“There is so much data on this that it’s unbelievable,” he said. “It’s just unequivocal that caffeine improves performance. It’s been shown in well-respected labs in multiple places around the world.”

Image Credit... Filip Kwiatkowski for The New York Times

The only new questions were how it exerts its effects and how little caffeine is needed to get an effect.

For many years, researchers thought the sole reason people could exercise harder and longer after using caffeine was that the compound helped muscles use fat as a fuel, sparing the glycogen stored in muscles and increasing endurance. But there were several hints that something else was going on. For example, caffeine improved performance even in short intense bursts of exercise when endurance is not an issue.

Now, Dr. Tarnopolsky and others report that caffeine increases the power output of muscles by releasing calcium that is stored in muscle. The effect can enable athletes to keep going longer or to go faster in the same length of time. Caffeine also affects the brain’s sensation of exhaustion, that feeling that it’s time to stop, you can’t go on any more. That may be one way it improves endurance, Dr. Tarnopolsky said.

The performance improvement in controlled laboratory settings can be 20 to 25 percent, Dr. Tarnopolsky said. But in the real world, including all comers, the improvement may average about 5 percent, still significant if you want to get your best time or even win a race.

For years, researchers believed that you needed about 5 to 6 milligrams of caffeine per kilogram of body weight. An 80-kilogram, or 176-pound man, for example, would need about 400 milligrams of caffeine, or 20 ounces of coffee.

Now, Louise M. Burke, the head of sports nutrition department of the Australian Institute of Sport in Canberra, reports that athletes get the full caffeine effect with as little as 1 milligram of caffeine per kilogram of body weight. Instead of 20 ounces of coffee, a 176-pound man could drink 4 ounces of coffee, or about two 12-ounce cans of Coke.