Eduard Vieta
Professor of Psychiatry at the University of Barcelona, Head of the Psychiatry and Psychology Department at Hospital Clínic in Barcelona, and researcher at the Biomedical Research Centre in Mental Health (CIBERSAM)
This is a very interesting and evolving topic. The results are not extraordinarily novel (only in a small part), but they confirm the changes in the microbiome already described and do so in one of the largest samples to date. Moreover, they do so by measuring the severity of depressive symptoms and controlling for the coexistence of other pathologies and the effects of treatment.
They identify changes in the bacterial populations of people with depression related to microorganisms that produce glutamate and serotonin, among other neurotransmitters, which are of great relevance in depression. Moreover, these changes are maintained across cultural, genetic and lifestyle differences in different ethnic groups. Despite the large sample size, some analyses could not be replicated but, in my opinion, it is a step forward in understanding the inflammatory changes associated with depression and the connection between our digestive flora and brain function.
Causality cannot be established, although it is assumed that chronic stress produces inflammation and that is what alters the microbiota, but we also don't know if some of the changes are compensatory.
It is still too early to talk about treatments. On the one hand, there are indications that a balanced diet (e.g. the Mediterranean diet) may improve mental health, although it is not an adequate treatment on its own for major depression. On the other hand, it would be premature and fanciful to consider faecal transplantation as a therapeutic alternative at this stage.