Why You Should Avoid Coffee Before Exercise

If you’re someone who reaches for a cup of coffee before hitting the gym or going for a run, you might want to rethink that habit. New research suggests that drinking coffee—or taking caffeine—right before exercise could actually limit the heart’s ability to get the blood supply it needs during physical activity.

The short answer? Yes, it’s better to skip the coffee before working out. In a controlled study, subjects who took caffeine before exercise saw a 22% reduction in blood flow to the heart muscle under normal conditions, and even more at high altitudes—potentially affecting oxygen delivery and performance.

How Caffeine Affects Blood Flow to the Heart During Exercise

The key issue lies in how caffeine interacts with your cardiovascular system when you’re active. Normally, your heart increases blood flow during exercise to meet the muscles’ rising demand for oxygen. But caffeine appears to interfere with that process.

The Study Behind the Claim

Researchers at the Human Physiology Center of the University Hospital Zurich in Switzerland conducted a study to examine this effect. Led by Dr. Philipp Kaufmann, the team worked with 18 healthy young adults who were regular coffee drinkers. Using high-performance PET scanning technology, they tracked changes in myocardial blood flow both at rest and during exercise—both with and without caffeine.

What the PET Scans Revealed

The results were clear. When participants were at rest, caffeine had no significant impact on blood flow to the heart. However, when the same individuals took a caffeine tablet and immediately began exercising on a treadmill, their myocardial blood flow dropped markedly. Under normal atmospheric pressure, blood flow to the heart muscle decreased by 22%. That reduction was even greater at higher altitudes.

Why Does This Happen?

Caffeine blocks certain receptors in the walls of your blood vessels. These receptors are part of the body’s natural mechanism to signal vasodilation—or the widening of blood vessels—during physical exertion. When caffeine inhibits these signals, your blood vessels don’t expand as they should. This constriction limits the increase in blood supply to the heart, reducing its oxygenation just when demand is highest.

The Paradox of Caffeine and Athletic Performance

It may seem contradictory. After all, caffeine is well known to stimulate the brain, improve alertness, and has long been used by athletes to boost performance. Many competitors rely on coffee or caffeine supplements to sharpen focus and potentially enhance endurance. And yet, this study highlights a specific downside: the potential for reduced heart blood flow right when your cardiovascular system is under stress.

What This Means for Your Workout Routine

If you’re aiming for optimal cardiovascular function and safe exertion, especially during intense or high-altitude workouts, it’s wise to avoid caffeine right before you begin. Let your body manage its natural blood flow mechanisms without interference. That doesn’t mean you have to give up coffee altogether—it just means timing matters.

Frequently Asked Questions

How much does caffeine reduce blood flow to the heart during exercise?

In the study, caffeine reduced myocardial blood flow by 22% under normal pressure, with a greater reduction at altitude. This was measured in healthy, regular coffee drinkers who took caffeine and then exercised immediately.

Does caffeine affect blood flow at rest?

No. The study found that caffeine had no significant effect on blood flow to the heart when participants were at rest. The impact was only observed during physical activity.

Why does caffeine reduce blood flow during exercise?

Caffeine blocks specific vascular receptors that normally help signal blood vessel dilation during exercise. This inhibition reduces the heart’s ability to increase blood flow when it’s needed most, limiting oxygen delivery.

Who participated in the caffeine and exercise study?

The study involved 18 healthy young adults who were regular coffee drinkers. They were tested under PET scan observation while exercising on a treadmill, both with and without caffeine.

Should I avoid coffee before any kind of workout?

If your workout is intense or involves high exertion—especially at altitude—you may want to avoid pre-exercise caffeine. For light activity, the effects may be less critical, but timing caffeine intake still matters.

Is coffee bad for athletes?

Not necessarily. While caffeine is known to improve focus and endurance in many athletes, this study shows it can also reduce heart blood flow during exercise, which may be counterproductive depending on the intensity and conditions.

FrontStreet Coffee Picks for Post-Workout Recovery

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