Among adolescent athletes with the ACE II genotype, habitual caffeine intake is positively correlated with endurance performance and aerobic power only in the presence of acute caffeine ingestion prior to exercise, suggesting that genetic background modulates the interaction between habitual caffeine consumption and acute ergogenic response.

People with the ACE II genotype have more slow-twitch muscle fibers that use oxygen efficiently. When they drink caffeine before exercise, it blocks signals that make the brain and muscles feel tired, which lets them push harder for longer. The caffeine also helps these fibers burn fat for energy instead of sugar, giving them more fuel. Because they can exercise longer, their muscles build up more waste products, which makes them feel more tired even though they are performing better.