Background: Heat is associated with physiological strain and endurance performance (EP) impairments. Studies have investigated the impact of caffeine intake upon EP and core temperature (CT) in the heat, but results are conflicting. There is a need to systematically determine the impact of pre-exercise caffeine intake in the heat.Objective: Use a meta-analytical approach to determine the effect of pre-exercise caffeine intake on EP and CT in the heat.Design: Systematic review with meta-analysis.Data sources: Four databases and cross-referencing. Data analysis:Weighted mean effect summaries using random-effects models for EP and CT, as well as meta regressions with robust standard errors to explore confounders.Study selection: Placebo-controlled, randomized studies in adults (≥ 18 yrs old) with caffeine intake at least 30 min before endurance exercise ≥ 30 min, performed in ambient conditions ≥ 27℃.Results: Respectively 6 and 12 studies examined caffeine's impact on EP and CT, representing 52 and 205 endurance-trained individuals. On average, 6 mg/kg body mass of caffeine were taken 1 h before exercises of ~ 70 min conducted at 34°C and 47% relative humidity. Caffeine supplementation improved EP by 2.0 ± 0.7% (95% CI: 0.6 to 3.5%) and increased the rate of change in CT by 0.10 ± 0.04°C/h (95% CI: 0.03 to 0.16°C/h), compared with the ingestion of a placebo. Conclusion:Caffeine ingestion of 6 mg/kg body mass ~ 1 h before an exercise in the heat provides a worthwhile improvement in EP of 2%, while trivially increasing the rate of change in CT by 0.10°C/h.
Five days of high-dose nitrate supplementation did not change physiological responses and failed to improve single and repeated time-trial performances in world-class short-track speed skaters. These data suggest that nitrate ingestion up to 6.5 mmol does not enhance recovery from supramaximal exercise in world-class athletes.
Maximal oxygen consumption (V˙O2max) is a major determinant of 5-km running time-trial (TT) performance. Glycerol-induced hyperhydration (GIH) could improve V˙O2max in recreationally active persons through an optimal increase in plasma volume. Moreover, ingestion of a large bolus of cold fluid before exercise could decrease thermal stress during exercise, potentially contributing to improved performance. We determined the effect of GIH on 5-km running TT performance in 10 recreationally active individuals (age: 24 ± 4 years; V˙O2max: 48 ± 3 mL/kg/min). Using a randomized and counterbalanced protocol, participants underwent two, 120-min hydration protocols where they ingested a 1) 30 mL/kg fat-free mass (FFM) of cold water (~4 °C) with an artificial sweetener + 1.4 g glycerol/kg FFM over the first 60 min (GIH) or 2) 7.5 mL/kg FFM of cold water with an artificial sweetener over the first 20 min (EUH). Following GIH and EUH, participants underwent a 5-km running TT at 30 °C and 50% relative humidity. After 120 min, GIH was associated with significantly greater fluid retention (846 ± 415 mL) and plasma volume changes (10.1 ± 8.4%) than EUH, but gastrointestinal (GI) temperature did not differ. During exercise, 5-km running TT performance (GIH: 22.95 ± 2.62; EUH: 22.52 ± 2.74 min), as well as heart rate, GI temperature and perceived exertion did not significantly differ between conditions. This study demonstrates that the additional body water and plasma volume gains provided by GIH do not improve 5-km running TT performance in the heat in recreationally active individuals.
Background: Heat is associated with physiological strain and endurance performance (EP) impairments. Studies have investigated the impact of caffeine intake upon EP and core temperature (CT) in the heat, but results are conflicting. There is a need to systematically determine the impact of pre-exercise caffeine intake in the heat. Objective: Use a meta-analytical approach to determine the effect of pre-exercise caffeine intake on EP and CT in the heat. Design: Systematic review with meta-analysis. Data sources: Four databases and cross-referencing. Data analysis: Weighted mean effect summaries using random-effects models for EP and CT, as well as meta regressions with robust standard errors to explore confounders. Study selection: Placebo-controlled, randomized studies in adults (≥ 18 yrs old) with caffeine intake at least 30 min before endurance exercise ≥ 30 min, performed in ambient conditions ≥ 27°C. Results: Respectively 6 and 12 studies examined caffeine's impact on EP and CT, representing 52 and 205 endurance-trained individuals. On average, 6 mg/kg body mass of caffeine were taken 1 h before exercises of 70 min conducted at 34°C and 47% relative humidity. Caffeine supplementation improved EP by 2.0 +/- 0.7% (95% CI: 0.6 to 3.5%) and increased the rate of change in CT by 0.10 +/- 0.04°C/h (95% CI: 0.03 to 0.16°C/h), compared with the ingestion of a placebo. Conclusion: Caffeine ingestion of 6 mg/kg body mass 1 h before an exercise in the heat provides a worthwhile improvement in EP of 2%, while trivially increasing the rate of change in CT by 0.10°C/h.
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