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

According to a new study of the genetics of caffeine metabolism, athletes with a particular variant of one gene show notable improvements in their endurance performance after swallowing caffeine.

But those with a different variant of that gene may perform worse if they first have caffeine, raising questions about who should be using the drug to bump up performance and about the broader interplay of nutrition, genetics and exercise.

For many of us, caffeine, usually in the form of coffee, is as necessary to the morning as sunrise.

But different people respond differently to the effects of caffeine. Some become jittery and later have difficulty sleeping. Others can drink the same amount of coffee and report increased alertness but no jitters or sleep disruptions.

The same range of reactions occurs in athletes. In multiple past studies, most people will work out longer, faster or more strenuously after they swallow a moderate dose of caffeine, but a few perform no better or even worse.