Introduction

In 2018, the IOC published a consensus statement regarding the effects of dietary supplements on exercise performance of athletes.1 The consensus statement placed meta-analyses at the top of the evidence pyramid.1 In sports nutrition research, meta-analyses provide a method of pooling available primary studies exploring the efficacy of a given supplement on a specific outcome (eg, performance of an exercise test). As such, meta-analyses are used to support establishing evidence-based guidelines and decision making for the effective prescription of nutritional supplements and ergogenic aids.

One supplement with a long history of use for its ergogenic effects on performance is caffeine.2 Caffeine ingestion is highly prevalent among athletes, especially since 2004, when it was removed from the World Anti-Doing Agency list of within-competition banned substances.3 For example, 74% of urine samples collected from 2004 to 2008 and analysed as a part of doping control contained caffeine.3 Given inconsistent evidence in the primary research that examined the effects of caffeine on exercise performance, several research groups explored this area using meta-analytical methods.4–15 While these meta-analyses generally report ergogenic effects of caffeine on exercise performance, even adequately conducted meta-analyses tend to focus on the ergogenic effects of caffeine within just a single performance domain. As an illustration, Grgic and Pickering10 only examined the effects of caffeine ingestion on isokinetic peak torque.

Given that each meta-analysis is typically focused only on a specific aspect of exercise performance, it is challenging to: (1) compare the effects of caffeine ingestion on different performance domains; (2) comparatively assess the availability and strength of evidence for different performance domains; (3) establish comprehensive recommendations on the use of caffeine in sports and exercise; and (4) provide overall recommendations for future research on the ergogenic effects of caffeine on exercise performance. Such recommendations may increase the uptake of evidence-based findings in the context of supplement prescription and guide future research in this area.

Consistency of meta-analytical findings is often lacking; even meta-analyses that have examined the same outcome may produce conflicting findings. For instance, Gonçalves Ribeiro et al 8 did not observe significant effects of caffeine ingestion on power. In contrast, a subsequent meta-analysis by Grgic9 reported that caffeine ingestion is ergogenic for this outcome. Such conflicting findings hinder firm evidence-based conclusions from individual meta-analyses. Ultimately, the methods employed in a specific meta-analysis (eg, the number of databases searched, the comprehensiveness of the search syntax and the methods used for analysing the data) determine the robustness of the pooled results. For example, a meta-analysis on the effects of caffeine supplementation on power conducted by Gonçalves Ribeiro et al 8 included only studies that were published between January 2010 and December 2015. Due to these restrictions, studies published before 2010 were excluded from consideration, and the authors provided no rationale for their approach. Only four studies that assessed power during Wingate tests were included in their review, and no significant pooled effects were found. Grgic9 conducted a similar meta-analysis without any restrictions regarding the year of publication; this analysis included 16 studies and reported significant improvements in both mean and peak power on the Wingate test following caffeine ingestion.

One proposed method to overcome some of the above, and other, potential limitations of meta-analyses is to perform umbrella reviews.16 Umbrella reviews (ie, reviews that include the syntheses and appraisal of existing systematic reviews and meta-analyses) provide a comprehensive view of the evidence landscape on a given topic because they encompass larger scale of evidence.16 Such reviews help us to understand the current strengths and limitations of the entire body of evidence by comparing and contrasting findings from the entirety of the published data. Such a treatise on the effects of caffeine on exercise may be a useful resource for researchers, sports nutritionists, athletes, coaches and others interested in the ergogenic effects of caffeine on acute exercise performance. To date, there are no published umbrella reviews focusing on the effects of caffeine on exercise performance.

The aim of the present paper is threefold: (1) to systematically review available meta-analytical evidence that has examined the effects of caffeine on exercise performance; (2) to addresses the quality, strengths and limitations of the meta-analytical evidence; and (3) to identify current gaps in the literature and make key suggestions for future research.