Synergy or Dominance? The Ergogenic Effects of Caffeine and Carbohydrate on High-Intensity Interval Exercise Performance: A Three-Level Meta-Analysis

Background/Objectives: This meta-analysis aimed to quantify the combined ergogenic effects of caffeine (CAF) and carbohydrate (CHO) on high-intensity interval exercise (HIIE) performance and to identify potential moderating factors. Methods: Four databases were systematically searched to identify randomized crossover trials assessing CAF combined with CHO (CAF + CHO) on HIIE performance (i.e., exercise time, distance, or total work). A three-level random-effects meta-analysis was performed to calculate pooled Hedge’s g (g) values. Moderator effects were explored through subgroup analyses, including control group (placebo-, CHO-, and CAF-controlled), CHO administration (mouth rinse and ingestion), and training status (recreationally active and trained). Results: Eleven studies met the inclusion criteria (n = 105; 8 female). CAF + CHO significantly enhanced HIIE performance (g = 0.44, 95% CI: 0.23–0.66). Subgroup analyses indicated that CAF + CHO mouth rinsing (g = 0.91, CI: 0.49–1.33) yielded superior effects compared to CAF + CHO ingestion (g = 0.33, CI: 0.14–0.52) (p for subgroup < 0.05). Performance improvements with CAF + CHO were observed for CHO- and placebo-controlled trials, but not in CAF-controlled trials, without significant subgroup effects (p for subgroup > 0.05). Importantly, evidence of publication bias was identified, and the overall certainty of evidence was graded as low according to the GRADE framework. Conclusions: CAF + CHO appears to be effective for enhancing HIIE performance, with greater benefits observed when CHO is administered via mouth rinsing rather than ingestion. Preliminary evidence suggests that CAF may play a key role in CAF + CHO strategies. However, given the limited number of female participants, the generalizability of these findings to both sexes is limited. Additional high-quality trials are needed to establish more definitive recommendations.