Background: The aim of this study was to conduct a systematic review and meta-analysis on the effects of caffeine supplementation on time trial performance and associated physiological responses. Methods: A total of 35 studies met the inclusion criteria of adopting double-blind randomized placebocontrolled crossover designs that included a closed-loop time trial ( ‡5 minutes) performed under a caffeine dose of 3-6 mg/kg administered 30-90 minutes beforehand. Meta-analyses were completed using a random-effects model, with effects on time trial performance presented as standardized mean difference (d) and with physiological responses presented as raw mean difference (D). The 95% confidence limits (CL 95 ) were calculated for all estimates. Results: Relative to placebo, caffeine had a positive effect on time trial performance (d = 0.32; CL 95 [0.19-0.44]). Moreover, the effect of caffeine on time trial performance corresponded with increases in heart rate (D = 3.3 beats/min; CL 95 [1.7-4.8]), oxygen uptake (D = 0.09 L/min; CL 95 [0.02-0.17]), blood lactate (D = 1.42 mmol/L; CL 95 [1.09-1.74]), and blood glucose (D = 0.94 mmol/L; CL 95 [0.58-1.30]). In contrast, caffeine had no effect on time trial measures of respiratory exchange ratio (D = 0.01; CL 95 [À0.01 to 0.02]), or ratings of perceived exertion (D = 0.1; CL 95 [À0.1 to 0.3]). Conclusions:The results reveal a clear effect of caffeine on moderate-to high-intensity time trial performance. When considered in conjunction with research using fixed-intensity exercise, the caffeine-induced increase in time trial intensity likely explains all of the associated increases in heart rate and oxygen uptake, and part of the increase in blood lactate and blood glucose.
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