Effects of caffeine on time-trial performance and associated physiological responses: a meta-analysis

Background: The aim was to conduct a systematic review and meta-analysis on the effects of caffeine supplementation on time-trial performance and associated physiological responses. Methods: 35 studies met the inclusion criteria of adopting double-blind, randomized, placebo-controlled, crossover designs that included a closed-loop time-trial (≥ 5 mins) performed under a caffeine dose of 3 – 6 mg∙kg-1 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 (δ) and with physiological responses presented as raw mean difference (D). 95% confidence limits (CL95) were calculated for all estimates. Results: Relative to placebo, caffeine had a positive effect on time-trial performance (δ = .32; CL95 [.19, .44]). Moreover, the effect of caffeine on time-trial performance corresponded with increases in heart rate (D = 3.3 b∙min-1; CL95 [1.7, 4.8]), oxygen uptake (D = .09 L∙min-1; CL95 [.02, .17]), blood lactate (D = 1.42 mmol∙L-1; CL95 [1.09, 1.74]), and blood glucose (D = .94 mmol∙L-1; CL95 [.58, 1.30]). In contrast, caffeine had no effect on time-trial measures of respiratory exchange ratio (D = .01; CL95 [-.01, .02]), or ratings of perceived exertion (D = .1; CL95 [-.1, .3]). Conclusion: 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.