Impact of probiotic supplementation on cortisol levels; GRADE-based dose-response meta-analysis

Dysregulated cortisol levels, driven by hypothalamic-pituitary-adrenal (HPA) axis hyperactivity, are linked to stress-related disorders, including anxiety, depression, and metabolic syndromes. Probiotics, through their modulation of the gut-brain axis, may reduce cortisol levels, but evidence remains inconsistent due to study heterogeneity. Therefore, this systematic review and meta-analysis of randomized controlled trials (RCTs) aimed to evaluate the effect of probiotic supplementation on serum cortisol levels in adults. Following PRISMA guidelines, we searched PubMed, Scopus, Web of Science, Embase, and Cochrane databases up to February 20, 2026. Eligible RCTs involved adult participants receiving oral probiotics compared to placebo or no intervention, with serum cortisol as an outcome. A random-effects model calculated weighted mean differences (WMDs), with subgroup and dose-response analyses to explore heterogeneity. Risk of bias was assessed using Cochrane RoB 2, and evidence quality via GRADE. From 41 RCTs (n = 24–214 participants), probiotics significantly reduced cortisol levels (WMD: -0.35 µg/dL, 95% CI: -0.42 to -0.27, P < 0.001), with high heterogeneity (I² = 96.9%, 95% CI: 96.1% to 97.5%). Single-strain probiotics were more effective than multi-strain. No significant dose-response relationships or publication bias were found. Evidence quality was very low due to heterogeneity and bias risks. Although probiotic supplementation, particularly single-strain formulations, was associated with a modest reduction in serum cortisol levels, the high heterogeneity (I²=96.9%) and very low certainty of evidence (GRADE) preclude firm conclusions. These findings support the need for further large-scale, well-standardized RCTs to clarify the clinical utility of probiotics in stress management.