Spirulina is a cyanobacterium that can be consumed by humans and other animals and is made primarily from two species of cyanobacteria: Arthrospira platensis and Arthrospira maxima.

Arthrospira is cultivated worldwide; used as a dietary supplement as well as a whole food; and is available in tablet, flake and powder form. It is also used as a feed supplement in the aquaculture, aquarium and poultry industries.

Nutrient and vitamin content

Protein

Dried spirulina contains about 60% (51–71%) protein. It is a complete protein containing all essential amino acids, though with reduced amounts of methionine, cysteine and lysine when compared to the proteins of meat, eggs and milk. It is, however, superior to typical plant protein, such as that from legumes.

The U.S. National Library of Medicine said that spirulina was no better than milk or meat as a protein source, and was approximately 30 times more expensive per gram.

Other nutrients

Spirulina's lipid content is about 7% by weight, and is rich in gamma-linolenic acid (GLA), and also provides alpha-linolenic acid (ALA), linoleic acid (LA), stearidonic acid (SDA), eicosapentaenoic acid (EPA), docosahexaenoic acid (DHA) and arachidonic acid (AA). Spirulina contains vitamins B1 (thiamine), B2 (riboflavin), B3 (nicotinamide), B6 (pyridoxine), B9 (folic acid), vitamin C, vitamin D, vitamin A and vitamin E. It is also a source of potassium, calcium, chromium, copper, iron, magnesium, manganese, phosphorus, selenium, sodium and zinc. Spirulina contains many pigments which may be beneficial and bioavailable, including beta-carotene, zeaxanthin, chlorophyll-a, xanthophyll, echinenone, myxoxanthophyll, canthaxanthin, diatoxanthin, 3'-hydroxyechinenone, beta-cryptoxanthin and oscillaxanthin, plus the phycobiliproteins c-phycocyanin and allophycocyanin.

Vitamin B₁₂ controversy

Spirulina is not considered to be a reliable source of Vitamin B12. Spirulina supplements contain predominantly pseudovitamin B12, which is biologically inactive in humans. Companies which grow and market spirulina have claimed it to be a significant source of B12 on the basis of alternative, unpublished assays, although their claims are not accepted by independent scientific organizations. The American Dietetic Association and Dietitians of Canada in their position paper on vegetarian diets state that spirulina cannot be counted on as a reliable source of active vitamin B12. The medical literature similarly advises that spirulina is unsuitable as a source of B12.

Possible health benefits and risks

Safety

Toxicological studies

Toxicological studies of the effects of spirulina consumption on humans and animals, including feeding as much as 800mg/kg, and replacing up to 60% of protein intake with spirulina, have shown no toxic effects. Fertility, teratogenicity, peri- and post-natal, and multi-generational studies on animals also have found no adverse effects from spirulina consumption. Spirulina intake has also been found to prevent damage caused by toxins affecting the heart, liver, kidneys, neurons, eyes, ovaries, DNA, and testicles. In a 2009 study, 550 malnourished children were fed up to 10 g/day of spirulina powder, with no adverse effects. Dozens of human clinical studies have similarly shown no harmful effects to spirulina supplementation.

Quality-related safety issues

Spirulina is a form of cyanobacterium, some of which are known to produce toxins such as microcystins, BMAA, and others. Some spirulina supplements have been found to be contaminated with microcystins, albeit at levels below the limit set by the Oregon Health Department. Microcystins can cause gastrointestinal disturbances and, in the long term, liver cancer. The effects of chronic exposure to even very low levels of microcystins are of concern, because of the potential risk of cancer.

These toxic compounds are not produced by spirulina itself, but may occur as a result of contamination of spirulina batches with other toxin-producing blue-green algae. Because spirulina is considered a dietary supplement in the U.S., there is no active, industry-wide regulation of its production and no enforced safety standards for its production or purity. The U.S. National Institutes of Health describes spirulina supplements as "possibly safe", provided they are free of microcystin contamination, but "likely unsafe" (especially for children) if contaminated. Given the lack of regulatory standards in the U.S., some public-health researchers have raised the concern that consumers cannot be certain that spirulina and other blue-green algae supplements are free of contamination.

Heavy-metal contamination of spirulina supplements has also raised concern. The Chinese State Food and Drug Administration reported that lead, mercury, and arsenic contamination was widespread in spirulina supplements marketed in China.

Safety issues for certain target groups

Due to very high Vitamin K content, patients undergoing anticoagulant treatments should not change consumption patterns of spirulina without seeking medical advice to adjust the level of medication accordingly.

Like all protein-rich foods, spirulina contains the essential amino acid phenylalanine (2.6-4.1 g/100 g), which should be avoided by people who have phenylketonuria, a rare genetic disorder that prevents the body from metabolizing phenylalanine, which then builds up in the brain, causing damage.