If you're a caffeine addict, chances are your blood flow isn't what it could be.

A new Swiss study suggests that the equivalent of two cups of coffee reduces the body's ability to boost blood flow to the heart muscle during exercise.

The small study, which included 18 young, healthy volunteers, is published in Tuesday's issue of the Journal of the American College of Cardiology.

Caffeine didn't affect blood flow within the heart muscle while the participants were at rest. But blood flow measurements taken immediately after exercise were significantly lower after the participants had taken caffeine tablets.

Although caffeine is a stimulant, these results suggest that coffee may not necessarily boost athletic performance.

"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," said Dr. Philipp Kaufmann, a researcher from the University Hospital Zurich. "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."

Kaufmann also said the results raise concerns about possible effects of caffeine in people with heart disease.

• Study Abstract