Caffeine in Ergogenic Trials: What the Evidence Shows

Caffeine (1,3,7-trimethylxanthine) is a naturally occurring methylxanthine found in coffee, tea, cocoa, and a range of food supplement formulations. Its primary pharmacological mechanism is non-selective competitive antagonism at adenosine A1 and A2A receptors in the central nervous system. Adenosine is an inhibitory neuromodulator: as it accumulates during sustained wakefulness or exercise, it progressively suppresses neural activity, increases perception of effort, and promotes sleep onset. By blocking these receptors, caffeine prevents adenosine from exerting its inhibitory effects — attenuating fatigue, reducing perceived exertion, and maintaining alertness and motor performance. **Absorption, distribution, and half-life** Oral caffeine is absorbed rapidly and nearly completely from the gastrointestinal tract. Peak plasma concentrations are typically reached within 30–60 minutes of ingestion, with a bioavailability approaching 100%. This predictable absorption profile makes dose-response research tractable — a characteristic that distinguishes caffeine from many other supplement categories where absorption varies considerably between individuals and formulations. The plasma half-life of caffeine in healthy adults is approximately 3–5 hours, though this varies substantially between individuals. The primary metabolic pathway is N-demethylation via the hepatic cytochrome P450 1A2 enzyme (CYP1A2), which converts caffeine to paraxanthine (approximately 84%), theobromine, and theophylline — all of which have some pharmacological activity. **CYP1A2 genotype and the ergogenic response — Guest et al. (PMID: 29509693)** A widely cited 2018 study by Guest NS et al., published in the Journal of the International Society of Sports Nutrition, investigated whether CYP1A2 genotype modifies the ergogenic response to caffeine in cyclists. The CYP1A2 -163C>A polymorphism (rs762551) produces a functionally meaningful difference: AA homozygotes (approximately 38–45% of the population) are classified as "fast metabolisers" — they clear caffeine more rapidly. C allele carriers (AC and CC genotypes, approximately 55–62% of the population) are "slow metabolisers" with longer caffeine exposure post-dose. In a randomised, double-blind, crossover trial, participants completed cycling time trials after supplementing with 2 mg/kg, 4 mg/kg caffeine, or placebo. Fast metabolisers (AA genotype) showed significant performance improvements at both doses. Slow metabolisers (AC and CC genotypes) showed no significant improvement at 2 mg/kg and, at 4 mg/kg, performance was actually significantly worse than placebo in the CC group. This finding has practical implications for interpreting heterogeneous trial results and individual response variation. A proportion of the null and adverse-response results in the caffeine literature may reflect the distribution of slow metaboliser genotypes within trial populations, rather than absence of an effect in the full population. Subsequent meta-analytic work has not consistently replicated the genotype effect at the meta-level, but the Guest et al. data remain an important reference point for understanding inter-individual variability in the caffeine response. **Tolerance and habituation** With regular caffeine consumption, adenosine receptor upregulation occurs, reducing the contrast between caffeinated and uncaffeinated states. This underlies caffeine tolerance and the withdrawal syndrome (headache, fatigue, difficulty concentrating) observed after abrupt cessation in habitual users. For ergogenic purposes, the literature has examined whether regular users show blunted performance effects — a question addressed in the endurance section below.