It’s well known that caffeine can improve athletic (and mental) performance, but not in everyone. A recent Canadian study helps illustrate and explain the “considerable inter-individual variability” of caffeine’s effects on exercise by looking at genetic factors. Many studies have found that caffeine, consumed in a beverage or a pill, can reduce fatigue and boost exercise endurance and performance. But some studies have found no benefits, on average, probably because people respond so differently to caffeine, depending not only on whether they are accustomed to it or not, but also on genes that determine how quickly their bodies metabolize caffeine (that is, break it down and clear it).
The new study, published in Medicine & Science in Sports & Exercise, involved 101 male athletes (average age 25), whose genes were analyzed, via cells from saliva, to see whether they were fast, moderate, or slow caffeine metabolizers (see below). During three sessions, a week apart, they pedaled 10 kilometers on a stationary bike at maximum speed. About half an hour before each ride, they swallowed one of three capsules: high-dose caffeine (equal to about two cups of coffee, adjusted for body weight), low-dose caffeine (equal to about one cup of coffee), or a placebo.
Overall, only the higher dose of caffeine improved performance time (by 3 percent, on average) compared to the placebo. But among the fast metabolizers, it improved performance time by 7 percent, while it actually impaired performance by 14 percent in the slow metabolizers; it had no effect in the moderate metabolizers. The new study involved only fit young men and cycling, so it’s not clear whether the same results would occur with other people or other activities.
The background story
Researchers have identified several genes that affect the metabolism of caffeine. One key gene is CYP1A2; each of us gets two copies of it, one from each parent. There are two common variants, A and C.
Nearly half of us have two copies of the A variant (AA), about 10 percent two C’s (CC), and the rest one A and one C (AC). Each copy of the A variant causes caffeine to be rapidly metabolized into other bioactive compounds, which appear to be responsible for performance benefits. Each C copy causes caffeine to be slowly metabolized, so more stays in the bloodstream and for a longer time.
Caffeine, before it is metabolized, is mainly responsible for well-known adverse effects such as jitteriness, as well as blood vessel constriction, which can lead to reduced blood flow to muscles and the heart. As a result, after consuming caffeine, people with the AA variants (fast metabolizers) get maximal benefit with the fewest side effects; people with CC variants (slow metabolizers) get minimal benefit with the most side effects; and people with AC (moderate metabolizers) get modest benefits with some side effects.
Bottom line: If you find that you perform better (or worse) after drinking a cup of coffee, you may have your genes to thank (or blame). Researchers are also studying how genetics may help determine how caffeine affects an individual’s cardiovascular and overall health.