Spirulina and chlorella supplements: evidence and contamination concerns

Spirulina is commonly called blue-green algae, but a review of its effects on weight and lipids explains that it was first classed as a plant because of its pigments and ability to photosynthesise, and was later placed among the cyanobacteria. It grows in salt and fresh water in warm regions, and the most commonly studied species are Spirulina platensis, Spirulina maxima and Spirulina fusiformis. Its blue pigment phycocyanin and other antioxidants are the focus of much of the research.

Chlorella is a green unicellular alga, produced commercially and sold worldwide as a dietary supplement. A 2020 review in Nutrients says chlorella products contain a range of nutrients including vitamins D and B12, folate and iron. Because the cell wall is made of cellulose that people cannot digest, supplements are made from broken cells.

The Open Heart review on weight loss and blood lipids concludes that the literature supports benefits of spirulina for reducing body fat, waist circumference, BMI and appetite, and for improving blood lipids, with possible effects on insulin resistance, glucose and blood pressure. It attributes the variation in trial results to differences in dose, duration of treatment and patients' other conditions, and describes spirulina as not established to have significant side effects.

Those findings come from a review of trials of varying size and design rather than from the large, long, independent studies that would justify treating it as a medicine. Cholesterol, weight and blood pressure are managed through a GP, so spirulina should not replace prescribed medicines or a review of them.

The Nutrients review reports that meta-analyses of chlorella supplementation on cardiovascular risk factors suggest improvements in total cholesterol, LDL cholesterol, systolic and diastolic blood pressure and fasting glucose, but not in triglycerides or HDL cholesterol. It suggests part of the effect may come from the indigestible fibre fraction, which binds bile acids in laboratory work, and from a mixture of nutrients and antioxidants.

The review also describes smaller studies and animal work on allergy, immune and mood effects, including a trial of a dried chlorella extract in people with major depressive disorder. The authors themselves say information on which compounds are responsible is limited, so these are leads for research rather than reasons to self-treat.

A 2022 study of spirulina dietary supplements sold on the Slovenian market notes that oversight of the supplement industry is limited, that adulteration, misbranding and undeclared ingredients are growing concerns, and that cadmium, mercury, arsenic and lead have been found in spirulina products in unusual quantities, linked to pesticide or fertiliser use near cultivation sites. The authors point out that supplements are not inspected as strictly as other food products or medicines.

A separate US study of blue-green algae supplements made from a different species found that just over half of the products tested had measurable microcystin, a liver toxin produced by some cyanobacteria, and a few exceeded a state limit. That paper is not about spirulina or chlorella specifically, but it shows why algae products have a reputation for variable quality. These studies are from other countries and cannot be applied to every UK product.

• Choose products that state the species, the country of origin and the serving size.

• Avoid unfamiliar online sellers; the FSA warns that counterfeits are harder to spot on marketplaces.

• Do not exceed the serving on the label, since the FSA says doing so can cause nausea and headaches.

• Stop the product and seek advice if you feel unwell.

The FSA says anyone who is pregnant, breastfeeding, has a medical condition or takes regular prescribed medicines should check with a GP or healthcare professional before starting a food supplement, because some supplements interact with medicines or conditions. That includes people on blood-pressure, cholesterol or diabetes treatment, where an added effect on blood pressure or glucose could matter.

In the UK, spirulina and chlorella are sold as food supplements. They should not be marketed with claims to prevent, treat or cure a medical condition, and any health claim on a pack should be an authorised one on the Great Britain nutrition and health claims register.