dietary supplements are not medicines. They are not intended to diagnose, treat, cure or prevent any disease. Information on this page is for general education and does not replace advice from a doctor, pharmacist or other qualified health professional.
Talk to your doctor before starting any supplement if you take prescription medication, have a diagnosed condition, or are scheduled for surgery.