A systematic review of studies on the faecal microbiota in anorexia nervosa: future research may need to include microbiota from the small intestine

Anorexia nervosa (AN) is a poorly understood and often chronic condition. Deviations in the gut microbiota have been reported to influence the gut-brain axis in other disorders. Therefore, if present in AN, it may impact on symptoms and illness progression. A review of the gut microbiota studies in AN is presented. A literature search on PubMed yielded 27 articles; 14 were selected and based on relevance, 9 articles were included. The findings were interpreted in the larger context of preclinical research and clinical observations. 8 out of 9 included studies analysed microbiota from faeces samples, while the last analysed a protein in plasma produced by the gut. Two studies were longitudinal and included an intervention (i.e., weight restoration), five were cross-sectional, one was a case report, and the last was a case series consisting of three cases. Deviations in abundance, diversity, and microbial composition of the faecal microbiota in AN were found. There are currently only a few studies on the gut microbiota in AN, all done on faeces samples, and not all describe the microbiota at the species level extensively. The Archaeon Methanobrevibacter smithii was increased in participants with a BMI < 25 in one study and specifically in AN patients in three studies. Methanobrevibacter smithii may, if detected, be a benchmark biomarker for future studies. We propose that microbiota samples could also be collected from the small intestine, where a major exchange of nutrients takes place and where the microbiota may have a biological impact on AN.