"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, M.D., F.A.C.C., from the University Hospital Zurich and Center for Integrative Human Physiology CIHP in Zurich,.

The researchers, studied 18 young, healthy people who were regular coffee drinkers. The participants did not drink any coffee for 36 hours prior to the study testing. 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 15,000 feet (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 pronounced in the group in the high-altitude chamber.

Blood flow normally increases in response to exercise, and the results indicate that caffeine reduces the body's ability to boost blood flow to the muscle of the heart on demand. The ratio of exercise blood flow to resting blood flow, called the myocardial flow reserve, was 22 percent lower in the group at normal air pressure after ingesting caffeine and 39 percent lower in the group in the high-altitude chamber. Dr. Kaufmann said that caffeine may block certain receptors in the walls of blood vessels, interfering with the normal process by which adenosine signals blood vessels to dilate in response to the demands of physical activity.

"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 researchers wrote.

Although caffeine is a stimulant, these results also indicate that coffee may not necessarily boost 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. It may not be a physical stimulant, and may even adversely affect physical performance," Dr. Kaufmann said. "It may not be as harmless as we thought before, particularly if you suffer from coronary artery disease or if you are in the mountains."

Although the participants were all healthy, Dr. Kaufmann said that the results raise concerns about possible effects of caffeine in people with heart disease. "Any advice would be based on results of healthy volunteers and would be a bit speculative; nevertheless, my advice would be: do not drink coffee before doing physical activities. We hope to be able to provide data soon on the situation of patients with coronary artery disease," he said.

Thomas H. Schindler, M.D. from the David Geffen School of Medicine at UCLA in Los Angeles, California, who was not connected with this study, said that if the results are confirmed, they could have important implications.

"In particular, this may play an important role in patients with obstructive coronary artery disease in the intermediate range between 50 percent and 85 percent narrowing of the epicardial luminal diameter. In this range of coronary artery disease-induced epicardial narrowing, the myocardial flow reserve (MFR) has been widely assumed to compensate for the epicardial narrowing and, thereby, to preserve the myocardial blood flow to the heart. A further reduction of the MFR, for example owing to caffeine intake, therefore could precipitate stress-induced myocardial ischemia, angina pectoris (reflecting an imbalance between myocardial oxygen supply and demand) or could also contribute to the manifestation of acute coronary syndromes. Consequently, as stated by Namdar et al., the current findings indeed raise safety questions in patients with already reduced MFR as seen in coronary artery disease, particularly before physical exercise and at high-altitude exposure," Dr. Schindler said.