During World War II, the famous Minnesota Starvation Experiment in healthy young men showed that semi-starvation (loss of  >25% of body weight) caused both physical symptoms, such as fatigue, dizziness and hair loss, and psychological symptoms, such as depression, and anxiety. Interestingly, long-term starvation also led to disordered eating behaviour, such as obsessing with food, overeating, and hiding food among some individuals. Unfortunately, in the 1940s, investigating the effects on the gut microbiome was not a yet topic of great interest, so the samples were not collected. We, as researchers would have been more than interested to know what occurred in the gut of these starving individuals.

Eating disorders, especially anorexia nervosa, are serious mental disorders, with the highest mortality rates of all psychiatric conditions. Treatment outcomes in anorexia nervosa are still poor. Only about 50% of people with anorexia will fully recover, and potential add-on therapies to the well-established psychotherapies and nutritional rehabilitation procedures are needed.

Rapidly accumulating evidence supports the important role of gut microbiome in regulating behaviour, depression and anxiety, gastrointestinal symptomology, appetite and metabolism. Interestingly, these are all core features frequently altered in individuals with anorexia nervosa and other eating disorders. We also know that diet is one of the primary influences on gut microbiota composition in both short– and long-term. However, it is only in the last few years that the hypothesis that gut microbiota has a role in the pathophysiology of anorexia nervosa has begun to been studied.

Gut microbial compositionseems to differ in individuals with anorexia nervosa compared to normal-weight or obese individuals. For example, anorexia nervosa patients have a decreased levels of carbohydrate utilising Roseburia, which produces an important short-chain fatty acid butyrate, and elevated levels of Verrucomicrobia, mostly accounted for by a particular  mucin-degrading bacteria Akkermansia muciniphila. Other interesting findings also support the altered gut microbiome in eating disorders. For example, 11 new bacterial species were found in an exploratory study in one anorexia nervosa patient. Moreover, levels of depression, anxiety, and eating disorder psychopathology seemed to associate with composition and diversity of the intestinal microbiota or short-chain fatty acid levels.

So far, research is still unable to describe what comes first. Is an altered gut microbiota simply an obvious result of long-term reduced food consumption, potential dietary deficiencies and weight loss? Or, alternatively, does microbial composition have the potential to cause these metabolic outcomes and possibly, to contribute to disordered eating behaviour? We need more large-scale follow-up studies to clarify these relationships.

It would be fascinating to consider a future where we could utilise probiotic foods or supplements in the treatment of anorexia nervosa. Nevertheless, there is no evidence supporting that yet. However, in theory, modulating the gut microbiota could support other treatments by improving gut health, reducing inflammation, potentially decreasing anxiety and depression, and also improving other common symptoms in anorexia nervosa, such as constipation.

Here at the Food & Mood Centre we are also aiming to study the role of the gut microbiome in eating disorders, as part of our future research projects.

Dr Anu Ruusunen is a Postdoctoral Research Fellow at the Food and Mood Centre, Deakin University, and she has worked as a clinical dietitian with individuals with eating disorders.