Spirulina is sometimes promoted for people following a plant-based diet because of its protein and vitamin B12 content. While it may have a role in this context, the value is often overstated.
Based on its dry weight, spirulina is about 60 to 75% protein (~65 g per 100 g dried spirulina) and contains all of the essential amino acids.[1] This is notable for a protein source that is not animal-based, but spirulina is usually only dosed up to 10 grams daily, which would provide 7.5 grams of protein at best. While this might help someone reach their protein goals, it only fulfills a fraction of daily protein requirements.
On average, a 10-gram dose of spirulina might contain around 20 micrograms of vitamin B12.[2] However, the majority of vitamin B12 in spirulina is actually a type of pseudovitamin B12 that is not readily usable by the human body.[3] One study found that 83% of the vitamin B12 in spirulina was in the pseudovitamin form, which would reduce the usable amount of vitamin B12 in a 10-gram dose from 20 micrograms to 3.4 micrograms. While 3.4 micrograms is a much smaller dose than that normally found in vitamin B12 supplements, it technically does meet the Recommended Daily Allowances (RDAs) for all age and sex groups.[4]