Zurich, Switzerland - Two cups of coffee before that run might make it easier to get out the door, but the caffeine in everybody's favorite pick-me-up does not appear to boost athletic performance, say researchers. According to the results of a new study, the equivalent of two cups of coffee in healthy volunteers significantly decreases exercise-induced myocardial flow reserve, with the effect even more pronounced at altitude.

"Many people take caffeine in foods and beverages before participating in sports," lead investigator Dr Philipp Kaufmann (University of Zurich, Switzerland) told heartwire. "It's a stimulant, and some people feel that it gives them more energy. People feel more awake, more alert, and able to concentrate better, at least for a little while, but apparently it is an energizer only in a subjective sense. While some people might even feel fitter perhaps, it's unlikely that physical fitness would improve, because blood flow actually decreases with caffeine."

Published in the January 17, 2005 issue of the Journal of the American College Cardiology, the study involved 18 healthy volunteers who were regular coffee drinkers and was designed to test the acute effect of caffeine on myocardial blood flow and the resulting myocardial flow reserve, at rest and exercise, and at normal levels of oxygen and at altitude.

Speaking with heartwire, Kaufmann said the influence of caffeine on myocardial blood flow during exercise is largely unknown. Coffee has been shown to attenuate the coronary hyperemic response by competitive blockade of the adenosine A2 receptor, and caffeine is commonly withheld before adenosine SPECT perfusion imaging so as not to mask ischemia detection. Adenosine, which promotes vasodilation, is released during physical activity in response to the extra demands on the heart, but this mechanism could be susceptible to adenosine-receptor antagonism by caffeine, said Kaufmann.

In this study, which was led by Dr Mehdi Namdar (University Hospital, Zurich, Switzerland), the investigators measured myocardial blood flow by PET scans in 10 subjects before and immediately after they rode a stationary exercise bicycle. In another eight subjects, the same measurements were taken after exercising in an environment designed to simulate altitude (approximately 4500 m) and to mimic the way coronary artery disease deprives the heart of sufficient oxygen. All subjects were then given 200 mg of caffeine, the equivalent of two cups of coffee, in the form of a commercially available tablet. Fifty minutes later, the time of expected peak serum caffeine levels, the study protocol was repeated, with blood flow measured by emission scans before and immediately after exercise.

Researchers found that caffeine significantly decreased exercise-induced hyperemic myocardial blood flow by 13%, resulting in a decrease of myocardial flow reserve of 22%. This effect was significantly more pronounced at high-altitude exposure, where myocardial flow reserve measured immediately after exercise was down 39% from baseline.

"I was surprised by the amount of decrease, because it is a large decrease, but we all know from daily life that it does not seem to affect our quality of life," said Kaufmann. "Everybody drinks coffee, and we're running for trains and buses and nothing happens. In these healthy people, there seems to be a lot of flow reserve and if there is a little bit less it doesn't really matter a lot. The concern is that in subjects already with a decreased flow reserve, if it would get even lower with caffeine, then that might put them in a riskier situation."

With no believed detrimental effect of caffeine in this group of healthy volunteers, Kaufmann said next steps involve studying patients with already compromised blood flow and myocardial reserve, such as patients with coronary artery disease. In the meantime, patients with coronary disease should be careful about drinking coffee before physical activity, especially at altitude, he said.