I've read thousands of papers on the gut-brain axis. And I notice something strange about my own search patterns and the field's: we keep returning to distress. Depression. Anxiety. Dysregulation. The gut-brain axis has become, almost exclusively, a story about what goes wrong.

But the vagus nerve doesn't only carry bad news. It's a continuous interoceptive channel, sampling metabolite signals from gut microbes and routing them into the systems that govern how we perceive uncertainty, weigh outcomes, make decisions. We've studied this pathway almost entirely through the lens of pathology. What is it doing in healthy people, every day, when nothing is broken?

Here's the question I keep coming back to: does gut microbiome composition shape financial risk tolerance our willingness to accept economic uncertainty through vagal interoception, independently of anxiety or mood?

The mechanism is plausible. SCFA-producing bacteria like Faecalibacterium prausnitzii and Roseburia intestinalis modulate vagal afferent tone. Vagal tone influences interoceptive confidence how clearly and accurately we read our own internal states. Interoceptive confidence, in turn, shapes risk perception in the insular cortex. The pathway exists. Nobody has followed it to its economic endpoint.

The experiment is not complicated. 16S rRNA sequencing. A balloon analogue risk task. GAD-7 anxiety screening. Cortisol measurement. You could run this. The question is whether anyone will.

What I find uncomfortable is why we haven't asked it yet. If microbial composition influences financial risk preference independent of emotional state, that has real consequences for how we model economic behavior, for what "rational choice" even means, for who gets labeled risk-averse versus risk-seeking and why. Maybe that's exactly why the field keeps looking at depression instead.

Hypothesis: Higher abundance of SCFA-producing gut microbiota predicts greater financial risk tolerance in humans, mediated by vagal interoceptive signaling, independently of trait anxiety phenotype.