TUESDAY, Jan. 17 (HealthDay News) -- Having a coffee fix just before a workout may not be the best idea, a new study suggests.

Researchers in Switzerland found that the amount of caffeine in just two cups of coffee limits the body's ability to increase blood flow to the heart during exercise.

"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," Dr. Philipp A. Kaufmann, of the University Hospital Zurich and Center for Integrative Human Physiology, said in a prepared statement.

The study included 18 young, healthy people who were regular coffee drinkers. They did not drink any coffee for 36 hours prior to study testing. The researchers used high-tech PET scans to measure the participants' heart blood flow before and after they rode a stationary bike. Ten of them did this in normal conditions, and eight did the exercise in a chamber that simulated being at about 15,000 feet altitude.

Both groups repeated the testing procedure after swallowing a tablet containing 200 milligrams of caffeine -- the amount contained in two cups of coffee.

As reported in the Jan. 17 issue of the Journal of the American College of Cardiology, the caffeine did not affect heart blood flow when the participants were inactive. However, measurements taken immediately after exercise showed a slowdown in heart blood flow after they'd taken the caffeine tablets, compared to their previous results.

Heart blood flow was 22 percent lower in those who exercised in normal air pressure and 39 percent lower in those who exercised in the high-altitude chamber, the researchers report.

They believe caffeine may block certain receptors in the walls of blood vessels, interfering with the normal signaling process that causes blood vessels to dilate in response to exercise.

"Although these findings seem not to have a clinical importance in healthy volunteers, they may raise safety questions in patients with reduced coronary flow reserve, as seen in coronary artery disease, particularly before physical exercise and at high-altitude exposure," the study authors wrote.

While some people regard caffeine as a stimulant, this study suggests it may not increase athletic performance.

"We now have good evidence that, at the level of myocardial blood flow, caffeine is not a useful stimulant. It may be a stimulant at the cerebral level in terms of being more awake and alert, which may subjectively give the feeling of having better physical performance. But I now would not recommend that any athlete drink caffeine before sports," Kaufmann said.

More information

Iowa State University has more about caffeine and athletes.