Gut Lactic Acid and Akkermansia Team Up to Ease Anxiety via Serotonin

Anxiety disorders are the world's most prevalent mental health condition, and disrupted serotonin (5-HT) signaling is a well-established contributor. Because ~95% of the body's serotonin is produced in the gut from tryptophan, the gut microbiome is a logical but underexplored lever for anxiety regulation. This study asked whether specific microbial metabolites—particularly lactic acid—could modulate tryptophan-to-serotonin conversion and anxiety outcomes.

The researchers recruited 110 elite Chinese shooting athletes in a closed training environment, a cohort uniquely suited to minimize confounding lifestyle variables. Participants completed validated anxiety questionnaires (GAD-7, STAI-T, CSAI-2) and provided fecal samples. Higher fecal lactate correlated significantly with lower anxiety scores, and Akkermansia abundance showed a significant negative correlation with anxiety levels. These observational findings motivated mechanistic animal experiments.

In mouse models using chronic unpredictable mild stress (CUMS) to induce anxiety, daily intraperitoneal sodium lactate injections reduced anxiety-like behavior in open-field and elevated plus maze tests. Crucially, antibiotic-depleted mice colonized with live A. muciniphila plus lactate showed the greatest anxiety reduction, outperforming either intervention alone or heat-killed bacteria controls. Mechanistically, the lactate + A. muciniphila combination increased intestinal TPH1 (tryptophan hydroxylase 1, the rate-limiting enzyme for 5-HT synthesis) and decreased IDO1 (indoleamine 2,3-dioxygenase 1, which diverts tryptophan toward kynurenine), thereby redirecting tryptophan flux toward serotonin. Colonic and plasma 5-HT levels rose, while kynurenine levels fell.

Lactate also enhanced A. muciniphila's production of propionate, a short-chain fatty acid with known anxiolytic properties. Oral calcium propionate supplementation in antibiotic-treated mice recapitulated the anxiety-reducing effects, confirming propionate as a downstream effector. Germ-free mouse experiments further validated that A. muciniphila colonization alone was sufficient to alter tryptophan metabolism, and that lactate amplified this effect.

These findings map a previously uncharacterized gut-brain axis pathway: exercise or dietary-derived enteric lactate nourishes and potentiates A. muciniphila, which in turn elevates propionate and rebalances tryptophan metabolism toward serotonin synthesis, collectively dampening anxiety. The study opens potential for microbiota-targeted interventions—such as lactate-boosting diets, probiotic A. muciniphila, or propionate supplementation—as adjunct strategies for anxiety management.