Anxiety disorders affect around 4% of the world’s population and is one of the most common mental illnesses. Recent studies have shown that the gut-brain axis plays an integral part in psychiatric health. In fecal transplant models, mice exhibit anxiety-like behaviors when receiving fecal microbiota from depressed individuals. This highlights the contribution that microbes play in human diseases.
A recent study published by Dr. Christopher R Brydges et al. in Scientific Reports investigated how microbe tryptophan metabolism affects depression and anxiety levels. Indole levels are significantly associated with age, height, and weight (indole-3-lactic acid (ILA) and the ILA/Indoxyl sulfate (IS) ratio are positively associated; and the indole acetic acid (IAA)/ILA ratio is negatively associated). Furthermore, the authors noted sex differences with significantly higher levels in males, specifically IAA, ILA, and the ILA/ indole-3-propionic acid (IPA) and ILA/IS ratios. There were noted treatment-specific changes in indole levels. Unfortunately, there were no baseline measurements that predicted treatment outcomes in this population, although individuals who remitted following medication treatment had a significant increase in IPA as well as a decrease in the ILA/IPA ratio. IS was significantly associated with depression and anxiety scores whereas ILA/IS and IPA/IS ratios were negatively correlated with depression and anxiety. The researchers next investigated brain connectivity and the correlation with IS. There was a significant correlation between IS levels and the functional connectivity of the Subcallosal Cingulate Cortex-right premotor region. Connectivity differences in this brain region have been shown to be associated with remission and treatment failure in Major depression disorder patients (Dunlop et al., 2017).
Taken together, this study adds to our understanding that crosstalk between microbes and their host can have an impact on overall human health and importantly on psychiatric health via alterations in tryptophan metabolism. Therefore, understanding dysbiosis and how microbial- and host-derived tryptophan metabolites impact human health and treatment success is critical for many disease/disorders including anxiety and depression.