Coffee Limits Blood Flow to 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," 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, including lead author Mehdi Namdar, M.D., F.A.C.C., studied only 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.

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.

Although this study included only 18 participants, the researchers said that the differences they saw were large enough for them to be confident that the effect of caffeine on heart muscle blood flow is real.

Although the participants were all healthy, Dr. Kaufmann said that the results raise concerns about possible effects of caffeine in people with heart disease.

The researchers noted that other studies of coffee and heart disease have produced mixed results.

The study is in the Jan. 17, 2006, issue of the Journal of the American College of Cardiology.

Dr. Schindler said that further studies will be needed to answer the important questions raised by this study.