Sunday, January 15, 2006

Caffeine limits blood flow during exercise

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, according to a new study.

"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 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 altitude. The high-altitude test was designed to mimic the way coronary artery disease deprives the heart muscle of sufficient oxygen.

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.

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.

"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. "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."

Here's a list of the caffeine content of foods and drugs.

With a study indicating an increase in inflammatory markers, such as c-reative protein, with consumption of two cups of coffee per day, it might be wise to rethink our love affair with this beverage. For it seems that coffee is the number one source of antioxidants in the U.S. diet. That probably means that consumers drink too much and, unfortunately, still do not eat enough fruits and vegetables.

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