Researchers discover caffeine limits blood flow to the heart during physical activity

• In healthy volunteers, the equivalent of two cups of coffee reduced the body's ability to boost blood flow to the heart muscle in response to exercise, and the effect was stronger when the participants were in a chamber simulating high altitude, according to a new study in the Jan. 17, 2006, issue of the Journal of the American College of Cardiology.
• "Whenever we do a physical exercise, myocardial blood flow has to increase in order to match the increased need of oxygen.
• 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 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.