Caffeine interferes with blood flow to the heart during exercise, research suggests.

"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," suggests study co-author, Phillip A. Kaufmann, MD, FACC.

Effects of Moderate Caffeine Intake Evaluated

Eighteen young, healthy, regular coffee drinkers were chosen for the study. The participants did not drink any coffee for thirty-six hours prior to testing, and were given caffeine tablets equivalent to two cups of coffee fifty minutes before the test.

In one part of the study, subjects rode a stationary exercise bicycle. In the second part of the study, the exercise was performed in a chamber simulating the thin air of 15,000 feet altitude. The high-altitude test was designed to mimic the way coronary artery disease deprives the heart muscle of sufficient oxygen. A control group remained inactive after taking the caffeine.

PET scans that show heart blood flow were performed on the participants before and immediately after each session.

Significant Effects

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 for the first group, and the effect was almost doubled for 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 heart muscle on demand. Caffeine may block certain receptors in the walls of blood vessels, interfering with the normal process by which vessels dilate in response to the demands of physical activity, suggests Dr. Kaufmann.

Caffeine Unsafe for Some?

The findings may not have a clinical importance in healthy persons, but they raise safety questions for patients with reduced coronary blood flow, particularly for those who live in areas of high elevation, the researchers note.