The microbiome and mental health: looking back, moving forward with lessons from allergic diseases

Relationships between gastrointestinal viscera and human emotions have been documented by virtually all medical traditions known to date. The focus on this relationship has waxed and waned through the centuries, with noted surges in interest driven by cultural forces. Here we explore some of this history and the emerging trends in experimental and clinical research. In particular, we pay specific attention to how the hygiene hypothesis and emerging research on traditional dietary patterns has helped re-ignite interest in the use of microbes to support mental health. At present, the application of microbes and their structural parts as a means to positively influence mental health is an area filled with promise. However, there are many limitations within this new paradigm shift in neuropsychiatry. Impediments that could block translation of encouraging experimental studies include environmental forces that work toward dysbiosis, perhaps none more important than westernized dietary patterns. On the other hand, it is likely that specific dietary choices may amplify the value of future microbial-based therapeutics. Pre-clinical and clinical research involving microbiota and allergic disorders has predated recent work in psychiatry, an early start that provides valuable lessons. The microbiome is intimately connected to diet, nutrition, and other lifestyle variables; microbial-based psychopharmacology will need to consider this contextual application, otherwise the ceiling of clinical expectations will likely need to be lowered.