Whether athletes can enhance their performance with caffeine may depend on their genes.

According to a new study, athletes with a particular variant of one gene show notable improvements after swallowing caffeine. But those with a different variant of that gene may perform worse if they first have caffeine, raising questions about the interplay of nutrition, genetics and exercise.

Gene CYP1A2 controls the expression of an enzyme that affects the breakdown of caffeine in the body. One variation — those who have two copies — prompts the body to rapidly metabolize caffeine and the drug gives them a jolt and is gone. By most estimates, about half of us are fast metabolizers.

People with one copy of the gene variant that slows caffeine metabolism and one of the faster-metabolizing type are considered to be moderate metabolizers. But about 10 percent of us have two copies of the slow-metabolizing variant and are slow metabolizers.

For the new study, Ahmed El-Sohemy, a professor at the University of Toronto who is the founder of genetic testing company Nutrigenomix, found that fast metabolizers rode a stationary bicycle nearly 7 percent faster after they had downed caffeine.

The moderate metabolizers performed almost exactly the same whether they had received caffeine or a placebo. But the slow metabolizers completed the 10-kilometer ride about 14 percent more slowly after the higher dose of caffeine than after the placebo.

El-Sohemy suspects that caffeine lingered in the slow metabolizers, narrowing their blood vessels and reducing the flow of blood and oxygen to tiring muscles.