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This reduction of blood flow will not affect healthy, fit people, but for people with coronary artery disease it can be harmful.

The study was run by researchers at the University Hospital in Zurich, Switzerland, under the watchful eye of study leader Dr Mehdi Namdar. The findings were presented at the annual meeting of the American College of Cardiology in Chicago.

If the team's results are comparable, people with coronary artery disease may soon be warned not to consume caffeine before exercise.

The altitude test
In the relatively small 16-participant study, researchers measured the effects of caffeine on the healthy volunteers during exercise and rest.

The participants had to ride an exercise bike for five minutes in a lab at sea level, increasing their speed every minute. Then, in order to simulate being at an altitude of 4 500 metres – the height of some mountains in the Alps and the Rockies – they had to inhale a low oxygen mixture and then ride the bike again. Each time their coronary blood flow was measured.

Then the participants were given 200 milligrams of caffeine, the same amount that is found in two cups of coffee. After 50 minutes, they had to remount and cycle at both altitudes again.

Caffeine reduced blood flow
Namdar's team found that when the volunteers were at rest, the caffeine actually increased blood flow through the heart. This was in comparison to when they had no caffeine in their bloodstream. But when they started to exercise, the caffeine reduced the blood flow.

During stressful exertions, such as exercise, the coronary flow reserve – an additional amount of blood – was reduced by 36 percent during the sea level caffeinated exercise and 56 percent during the simulated high altitude exercise (both were measured in comparison to the caffeine-free workout).

Healthy people can have their daily cuppa
Healthy people will not be affected by the caffeine-induced reduction, because their blood vessels naturally step up blood flow during exercise. But people with heart disease have damaged blood vessels that may not compensate for the reduced blood flow during exercise and this could be dangerous for them.

The people that will be especially affected by caffeine before exercise are those with heart disease, because their arteries have narrowed and therefore they have limited coronary flow reserve.

How long does a caffeine-rush last?
Namdar's team found that consuming caffeine at high altitudes would compromise the heart's ability to respond to stresses like physical exertion and limited oxygen supply. This forces the heart to work harder.

No one is sure how long the effect of caffeine lingers on, but past research shows that the effect of caffeine peaks 30 minutes after ingestion and lasts for about 10 hours, at low but detectable levels.

Namdar reiterated that healthy people seem to have enough coronary flow reserve to combat the effect of caffeine on the blood flow. But those with heart disease do not.

Re-testing their hypothesis
The research team is busy running another study to test their hypothesis on volunteers with coronary artery disease.

The altitude being simulated in their new study is similar to that experienced by people in an airplane. - (Health24)

Read more:
Exercise and heart disease
Exercise: no pain, no gain
 

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