Caffeine makes a splash: a systematic review and multilevel meta-analysis exploring the effects of caffeine intake on swimming performance.

چکیده اصلی

BACKGROUND: Evidence for acute caffeine supplementation in swimming remains inconsistent across reviews, likely due to between-study heterogeneity and methodological differences. METHODS: Five databases were searched for randomized placebo-controlled crossover trials exploring the effects of caffeine on swimming performance in competitive swimmers. Performance outcomes were pooled as standardized mean differences (SMD; Hedges' g), with time outcomes sign-reversed so that positive values indicated improved performance. Dependence among multiple outcomes within studies was addressed using three-level random-effects models (REML). As a supplementary, interpretable metric, log ratios of means (lnRoM) were meta-analysed and expressed as percentage change. Post-exercise blood lactate (mmol/L) was pooled as mean differences (MD). Risk of bias was assessed using RoB 2 and certainty of evidence using GRADE. Prespecified exploratory moderator analyses examined potential effect modification (dose, timing, distance, administration form, stroke, gender, and athlete level). RESULTS: Thirteen studies were included (144 men and 48 women), which contributed 28 performance effect sizes. Caffeine improved swimming performance (SMD = 0.57, 95% CI: 0.20 to 0.94; p = 0.005), corresponding to a +1.71% improvement in lnRoM (95% CI: +1.01% to +2.41%; p < 0.001). Heterogeneity was substantial (I²total = 64.7%) and primarily between studies. Eight studies contributed 24 blood lactate effect sizes. In the meta-analysis, caffeine increased post-test lactate (MD = 0.85 mmol/L, 95% CI: 0.22 to 1.49; p = 0.016) with high heterogeneity (I²total = 76.5%). Exploratory moderator analyses suggested a possible dose-related pattern for swimming performance, with larger pooled effects at ≥6 mg/kg (SMD = 0.95, 95% CI: 0.52 to 1.38) than at <6 mg/kg (SMD = 0.22, 95% CI: -0.15 to 0.60; p difference = 0.017). Other performance moderators did not show clear between-subgroup differences. For lactate, exploratory subgroup analyses suggested larger increases in freestyle and short-distance events, although these patterns were more sensitive to modelling assumptions and should be interpreted cautiously. CONCLUSION: Acute caffeine ingestion confers a moderate ergogenic benefit for swimming performance (SMD = 0.57, 1.7% improvement) and is associated with higher post-test blood lactate (MD = 0.85 mmol/L). Dose (≥6 mg/kg) may be associated with larger performance benefits, whereas lactate responses appear more pronounced in freestyle and shorter events. However, these subgroup findings are hypothesis-generating and require exploration.

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