Grgic, Jozo ORCID: 0000-0002-6929-2844 (2020) The acute effects of caffeine supplementation on muscle strength, power and endurance. PhD thesis, Victoria University.

Abstract

Caffeine is a highly popular ergogenic aid, often consumed by athletes and non-athletes alike. The aim of this thesis was to explore: (a) the effects of caffeine on different exercise tasks; (b) the effects of varying doses of caffeine on resistance exercise performance; and (c) the effects of ADORA2A and CYP1A2 genotype variations on the individual response to caffeine ingestion. This thesis is comprised of eight published studies – four reviews and four primary studies. The first study was an umbrella review of 21 published meta-analyses on the ergogenic effects of caffeine on exercise performance. This review showed that caffeine ingestion was ergogenic for aerobic endurance, muscle strength, muscle endurance, power, jumping performance, and exercise speed. The ergogenic effects of caffeine on muscle endurance, muscle strength, anaerobic power, and aerobic endurance were substantiated by moderate-quality evidence from moderate-to-high quality systematic reviews. The evidence for other outcomes was of low or very low quality and it was based on moderate-quality reviews. The second study was a narrative review that critically evaluated the evidence on the topic of caffeine supplementation when performing resistance exercise. This study provided a comprehensive overview of caffeine’s effects on resistance exercise performance and its influence on the associated physiological responses. The third study was a meta-analysis that explored the effects of caffeine on maximum strength (one repetition maximum) and vertical jump height. This analysis found that caffeine ingestion provides an ergogenic effect on both outcomes. The fourth study was a meta-analysis that explored the acute effects of caffeine on Wingate (all-out, 30-s cycle sprint) test performance, showing ergogenic effects of caffeine on mean and peak power in this test. Based on these reviews of literature, it was identified that more research is needed to explore the effects of caffeine supplementation in trained individuals, the optimal dose of caffeine for improving anaerobic exercise performance, and the influence of genotype variations on the responses to caffeine ingestion. To fill the evidence gap, the fifth study explored the acute effects of caffeine ingestion (6 mg/kg) on strength, power, muscular endurance, rating of perceived exertion (RPE), and pain perception in resistance-trained men. This study demonstrated that caffeine ingestion acutely reduced RPE and enhanced upper-body power and lower-body strength. Given that quite a high dose was used in the fifth study, and that several reviews suggested there may be a caffeine dose effect, the sixth study explored the acute effects of three different doses of caffeine (2 mg/kg, 4 mg/kg, and 6 mg/kg) on upper- and lower-body muscular strength and endurance. While caffeine ingestion enhanced lower-body strength and muscular endurance, this study found no clear association between the caffeine dose and the magnitude of ergogenic effects. However, a relatively large individual variation in responses to caffeine was noted. The final two studies were, therefore, conducted to explore possible genetic determinants of individual responses to caffeine supplementation. The seventh study explored the influence of caffeine ingestion on movement velocity, muscular endurance, jumping, and sprinting performance in a sample of 20 ADORA2A (rs5751876) C allele carriers (CC/CT genotype). In contrast to previous findings on this topic, this study showed that C allele carriers exhibited ergogenic responses to caffeine in the majority of exercise outcomes. The eighth study explored the influence of variation in CYP1A2 (rs762551) genotype in a sample of 22 men (AA homozygotes n = 13; C allele carriers n = 9) on the acute effects of caffeine ingestion on exercise performance, including velocity, power, and muscle endurance. Compared to placebo, caffeine ingestion improved exercise performance in most outcomes, but there was no significant genotype × caffeine interaction. Overall, the main findings of this thesis are that: (a) caffeine ingestion acutely enhances performance in various exercise tasks; (b) lower doses of caffeine may produce ergogenic effects comparable to those of higher doses of caffeine; and (c) the individual responses to caffeine ingestion may not be moderated by ADORA2A and CYP1A2 genotype variation. The findings on ergogenic effects of different doses of caffeine and the influence of genotype on individual responses to caffeine need to be confirmed in future studies with larger sample sizes. These findings may be useful to athletes, coaches, and sports nutritionists in making evidence-based decisions about caffeine supplementation.

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