Evidence check: Cortisol in the context of gut health

Cortisol, the primary stress hormone in humans, is regulated by the hypothalamic-pituitary-adrenal (HPA) axis. The gut-brain axis links the gut microbiome and stress response: a compromised gut barrier ("leaky gut") can increase systemic inflammation, which in turn may alter HPA axis activity and cortisol levels. Conversely, chronically elevated cortisol can increase intestinal permeability and shift microbial composition. However, the evidence is largely derived from animal models and small human studies; direct causal evidence in humans remains limited. Circadian cortisol measurements (saliva, serum) are established biomarkers for stress, but their interpretation in the context of gut health is complex and not standardized. Commercial tests that claim to link cortisol levels directly to gut issues often overstate clinical significance. A balanced diet, stress management, and adequate sleep are currently the most evidence-based approaches to support both systems.