Hormonal
Caffeine improves 10-km cycling time by 4.8-6.8% in athletes with the CYP1A2 AA genotype (fast metabolizers), but impairs performance by 13.7% in those with the CC genotype (slow metabolizers).
If you use caffeine for endurance sports, your CYP1A2 genotype matters. Fast metabolizers (AA) benefit from 2-4 mg/kg, seeing 5-7% performance gains. Slow metabolizers (CC) should avoid high doses (4 mg/kg) as it can worsen performance by nearly 14%. If you suspect caffeine hurts your performance, genetic testing or a self-experiment comparing placebo vs. caffeine is recommended before committing to a regimen.
Among those with the AA genotype, cycling time increased by 13.7% versus placebo... In those with the CC genotype, 4 mg/kg decreased by 4.8% at 2 mg/kg... and by 6.8% at 4 mg/kg... Caffeine had no effect in those with the AC genotype and diminished performance at 4 mg/kg in those with the CC genotype.
Why this rating
Randomized, double-blind, placebo-controlled trial with a large sample size (n=101) for a genetic subgroup analysis.
Source
Caffeine, CYP1A2 Genotype, and Endurance Performance in Athletes
Nanci S. Guest et al. · Medicine & Science in Sports & Exercise · 2018
DOI 10.1249/mss.0000000000001596
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