Effects of the Edible Microalga Chlorella on Gut Microbiota and on Brain Health: Current Evidence and Emerging Links

Background: Chlorella, a unicellular green alga, is currently one of the most popular algae supplements due to its high content of bioactive compounds. Chlorella’s wide range of macro- and micronutrients, including chlorophyll compounds and carotenoids, has been suggested to influence various disorders related to the digestive and nervous systems. This review’s primary purpose was to critically analyze the effects of Chlorella intake on gut microbiota and brain function. Methods: The authors conducted a systematic review with narrative synthesis of peer-reviewed articles written in English and published in PubMed, Web of Science, and Scopus spanning the years 2009 to 2026 (PROSPERO registration number CRD42024527705). The search protocol was performed following PRISMA guidelines. Primary outcomes encompassed physiological variables, such as gut microbial composition, short-chain fatty acids, brain-derived neurotrophic factor, and hippocampal cell density. Secondary outcomes were assessed through neurobehavioral tests and psychological questionnaires. Results: Out of the 1333 articles identified, 47 studies were deemed eligible, and 21 met the predefined criteria, subsequently incorporated into this systematic review. In total, 10 articles documented interventions involving Chlorella and their effects on the gut microbiota, whereas 11 articles investigated several variables pertinent to brain function. Most of the studies included were conducted in animal models, with only a limited number of human trials. Nineteen studies (90%), predominantly preclinical, reported positive associations between Chlorella consumption, gut microbiota modulation, and physiological or neurobehavioral markers related to the gut–brain axis. Conclusions: Chlorella consumption may modulate gut microbiota composition and function, potentially influencing brain-related processes. However, the available literature lacks studies simultaneously addressing both gut microbiota and brain health parameters limiting the understanding of the underlying physiological mechanisms.