A Brigham Young University study shows drinking caffeinated beverages before a hard workout might give you a psychological boost, but it does nothing to increase your endurance or to help your body burn off fat more easily.

Researchers said their findings dispel a longstanding myth that caffeine can improve athletic performance.William Winder, a BYU professor of zoology, and graduate student Josephine Arogyasami found that rats that were given caffeine didn't run any longer than rats in a control group. Both groups used glycogen (the body's primary energy source) at the same rate, indicating that the rats on caffeine didn't burn fat any better either.

"It was postulated that if caffeine makes fat more available for use by the muscle, then that will spare the use of glycogen and other carbohydrates during long-term exercise bouts," said Winder.

"We were very surprised to find that there was no - absolutely no - carbohydrate-sparing effect in the muscle or liver due to the caffeine."

Winder and Arogyasami conducted several studies to determine how different dosages of caffeine affected trained and untrained rats as well as rats that had gone without food. In every situation, the caffeine failed to make a significant change in the amount of glycogen utilized.

In a 1978 study on humans, other researchers concluded that caffeine increased performance by accelerating utilization of fat by the muscles, giving the body more energy resources to draw from. However, the study didn't include actual muscle glycogen measurements, and no recent studies have been able to substantiate a carbohydrate-sparing effect in humans.

"There may be differences between how the rat responds and how humans would respond. These studies really need to be repeated in human subjects using the muscle biopsy technique so glycogen can be measured in the muscles of human subjects," Winder said.

Although the BYU study provides evidence that caffeine does not boost energy levels, Winder said some athletes may experience a psychological effect.

"In a human subject, of course, if he thinks that it will be beneficial to him and increase his endurance, there may be a psychological effect that would cause him to push harder and would give an apparent positive result."