Flow-Mediated Dilation in Competitive Athletes: Non-linear Associations and Haemodynamic Characteristics.

AIMS: The vascular endothelial effects of chronic competitive training remain controversial. Traditional sport classifications may not capture complex haemodynamic demands, while training-related outward arterial remodelling may reduce percentage flow-mediated dilation (FMD) and obscure endothelial adaptation. We compared resting FMD between competitive athletes and non-athletic controls and explored potential sport-, participant-, and vascular-level moderators. METHODS: Five databases were searched from inception to 11 March 2026. Controlled studies reporting resting ultrasound-assessed conduit-artery FMD in competitive athletes and non-athletic controls were included. Dependent effect sizes were synthesised using a three-level random-effects model. Subgroup analyses and meta-regression examined potential moderators, with secondary exploratory quadratic models applied to selected continuous variables. Evidence certainty was assessed using GRADE. RESULTS: The pooled athlete-control difference in FMD was not statistically significant (SMD = 0.33, 95% CI -0.01 to 0.66; P = 0.06), with substantial heterogeneity. In secondary analyses, sport modality was associated with variation in effect estimates, and endurance athletes showed higher FMD than controls; evidence for strength and mixed-modality sports was less conclusive. Exploratory quadratic meta-regression suggested possible curved associations involving age and maximal oxygen uptake (VO2max), although the fitted turning points were not validated physiological thresholds. Substantial heterogeneity remained across most analyses. CONCLUSION: No clear overall difference in FMD was observed between athletes and controls. Higher FMD among endurance athletes in secondary analyses should be interpreted cautiously given the observational designs, residual heterogeneity, and moderate certainty of evidence.