The primary aim of this review was to critically analyze the existing literature to elucidate the potential of caffeine to enhance short-term high-intensity exercise performance. It is well known that acute caffeine ingestion improves endurance performance, yet existing data regarding caffeine's effect on exercise dependent on nonoxidative metabolism are equivocal. The PEDro scale (58) was used to systematically analyze the validity of existing studies. Results revealed that 54-65% of studies demonstrate improved performance as a result of caffeine intake. The mean improvement in performance ranged from 6.5 to 9.4%, although the variability across studies was sizable. This suggests that only some individuals may experience improved performance with acute caffeine intake.
Caffeine ingested in capsule or powder form and as a constituent in energy drinks or supplements was shown to be ergogenic (Tables 1 and 2) for various high-intensity exercise protocols such as resistance training, sprinting, or activities simulating team sports. The lowest ergogenic caffeine doses for short-term high-intensity exercise were ingested as gum (9) (100 mg) or as part of a supplement (8). Nevertheless, the supplement ingested in the study of Beck et al. (8) did not augment 1RM strength in untrained men (7). Unpublished data (13) from a double-blind crossover study in strength-trained men reveal no change in knee extension/flexion torque, total work, or power output when a 2 mg·kg−1 dose of caffeine was ingested 70 minutes before “all-out” isokinetic dynamometry vs. a higher dose (5 mg·kg−1) or placebo. These data corroborate a previous study (48) in untrained men showing no effect of caffeine (300 and 600 mg) on peak torque. Intake of pure caffeine in doses from 3 to 6 mg·kg−1 has been shown to improve performance in team sports (15,65,67), resistance training (43,48,76), and sprint/power-based activity (3,18,60). Overall, lower doses typically ingested via commercially available energy drinks or supplements seem to be as effective as higher doses and may minimize onset of negative symptoms experienced with doses greater than 6 mg·kg−1 that are deleterious to training or athletic performance.
One potential explanation for the equivocal data regarding caffeine's ability to alter high-intensity exercise may be differences in subjects' training status. Of the studies revealing a significant improvement in short-term high-intensity exercise performance, many (9,19,42,49,72,76) included trained athletes (competitive cyclists, football players, elite athletes, and competitive swimmers) and not untrained, recreationally active, or strength-trained subjects who are typically college students. It is likely that athletes have greater motivation to perform fatiguing exercise and can provide more consistent performance day-to-day (12), which may reduce variability and thus increase statistical power. In many studies (32,49,76), subjects were low-caffeine consumers (<100 mg per day), which may potentiate the ergogenic effect of the drug compared with subjects tolerant to the effects of caffeine (50). However, further study is needed to elucidate this possibility.
An additional explanation is that like creatine, there may be individual “responders” and “nonresponders” to the effects of caffeine during short-term exercise. This may be related to caffeine habituation, although there is evidence (32,34) that caffeine tolerance does not affect subsequent endurance performance. To our knowledge, this has not been tested during short-term high-intensity exercise. In 1 study in 22 strength-trained men (5), 12 men lifted more weight on the 1RM bench press and 11 on the leg press, respectively, with caffeine (6 mg·kg−1), yet 5 and 8 men lifted more weight in the placebo trial. Similar individual variability in performance in response to acute caffeine intake was also observed in men completing resistance training (43). Unpublished data (70) in 14 strength-trained men revealed that 6 of 9 men labeled as “responders” expressed chronic caffeine intakes greater than 225 mg per day, although there was no relationship between caffeine concentration and magnitude of performance improvements. To further explore this discrepancy, scientists should regularly assess caffeine concentration in studies in which changes in short-term exercise performance are examined. However, this measurement is not frequently obtained in the majority of research investigating ergogenic effects of caffeine for short-term high-intensity exercise performance.
Recent data suggest that variations in genotype may alter caffeine metabolism and potentially magnitude of performance in response to caffeine ingestion. Caffeine is metabolized in the liver by cytochrome P450 1A2 (14), which shows marked variability between individuals (40). A single substitution in the gene causes some persons to be slow caffeine metabolizers, whereas those who are homozygous for the allele metabolize caffeine more rapidly (66). A recent report (21) demonstrated that habitual caffeine consumption is related to these genotypes, which may explain the discrepancy in individual responses to caffeine's physiological effects. However, no study has simultaneously examined the effect of differences in genotype on high-intensity exercise performance after caffeine intake.
