Abstract:
Caffeine is now an unrestricted ergogenic aid for competitive athletes. Previous reviews of caffeine's effects on exercise performance have been limited to qualitative analysis. The purpose of this paper was therefore to quantitatively meta-analyze the effects of caffeine on exercise performance. We identified 90 estimates of performance effects of caffeine in 32 peer-reviewed studies. All estimates were converted to mean power in an equivalent time trial then subjected to a mixed-model meta-analysis. The fixed effects were gender, training status (elite athlete, non-elite athlete, non-athlete), dietary caffeine status (habitual consumer, non-consumer), caffeine abstention period, caffeine dose (mg/kg body mass), type of caffeine (pure or in coffee), delay between ingestion and performance test, duration of test, and presence or absence of fatiguing exercise before the test. The random effects accounted for within- and between-study variance. We found that caffeine enhanced mean power by 2.8% (90% confidence limits ± 1.1%) in male non-elite athletes who are habitual caffeine consumers abstaining from caffeine for 2 d before consuming 6 mg/kg of caffeine capsules 1 h before performing a 30-min time trial without intervening fatiguing exercise. The effects for other athletes and conditions were: females, 3.1% (± 2.7%); elite athletes, 2.9% (± 1.4%); non-athletes, 1.3% (± 1.2%); habitual non-consumers, 4.0% (± 1.4%); 7 d of abstention, 3.4% (± 2.6%); 0.3 mg/kg of caffeine, 1.6% (± 5.3%); caffeinated coffee, 1.0% (± 1.6%); 2-h delay before exercise, 2.9% (± 1.2%); 6-s exercise test, 1.6% (± 1.7%); prior fatiguing exercise, 3.0% (± 1.6%). Each of these effects of caffeine varied typically between studies by ± 1.4% (the between-study random effect; 90% confidence limits ± 0.9 to ± 3.5%). We conclude that caffeine has a greater effect on performance with athletes, with habitual non-consumers of dietary caffeine, when administered as pure caffeine, and in endurance exercise, but there is considerable uncertainty about the magnitude of the effects on individuals. More research is needed to reduce this uncertainty and to determine the performance effects of caffeine with females, following longer periods of dietary abstention, in low doses, and for brief exercise. There has also been no research on effects of caffeine on the repetitive fatiguing exercise typical of team sports.
