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

Clinical Snapshot

Key Findings

• P Value: Performance: p = 0.005; Percentage change: p < 0.001; Blood lactate: p = 0.016; Dose subgroup difference: p = 0.017

• Effect Size: SMD (Hedges' g) = 0.57 (moderate effect); percentage improvement = +1.71% (lnRoM). Post-exercise blood lactate increased by MD = 0.85 mmol/L. Dose subgroup: ≥6 mg/kg SMD = 0.95 (95% CI: 0.52–1.38) vs. <6 mg/kg SMD = 0.22 (95% CI: −0.15 to 0.60)

• Primary Outcome: Acute caffeine supplementation significantly improved swimming performance compared to placebo across 13 randomised crossover trials (28 performance effect sizes from 192 participants)

• Nnt Or Sensitivity: No NNT applicable (continuous performance outcome). The 1.71% mean improvement in swim time is potentially race-decisive at elite level where winning margins are frequently <1%. Dose-response pattern suggests ≥6 mg/kg body mass may be required for clinically meaningful ergogenic effect (SMD 0.95 vs. 0.22 at lower doses)

• Confidence Interval: Performance SMD: 95% CI 0.20 to 0.94. Percentage change: 95% CI +1.01% to +2.41%. Blood lactate MD: 95% CI 0.22 to 1.49 mmol/L

Clinical Application

Caffeine is widely available, inexpensive, and legally permitted under World Anti-Doping Agency (WADA) regulations (removed from the prohibited list in 2004). Practical administration (capsule, anhydrous powder, or caffeinated beverage) is feasible in competitive settings. Timing (typically 60 minutes pre-exercise) and dose (≥6 mg/kg suggested by subgroup data) require individualisation. Gastrointestinal tolerance and habitual caffeine use should be assessed prior to competition use. Caffeine supplements are available over-the-counter in Australia and are not subject to TGA prescription requirements at standard ergogenic doses. Swimming Australia and the Australian Institute of Sport (AIS) Sports Supplement Framework classify caffeine as a Group A supplement (strong evidence for use in specific situations), consistent with the findings of this meta-analysis. RACGP guidelines do not specifically address ergogenic supplementation, but general practitioners advising competitive athletes should be aware of the cardiovascular and sleep-disruption risks at higher doses (≥6 mg/kg equates to approximately 420–480 mg for a 70–80 kg athlete). The AIS recommends individualised trialling of caffeine in training before competition use. PBS listing is not applicable. Competitive swimmers (elite and sub-elite) seeking legal ergogenic supplementation. Most applicable to adult male swimmers based on sample composition. Applicability to female swimmers, masters athletes, and junior swimmers requires further study. Results are most relevant to short-to-middle distance events, though stroke- and distance-specific subgroup findings remain exploratory.

Abstract

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.

References

1. 1.Wang, Z., Qiu, B., Bao, J., Diao, P., Hamlin, M., Del Coso, J., Grgic, J., & Petersen, C. (2026). Caffeine makes a splash: a systematic review and multilevel meta-analysis exploring the effects of caffeine intake on swimming performance. Journal of the International Society of Sports Nutrition. https://doi.org/10.1080/15502783.2026.2692016