Purported mechanisms of caffeine's ergogenic effects during high-intensity exercise are demonstrated in Figure 1. Early data (22,44) revealed that caffeine increased lipolysis and spared muscle glycogen during endurance exercise. In contrast, other studies (35,45) revealed that caffeine does not spare glycogen. High-intensity short-term exercise is not limited by carbohydrate availability, so other mechanisms must explain the ergogenic effect of caffeine.
An alternative site for caffeine's ergogenic effects may lie in the brain. Davis et al. (25) reported that caffeine delays fatigue via stimulation of the CNS by acting as an adenosine antagonist. Adenosine, a cellular component that increases with muscular contraction, inhibits neuron excitability and synaptic transmission via binding to its receptors (52), leading to decreased arousal and increased sleep (62). In male rats, run time to fatigue was 60% longer with caffeine vs. an adenosine agonist (25). Adenosine receptors exist primarily in type I fibers (37), which may decrease the likelihood of performance gains in activities dependent on type II fibers, such as heavy resistance training or sprinting. Consequently, it is unknown if enhanced short-term high-intensity exercise performance occurs via adenosine antagonism.
Acute caffeine ingestion also modifies perceptual responses that may alter performance. In a meta-analysis (27) of 21 studies using a placebo-controlled double-blind design, data revealed that caffeine decreased rating of perceived exertion (RPE) by 5.6% during prolonged exercise, which explained approximately 33% of improved performance. However, RPE assessment may not apply to the high-intensity intermittent nature of team sports or activities such as sprinting and/or resistance training in which recording exertion during a brief intense exercise bout is impractical. RPE recorded at the completion of knee extension and biceps curls was similar between caffeine and placebo (46). Unpublished data (70) in strength-trained men show no difference in RPE (11) during completion of 40 repetitions of 1-leg knee extension and flexion at 180°·s−1 after ingestion of a low (2 mg·kg−1) (5.36 ± 1.57) or moderate dose (5 mg·kg−1) (5.46 ± 1.51) of caffeine vs. placebo (5.27 ± 1.79). Overall, caffeine does not seem to alter RPE during or at completion of short-term intense exercise, although it may be due to challenges with recording RPE during exercise. Because of the paucity of data, further investigation is merited.
An effect of caffeine on psychological function is also plausible, as it alters the CNS by promoting serotonin release, increasing sympathetic activity, and decreasing the activity of inhibitory neurons due to adenosine antagonism (69). This may explain the reduced tiredness and improved mood and alertness shown with acute caffeine ingestion (41). An intriguing hypothesis is that compared with placebo, caffeine may reverse the serious withdrawal effects, such as lethargy, irritability, and headaches, reported with 24- to 48-hour caffeine abstention as commonly required in scientific protocols. Data from a recent study (70) revealed improved resistance training performance in 67% of heavy caffeine users (>225 mg per day) vs. men with lower habitual intakes (∼125 mg per day) who typically performed better in the placebo condition. These men commonly reported that they felt “less tired” and had “more energy” in the caffeine trial, although only 28% of subjects correctly identified the caffeine treatment. Further research using psychological scales quantifying mood before and during bouts of high-intensity exercise is warranted to further explore this potential mechanism.
In summary, the exact mechanism explaining ergogenic effects of caffeine for short-term high-intensity exercise is relatively unknown, especially at physiological caffeine concentrations. It is likely multifactorial with central factors such as adenosine antagonism the most probable mechanism, yet enhanced performance may also be related to alterations in perceived exertion, reaction time, cognition, and/or mood.
There is a paucity of research investigating efficacy of caffeine in sports including sprinting, track and field, football, and hockey. Any benefit of chronic caffeine intake for completion of day-to-day training, and subsequent performance at games or meets, also requires further investigation.
The focus of this review was to critically examine studies investigating the effect of acute caffeine ingestion on performance in activities including sprinting, all-out cycling, team sports, and resistance training that are dependent on nonoxidative metabolism. Data reveal that:
Gratitude is expressed to many participants who completed various studies in the author's laboratory. Furthermore, this work could not have occurred without the assistance of an outstanding collection of undergraduate students. The authors also thank the reviewers for thorough comments that improved the quality of this review. The results of the present study do not constitute endorsement of the product by the authors or the National Strength and Conditioning Association.
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