“If we put you on a treadmill and told you to go as long as you can, you’ll run farther if you’ve had caffeine than if you haven’t,” says Matthew Ganio, who heads the department of health, human performance, and recreation at the University of Arkansas. (Researchers typically give people between 1.4 and 2.7 milligrams of caffeine per pound of body weight an hour before exercise. For a 150-pound person, that means roughly 200 to 400 mg of caffeine. And more isn’t better. Higher doses don’t give more of a boost and can cause side effects.)