Caffeine tolerance kills its benefits

Caffeine is well-known for being an adenosine receptor antagonist^[1]. The connection between caffeine and adenosine is shown in Figure 1. Because adenosine mediates the perception of drowsiness, caffeine consumption results in alertness. On any given day, about 89% of the U.S. population consumes an average dose of 186 milligrams of caffeine. Athletes appear to love the stuff too, with one study of triathletes reporting that 89% were planning on using caffeine throughout their race.

The ergogenic effects of caffeine are well documented. The International Society of Sports Nutrition position stand^[2] on caffeine and exercise performance states that benefits are observed primarily in sports of a prolonged duration with and without intermittent bouts of high-intensity activity, such as endurance running, soccer, and rugby. However, at least one study^[3] has reported less pronounced ergogenic effects of caffeine supplementation in regular consumers (more than 300 milligrams per day) compared to nonusers (less than 50 milligrams per day). Importantly, not all studies have supported this finding^[4][5][5], and it remains unclear whether regular caffeine consumption impacts its ergogenic effects.

The major limitations of research to date are the use of comparisons between groups based on self-reported caffeine intake or researchers designing caffeine-controlled interventions lasting no more than several days. The issue with the former is that self-reported caffeine intake is prone to inaccuracy. The issue with the latter is that it may take more than several days for the effects of caffeine tolerance to become apparent. Both of these limitations were addressed in the study under review, which used a caffeine-controlled diet over the course of a month to investigate the effect of prolonged habitual caffeine consumption on endurance exercise performance.

Caffeine is a popular and effective ergogenic aid for endurance exercise, but whether habitual caffeine consumption impacts the effects of an acute caffeine dose remains unclear. The study under review sought to test how prolonged regular caffeine use influences exercise performance.