Bulimia Nervosa and Depression, from the Brain to the Gut Microbiota and Back

Bulimia nervosa (BN) is a condition marked by a typical cyclical behavioural activity, characterized by restrictions, binges and vomiting, as well as a disturbance of the emotional value of food. Food stimuli acquire excessive relevance, giving rise to a succession of states of excitement and anxiety. The depressive condition accompanies very often BN. Most people with BN also experience one or more anxiety disorders. The aim of the review is to identify a link at a central and peripheral level that connects an eating disorder with a mood state. Altered nervous mechanisms are involved in BN. Among the cerebral areas, the insula is functionally compromised in BN. The insula is also implicated in depressive states. The insula is the primary gustatory cortex, where gustatory sensory information such as taste discrimination and higher cognitive functions such as food anticipation and reward are processed. The insula is anatomically connected to a wide range of cortical, limbic and paralimbic structures, and functionally implicated in high-order cognition, emotional responses, and empathic processes. The insula plays a crucial role in empathy, or in the ability to share the emotional states of others, and in particular negative emotions. In fact, the insular cortex is also activated in conditions of anxiety and depression. One of the pathophysiological factors that influences bulimia and depression is the composition of gut microbiota, as there is a strong association between the microbial signature and the brain function. Gut dysbiosis condition may contribute to the development of eating disorders, including BN. Dysbiosis may promote intestinal inflammation, alter gut permeability, and trigger immune reactions in the hunger/satiety regulation center contributing to the pathophysiological development of eating disorders. From this emerges the importance of adequate probiotic integration as a preventive and/or therapeutic tool in these pathologies.