The Gut-Brain Axis and Precision Mental Health

Description

DOI: https://doi.org/10.5281/zenodo.22659999

Aviral Goswami (Department of Psychology, H P University, Shimla)

The gut microbiome is being explored as a modifiable contributor to depression and anxiety via bidirectional signaling along the vagus nerve, the hypothalamic-pituitary-adrenal axis, and microbial neurotransmitter and short-chain fatty acid production. This review synthesizes existing meta-analytic evidence for the treatment of depressive and anxiety symptoms with gut-microbiome-targeted therapies (probiotics, prebiotics, and synbiotics), and finds that eight independent meta-analyses addressing essentially the same underlying question report standardized mean differences ranging from a nonsignificant null (SMD = -0.02, standalone treatment) to a very large effect (SMD = -1.76), with between-study heterogeneity as high as I^2 = 96%. This divergence is not noise: it is associated with identifiable moderators including whether the intervention is adjunctive to antidepressants or given as monotherapy (adjunctive SMD = 0.83 vs. standalone SMD = -0.02), whether the sample is clinically diagnosed with major depressive disorder or an unmedicated or subclinical population (SMD = -0.96 vs. -0.34 to -0.38), the proportion of female participants, industry funding status, and probiotic strain composition. Such a pattern  a single pooled estimate misrepresenting a genuinely heterogeneous underlying structure  is a direct parallel to the central methodological finding of the correlational study motivating this review on Himachal Pradesh college students (Goswami & Zinta, 2026). This study also found that a pooled model assuming an invariant association structure could not answer the predictive question it was designed to address. The review concludes that gut-brain axis research on mental health has already made an effective case for precision psychiatry: a stratified, mechanism- and moderator-based approach rather than a single population-wide efficacy estimate. No study was identified that examined gut-microbiome-mental health associations in an Indian or Himalayan population, although regionally distinct dietary and fermented-food patterns plausibly constitute an additional, currently untested moderator.