According to a new study published in the Journal of the American College of Cardiology, the equivalent of two cups of coffee reduces the body's ability to boost blood flow to the heart muscle in response to exercise, and the effect is stronger at higher altitudes.

"Whenever we do a physical exercise, myocardial blood flow has to increase in order to match the increased need of oxygen. We found that caffeine may adversely affect this mechanism. It partly blunts the needed increase in flow," said Philipp A. Kaufmann, from the University Hospital Zurich and Center for Integrative Human Physiology in Zurich.

Researchers led by Mehdi Namdar looked at 18 young healthy persons who were regular coffee drinkers. Prior to going through the tests, the participants didn't drink coffee for 36 hours.

In one part of the study, PET scans that showed blood flow in the hearts of 10 participants were performed before and immediately after they rode a stationary exercise bicycle.

In the second part of the study, the same type of myocardial blood-flow measurements were done in 8 participants who were in a chamber simulating the thin air at about 4,500 meters altitude. The high-altitude test was designed to mimic the way coronary artery disease deprives the heart muscle of sufficient oxygen.

In both groups, the testing procedure was repeated 50 minutes after each participant swallowed a tablet containing 200 milligrams of caffeine, the equivalent of two cups of coffee.

The caffeine dose did not affect blood flow within the heart muscle while the participants were at rest. However, the blood flow measurements taken immediately after exercise were significantly lower after the participants had taken caffeine tablets. The effect was more pronounced in the group in the high-altitude chamber.

The conclusion of the study is that although caffeine is a stimulant, it may not necessarily boost athletic performance.