Blood flow to the heart has to increase during exercise in order to match the increased need of oxygen. But when 18 healthy people were given the equivalent of two cups of coffee, scientists found that blood flow increase during exercise was much lower than when they exercised without having consumed coffee.
This effect was even stronger when the participants were in a chamber simulating high altitude, said the scientists at the University Hospital in Zurich.
They say that although caffeine drinks are known to stimulate the brain, their results show that caffeine is unlikely to boost athletic performance.
This counters previous research suggesting that caffeine-based drinks may help sportspeople. A UK team previously reported that introducing caffeine into sports drinks increased the rate in which carbohydrate is delivered to the athlete. Many professional sportspeople already take caffeine tablets to boost their performance.
The Swiss researchers said however that their research could also "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".
Reported in the 17 January issue of the Journal of the American College of Cardiology (vol 47, pp405-410), the research investigated the effects on young people who were regular coffee drinkers but they did not drink any coffee for 36 hours prior to the tests.
PET scans were used to show blood flow in the hearts of 10 participants before and immediately after they rode a stationary exercise bicycle. Then they were given 200mg of caffeine in a tablet, and the exercise test was repeated 50 minutes later.
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 when the participants had taken caffeine tablets.
The ratio of exercise blood flow to resting blood flow, called the myocardial flow reserve, was 22 per cent lower in the group at normal air pressure after ingesting caffeine.
This was even lower in a second group who exercised in a chamber simulating the air at about 15,000 feet (4,500 meters) altitude. (39 per cent lower).
Author Dr Philipp 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.
"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," he said.
Dr Kaufmann added that the study was not designed to measure athletic performance but even though it was small, the differences were large enough for the team to be confident that the effect of caffeine on heart muscle blood flow is real.
They pointed out that longer studies of people with heart disease will be needed in order to understand whether the blood flow effects have important health consequences.