Coffee and BloodFlow During Exercise

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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, and the effect was stronger when the

participants were in a chamber simulating high altitude, according to

a new study in the Jan. 17, 2006, issue of the Journal of the

American College of Cardiology.

 

" 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 researchers, including lead author Mehdi Namdar, M.D., F.A.C.C.,

studied 18 young, healthy people who were regular coffee drinkers.

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 (4,500 meters) altitude.

The high-altitude test was designed to mimic the way coronary artery

disease deprives the heart muscle of sufficient oxygen. In both

groups, the testing procedure was repeated 50 minutes after each

participant swallowed a tablet containing 200 milligrams of caffeine,

the equivalent of two cups of coffee.

 

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.

 

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 muscle of the heart on demand. 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. Dr. 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.

 

" 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.

 

Although caffeine is a stimulant, these results also indicate that

coffee may not necessarily boost athletic performance.

 

" 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, " Dr. Kaufmann said. " 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. "

 

Dr. Kaufmann noted that this study was not designed to measure

athletic performance.

 

Although the participants were all healthy, Dr. Kaufmann said that

the results raise concerns about possible effects of caffeine in

people with heart disease.

 

" Any advice would be based on results of healthy volunteers and would

be a bit speculative; nevertheless, my advice would be: do not drink

coffee before doing physical activities. We hope to be able to

provide data soon on the situation of patients with coronary artery

disease, " he said.

 

The researchers noted that other studies of coffee and heart disease

have produced mixed results.

 

Although this study included only 18 participants, the researchers

said that the differences they saw were large enough for them 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.

 

Thomas H. Schindler, M.D. from the David Geffen School of Medicine at

UCLA in Los Angeles, California, who was not connected with this

study, said that if the results are confirmed, they could have

important implications.

 

" In particular, this may play an important role in patients with

obstructive coronary artery disease in the intermediate range between

50 percent and 85 percent narrowing of the epicardial luminal

diameter. In this range of coronary artery disease-induced epicardial

narrowing, the myocardial flow reserve (MFR) has been widely assumed

to compensate for the epicardial narrowing and, thereby, to preserve

the myocardial blood flow to the heart. A further reduction of the

MFR, for example owing to caffeine intake, therefore could

precipitate stress-induced myocardial ischemia, angina pectoris

(reflecting an imbalance between myocardial oxygen supply and demand)

or could also contribute to the manifestation of acute coronary

syndromes. Consequently, as stated by Namdar et al., the current

findings indeed raise safety questions in patients with already

reduced MFR as seen in coronary artery disease, particularly before

physical exercise and at high-altitude exposure, " Dr. Schindler said.

 

Dr. Schindler said that further studies will be needed to answer the

important questions raised by this study.

 

Disclosure Box

Dr. Kaufmann was supported by a grant from the Swiss National Science

Foundation.

 

Amy Murphy

amurphy

American College of Cardiology

www.acc.org