A causal inference framework to bridge association and mechanism in the gut-brain axis

The gut-brain axis represents a major paradigm shift in how we evaluate diseases in neuroscience, with microbial dysbiosis affecting many neurological and psychiatric disorders. However, the clinical translation of these findings into effective therapies is currently stalled at a methodological impasse. This Causality Conundrum arises due to the fact that current models fail to resolve the bidirectional noise and cyclic feedback loops inherent in the gut-brain axis. These insufficiencies have made the field rely on cross-sectional cohorts and functionally blind 16S rRNA sequencing, creating a Resolution-Causality Gap, trapping the field in a cycle of correlation. Therefore, this perspective study argues for a new framework "Causality Funnel" for establishing causality in microbiome research. The framework introduces a multi-staged resource-prioritization protocol rooted in the epidemiological principle of triangulation. It prioritizes human-centric discovery using powerful causal inference methods like Mendelian Randomization, followed by multi-omics for molecular mechanism identification, and concluding with definitive validation in reductionist gnotobiotic models. By strategically using resource intensive research only on high-confidence hypotheses the field can move from human data to validating mechanisms through resource-efficient discovery. Furthermore, by anchoring this protocol in disease exemplars such as pediatric epilepsy and neurodevelopmental trajectories the field can navigate in a much more effective way, providing a road map that moves beyond just finding associations and accelerating the development of a new generation of targeted, evidence-based neurotherapeutics.