Study: Caffeine limits myocardial blood flow during exercise

As reported on Science Daily, researchers in Switzerland have found that the equivalent of two cups of coffee blunts the increase in blood flow to the heart muscle itself that occurs with exercise, and that the effect is increased at conditions simulating high altitude. These findings (free full text) were part of a study published over three years ago in the Journal of the American College of Cardiology. I have no idea why mention of it appeared in one of my news feeds just yesterday, but it’s interesting enough to warrant comment.

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.

On the surface, this comes as a surprise to those of us who routinely load up on coffee before endurance training and competitions. In fact, I can’t name on personal-best time from the mile to the 50K that did not involve some amount of caffeine ingestion beforehand.

I can, however, make a reasonable hypothesis about these results. Coronary arterial blood flow, unless severely compromised by stenosis or spasm, would not be a limiting factor in exercise, especially in untrained persons like the ones in this study. In people free of airway obstruction and anemia, the limiting factor in exercise is likely to be end-organ utilization; that is, people’s muscles are not sufficiently dense in mitochondria to process all of the oxygen delivered to them. Aerobic training increases both mitochondrial density in working muscles and the enzyme activity associated with aerobic metabolism, but either this or oxygen delivery to active tissues generally remain the limiting factor in performance.

Ventilation even at fast (aerobic) running speeds supplies the body with more than the full amount of oxygen it can out to use, at least at sea level; this oxygen very quickly diffuses across the alveolar membranes in the lungs and saturates available red-blood cells. Athletes can remove the limitations imposed at this point in the chain by taking banned “blood boosters” such as erythropoetin (EPO) and its synthetic derivatives; when highly trained people do this, the limiting factor in their exercise output is generally regarded as “substrate failure”–again, a concern within the working muscles themselves.

Throughout all of this, the heart is pumping perhaps up to 10 percent of its own output to itself in order to sustain near-maximal aerobic effort. Simply put, coronary artery blood flow would have to decline significantly in order for this to become limiting, and this is in trained people. Healthy normals like the 18 subjects in the study would never notice the difference.
Anyway, you’ll pry my coffee out of my cold, dead fingers long before you’ll see me give it up for fear of negative cardiac consequences, with or without competitive running in the mix.

3 thoughts on “Study: Caffeine limits myocardial blood flow during exercise”

  1. did you see the article on the nytimes site, about how caffeine increases performances?

  2. One of the problems I have with this study is that there is a good chance that this isn’t even applicable to everyday life. The 36 hours of no caffeine is probably a good way to get a feel for what caffeine can do to you when all traces have cleared from your body, but this is hardly the way people use caffeine in the real world.
    I understand this was done to prevent the persons normal use from influencing the outcome. However, it produces results that, although real, are probably not even a factor for anyone. Seriously, how many people go cold turkey, and then shock their system with 2 cups of coffee?

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