Objective: This systematic review and meta-analysis evaluated the effects of caffeine (CAF) on high-intensity interval exercise (HIIE) performance and examined potential moderators. Methods: Several databases were searched for studies of CAF on HIIE performance. Pooled effects were calculated using Hedge`s g (g) via a three-level random-effects meta-analysis. Subgroup analyses were performed based on sex, training status, CAF dose, CAF form, and interval type. A meta-regression analysis was conducted to investigate the potential moderating effect of the rest/work ratio on HIIE performance. Results: Twenty studies were included (n = 320; 57 females). CAF significantly improved HIIE performance (g = 0.28, 95% CI = 0.14 to 0.43), concurrently elevating blood lactate (g = 0.50, 95% CI = 0.22 to 0.79) and glucose (g = 0.56, 95% CI = 0.17 to 0.96). Subgroup analyses demonstrated significant improvements across all sexes (g = 0.30-0.34), trained athletes (g = 0.44), CAF dose (low and moderate) (g = 0.28-0.33), CAF form (capsule and beverage) (g = 0.32-0.33), and HIIE protocol (repeated short sprints and short intervals) (g = 0.16-0.36), with no subgroup differences (all p > 0.05). The rest/work ratio was a significant moderator of HIIE performance (ß2 = 0.003, p = 0.017). Conclusion: CAF ingestion significantly enhances HIIE performance, with ergogenic benefits observed across both sexes and trained athletes. Effective ergogenic benefits can be achieved with a low dose (~3 mg/kg), administered as a capsule or beverage. Notably, meta-regression indicates that the rest/work ratio is a critical moderator of HIIE performance, with evidence of a nonlinear association.
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