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The enduring mystery of caffeine’s ergogenic effects

Caffeine’s ability to boost exercise performance is well known. Exactly how it does so is a little less clear.

Study under review: Caffeine and Physiological Responses to Submaximal Exercise: A Meta-Analysis

Introduction

Caffeine is the most widely consumed psychoactive substance in the world. In the U.S., nearly 90% of adults[1] consume caffeine in some form because of its stimulatory effects on the central nervous system, which helps combat feelings of fatigue[2], but also because it’s mildly habit-forming.

It’s also used by many people for its beneficial effects on physical performance. As shown in Figure 1, caffeine is among the most commonly used supplements, and is also a frequent ingredient in pre-workout supplements. Most of the evidence to date[3] has suggested that supplementing caffeine 30-90 minutes before high-intensity aerobic exercise can reliably increase performance.

Some of caffeine’s effects can be attributed to its binding to and blocking adenosine receptors. Adenosine is a molecule that promotes sleepiness by binding to its receptors within the brain and preventing the release of other neurotransmitters that promote wakefulness. In short, caffeine allows neurotransmitters that are associated with wakefulness to function with less interference.

However, adenosine and its receptors are present throughout the body, not just the brain, where they have different physiological effects. Unfortunately, the ubiquity of adenosine as an extracellular signaling molecule makes it difficult to pinpoint the exact mechanisms through which caffeine benefits athletic performance. Another problem in trying to assess the mechanisms through which caffeine may exert its performance-enhancing effects is that the physiological responses caffeine might affect can also be influenced by changes in exercise intensity, which can also affect performance outcomes.

Previous studies have attempted to address this problem by having participants engage in fixed intensity submaximal exercise (60-80% of VO2max), thus addressing the issue of increased intensity affecting performance. However, these studies have generally produced mixed results, with some studies reporting caffeine resulting in significant increases in outcomes like minute ventilation[4] and respiratory exchange ratio[5] and others finding no effect[6]. It’s possible that these studies may not have had large enough sample sizes to properly detect whether or not caffeine can affect these outcomes, compared to placebo.

The study under review attempts to address this issue by pooling the results of studies from the literature to increase the probability of finding a real effect. Its purpose was to identify the effects of caffeine on physiological responses typically associated with submaximal exercise to better grasp how caffeine works mechanistically.

Caffeine is a popular psychoactive substance used by many people to increase alertness and exercise performance. The ways in which caffeine exerts its ergogenic effects are unclear, due to the ubiquity of adenosine and its receptors. Previous studies have had difficulty in determining the effects of caffeine on physiological responses because they often produced imprecise estimates due to small sample sizes. The study under review attempts to resolve these issues by searching the literature and pooling the results of studies that have explored the effects of caffeine supplementation on physiological responses to submaximal exercise.

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