Mixed
Caffeine improves endurance performance only in individuals with the CYP1A2 AA genotype (fast metabolizers), while impairing performance in those with the CC genotype.
If you use caffeine for performance, know your CYP1A2 genotype. If you are a 'fast metabolizer' (AA), standard doses (2-4 mg/kg) likely boost your endurance. If you are a 'slow metabolizer' (CC), caffeine may actually hurt your performance and increase health risks like hypertension. If you are intermediate (AC), you likely see no benefit. Genetic testing can prevent wasted effort or performance loss.
There was a significant caffeine-gene interaction where improvements in performance were seen at both caffeine doses, but only in those with the AA genotype who are 'fast metabolizers' of caffeine... Among those with the CC genotype, 4 mg/kg caffeine impaired performance by 13.7%, and in those with the AC genotype there was no effect of either caffeine dose (19).
Why this rating
Based on randomized controlled trials (RCTs) with clear genotype stratification, though sample sizes vary.
Source
Sport Nutrigenomics: Personalized Nutrition for Athletic Performance
Nanci S. Guest et al. · Frontiers in Nutrition · 2019
DOI 10.3389/fnut.2019.00008
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