Caffeine as a context-dependent metabolic, perceptual, and recovery modulator in exercise: implications for substrate utilization, performance, and individual responsiveness

Caffeine is a widely studied ergogenic aid in sport and exercise science, yet its effects on exercise metabolism, performance, and recovery remain context dependent. Although caffeine is commonly used to enhance alertness, reduce perceived exertion, and improve exercise capacity, its effects vary with dose, ingestion timing, delivery form, exercise modality, habitual caffeine intake, training status, sex-related factors, genotype, and sleep vulnerability. This narrative review provides an evidence-calibrated synthesis of caffeine’s central, metabolic, perceptual, performance-related, and recovery-related effects, emphasizing substrate utilization and individual responsiveness. It distinguishes relatively well-supported mechanisms from conditional or inconsistent effects, examines major sources of heterogeneity across studies, and evaluates acute ergogenic benefits alongside potential recovery costs. Evidence indicates that caffeine’s most consistent mechanisms involve adenosine receptor antagonism, increased central nervous system activation, and reduced perception of effort or fatigue. Peripheral and metabolic mechanisms, including catecholamine-mediated substrate mobilization, changes in fat oxidation, calcium handling, and glycogen-related responses, are biologically plausible but appear more conditional and protocol dependent. Importantly, caffeine-induced increases in fat oxidation should not be interpreted as direct evidence of glycogen sparing or performance enhancement, because their practical relevance depends on exercise intensity, duration, carbohydrate availability, and the performance outcome assessed. Caffeine shows the strongest ergogenic evidence in endurance exercise, while effects on strength, power, repeated sprint ability, and sport-specific skills are smaller and more variable. Recovery-related applications require particular caution: caffeine may support perceived readiness or selected glycogen-related outcomes when combined with carbohydrate in short-turnaround contexts, but late-day ingestion may impair sleep and thereby compromise broader recovery. Overall, caffeine should be understood as a context-dependent metabolic and perceptual modulator rather than a universally ergogenic supplement. Applied use should prioritize the lowest effective dose, individualized timing, sport-specific trialling, adverse-effect monitoring, and careful consideration of recovery costs. Future research should integrate metabolic markers, performance outcomes, sleep, and individual moderators within ecologically valid sport protocols to clarify when caffeine is beneficial, neutral, or potentially counterproductive.