Effects of Caffeine Dose and Timing on Exercise Performance

TLDR: It was concluded that a high CHO meal consumed prior to caffeine ingestion significantly reduced serum caffeine concentrations and delayed time to peak concentration and in light of these findings, the third study aimed to determine whether performance would differ when exercise coincided with peak serum concentrations following caffeine intake.

Abstract: The central aim of the thesis was to investigate the influence of caffeine dose, supplementation timing and postprandial timing on exercise performance. The first study investigated the dose-response relationship between caffeine and 2000 m rowing performance. In this randomised, placebo controlled, double-blind crossover study, ten competitive male rowers consumed 2, 4 or 6 mg·kg-1 caffeine or a placebo 60 min prior to completing a 2000 m time trial on a rowing ergometer. The trials were preceded by a 24 h standardised diet [including a pre-exercise meal of 2 g·kg-1 carbohydrates (CHO)] and subjects were tested pre-exercise for hydration, caffeine abstinence and for blood glucose concentrations. Time trial performance was not significantly different across the three caffeine doses or placebo (p = 0.25). Following the three caffeine trials, post-exercise plasma glucose and lactate concentrations were higher compared to the placebo trial (p < 0.05). Plasma caffeine concentrations 60 min following ingestion were lower than values reported previously by others following the same dose; it was concluded that pre-exercise feeding may significantly affect plasma caffeine concentrations and the potential for caffeine to improve performance. Study Two addressed this issue of food intake by examining the effect of a standardised high CHO meal on serum caffeine concentration following caffeine intake. Fourteen healthy males randomly completed four trials, each separated by five days. Subjects fasted for 12 h prior to each trial; on arrival at the laboratory subjects either remained fasted (on two occasions) or consumed a high CHO meal (2 g∙kg-1 CHO, 42 ± 1 kJ∙kg-1) prior to consuming either 6 or 9 mg∙kg-1 anhydrous caffeine. Venous blood was sampled for the analysis of serum caffeine at baseline and at 6 time-points over 4 h following caffeine intake. Subjects remained at rest throughout each trial. There was a significant difference among treatment conditions (p < 0.001) in peak, and time to peak caffeine concentration. Peak caffeine concentration occurred 60 min following ingestion for both the 6 and 9 mg∙kg-1 fasted (p < 0.001) trials, whilst time to peak concentration occurred 120 and 180 min following ingestion for the 6 and 9 mg∙kg-1 fed trials, respectively (p < 0.001). Peak concentration was greater in the 9 mg∙kg-1 fasted trial than the corresponding fed condition, and both were greater than the 6 mg∙kg-1 fed and fasted conditions. It was concluded that a high CHO meal consumed prior to caffeine ingestion significantly reduced serum caffeine concentrations and delayed time to peak concentration. In light of these findings, the third study aimed to determine whether performance would differ when exercise coincided with peak serum concentrations following caffeine intake compared to when exercise for all participants began 60 min following supplementation. The study was a double-blind, randomised, within-subjects design with three experimental conditions. Following a 12 h fast, 14 male trained cyclists and triathletes consumed a pre-exercise meal (52 ± 3 kJ·kg-1 including 2 g∙kg-1 CHO); subjects then ingested either placebo or caffeine (6 mg.kg-1) capsules timed to coincide peak serum caffeine concentration with exercise onset (Cpeak), or 60 min (C1hr) prior to a simulated 40 km cycling time trial. Venous blood was sampled at baseline, 65 and 20 min prior to exercise and six min post-exercise for the analysis of serum caffeine, plasma glucose, catecholamine and blood lactate concentrations. Peak serum caffeine concentrations occurred 120 (n = 12) and 150 min (n = 2) following caffeine ingestion. Time to complete the 40 km time trial was significantly faster (+ 2%; p = 0.002) in the C1hr trial compared to placebo. A 1% improvement in performance was noted in the Cpeak condition versus placebo, although this was not statistically significant (p = 0.24). Whilst no differences in metabolic markers were found between Cpeak and placebo conditions, significant increases in plasma glucose (p = 0.005), norepinephrine and epinephrine (p = 0.002) concentrations were observed in the C1hr trial post-exercise versus placebo. Collectively, the present studies confirm that timing of caffeine supplementation and postprandial timing significantly influence peak serum caffeine concentrations and its effect on subsequent exercise performance. Consumption of a high CHO meal significantly slowed the appearance of caffeine in the blood and also reduced peak concentrations compared to a fasted state. Various influences on serum caffeine concentrations and the wide individual variations may explain, at least in part, inconsistencies in the literature and why some individuals appear to respond better than others in terms of performance improvements. The large inter-subject variability across all studies highlights significant variation in caffeine concentrations in the blood following caffeine ingestion. Contrary to the tacit assumptions within the literature, optimising conditions to ensure peak availability of caffeine within the bloodstream was not found to be related to enhanced performance. Collectively, the present results have an important role in informing the usage of caffeine in sports and the design of future studies involving caffeine supplementation.