Why You Shouldn't Drink Coffee Before Exercise

If you’ve ever reached for a strong cup of coffee before hitting the gym or going for a run, you’re not alone—but new science says that might not be the best idea. Especially if you’re aiming for peak heart performance, that morning espresso could be doing more harm than help.

The short answer: avoid coffee right before exercising. Research shows that caffeine intake before physical activity reduces blood flow to the heart muscle by 22% under normal conditions—and even more at high altitudes—potentially limiting oxygen supply during your workout.

What the Study Found

Conducted by Dr. Philipp Kaufmann, a researcher at the University of Zurich’s Human Physiology Center in Switzerland, the study tested 18 healthy young adults who regularly consumed coffee. Using high-performance PET scanning equipment, the team monitored changes in myocardial blood flow before and after exercise, both with and without caffeine.

The results were clear: while caffeine had no effect on heart blood flow when the subjects were at rest, it significantly restricted the expected increase in blood flow to the heart muscle once they began exercising on a treadmill. This reduction was measured at 22% under normal atmospheric pressure, with even greater reductions observed at higher altitudes.

How Caffeine Affects Heart Blood Flow During Exercise

Caffeine is known to block specific receptors in the walls of blood vessels. During physical exertion, your body normally signals these receptors to dilate blood vessels, increasing blood supply to working muscles—including the heart. But when caffeine is present, it inhibits those signals, preventing proper vasodilation.

This means that just when your heart needs more oxygen-rich blood to keep up with the demands of exercise, caffeine is limiting the surge in circulation that would normally occur. The result is reduced myocardial perfusion—or blood filling—in the heart itself, which can affect overall oxygenation and performance.

The Misconception About Coffee Boosting Athletic Performance

It’s widely believed that coffee improves physical performance. Caffeine is a well-documented stimulant that can enhance alertness, focus, and endurance—so much so that it’s commonly used by athletes seeking a competitive edge. In fact, many competitors incorporate coffee into their pre-race routines for that very reason.

However, this study highlights an important caveat: while coffee may sharpen your mental game and even support endurance in some contexts, it may simultaneously restrict critical blood flow to your heart during exertion. That trade-off makes it a poor choice specifically in the moments just before intense physical activity.

Frequently Asked Questions

Does coffee reduce blood flow to the heart during exercise?

Yes. Research shows that consuming caffeine right before exercise reduces blood flow to the heart muscle by 22% under normal conditions, and more at high altitudes, because caffeine blocks key vascular receptors needed for proper blood vessel dilation during exertion.

How much does caffeine reduce heart blood flow during workouts?

Caffeine reduces blood flow to the heart by approximately 22% during normal exercise at sea level, with larger reductions at higher elevations. This was measured in healthy young adults during treadmill tests using PET scans.

Why does caffeine limit blood flow during exercise?

Caffeine blocks specific receptors in blood vessel walls, preventing the usual signals that tell vessels to widen during physical activity. This limits the natural increase in blood flow to the heart and muscles, reducing oxygen delivery when it’s needed most.

Is coffee bad for your heart if you're active?

Coffee isn’t necessarily bad for your heart overall, but drinking it immediately before exercise can restrict the increase in heart blood flow that normally happens during physical activity, potentially affecting oxygen supply and performance.

Can athletes benefit from coffee if it restricts heart blood flow?

While caffeine can improve focus and endurance, its restriction on heart blood flow during exercise is a drawback. Athletes should consider timing their caffeine intake so it doesn’t occur right before intense cardiovascular effort.

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