Why You Should Avoid Coffee Before Exercise

If you’ve ever downed a cup of coffee before hitting the gym, you’re not alone. Many athletes and fitness enthusiasts rely on caffeine for a boost. But new research suggests that having coffee right before exercise might not be as harmless—or helpful—as once believed, especially when it comes to your heart.

The short answer? Avoid drinking coffee immediately before working out. In a controlled study, researchers found that caffeine reduced blood flow to the heart muscle by 22% during exercise at normal altitude, and even more at high altitudes—potentially affecting oxygen supply to your body.

How the Study Was Conducted

Researchers at the Human Physiology Center of Zurich University Hospital in Switzerland, led by Dr. Philipp Kaufmann, tested the effects of caffeine on 18 healthy young adults who regularly consumed coffee. Using high-performance PET scanning technology, they monitored blood flow to the heart both at rest and during treadmill exercise—before and after participants took a caffeine tablet.

What the PET Scans Revealed

The scans showed that while caffeine had no significant impact on myocardial blood flow when the subjects were at rest, it had a measurable effect once they began exercising. Specifically, blood flow to the heart muscle decreased by 22% under normal atmospheric conditions. This reduction was even greater at higher altitudes. The PET imaging allowed scientists to observe these changes in real-time with high precision.

Why Caffeine Restricts Heart Blood Flow During Exercise

Caffeine blocks certain receptors in the walls of blood vessels. Normally, during physical activity, these receptors help trigger vasodilation—the widening of blood vessels—to increase blood supply to working muscles, including the heart. Caffeine interferes with this signaling process, preventing proper blood vessel expansion. As a result, the heart receives less blood and oxygen during exertion, which could impair oxygenation and performance.

The Known Benefits—and New Limits—of Pre-Workout Coffee

It’s well-established that caffeine stimulates the brain, enhances alertness, and can improve physical performance. Many athletes use it strategically to boost endurance and focus. However, this study highlights an important caveat: while caffeine may sharpen your mind and delay fatigue, it may also limit the heart’s ability to get the blood flow it needs during exercise.

Key Takeaways for Coffee Drinkers Who Exercise

• Caffeine reduces myocardial blood flow by up to 22% during normal exercise.
• The effect is more pronounced at high altitudes.
• The reduction is caused by caffeine blocking receptors that help dilate blood vessels.
• This can limit oxygen delivery to your heart and muscles mid-workout.
• Coffee still offers cognitive and performance benefits—but timing matters.

Frequently Asked Questions

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

Caffeine reduces blood flow to the heart muscle by up to 22% during normal exercise at regular altitude, according to a PET scan study. The decrease is even greater at high altitudes, though exact numbers for altitude weren’t specified in the article.

Does caffeine affect heart blood flow when you’re not exercising?

No, the study found that caffeine does not significantly affect blood flow to the heart when you are at rest. The impact is only observed during physical activity, particularly exercise.

What type of caffeine was used in the study?

The study used caffeine in the form of tablets given to participants immediately before they exercised on a treadmill. The exact dosage wasn’t specified, but it was enough to produce measurable effects on blood flow.

Why does caffeine reduce blood flow to the heart during exercise?

Caffeine blocks specific receptors in blood vessel walls that are responsible for signaling vasodilation. This interference prevents blood vessels from widening properly during exercise, reducing the amount of blood—and oxygen—that reaches the heart.

Should I stop drinking coffee altogether before workouts?

You don’t necessarily have to stop, but it’s wise to avoid coffee right before exercising, especially intense workouts. The key is timing: allow enough time for caffeine to metabolize if you want to avoid restricted heart blood flow during exertion.

Recommended FrontStreet Coffees for Post-Workout Recovery

After exercise, try FrontStreet Coffee’s Ethiopia Yirgacheffe for its bright citrus and floral notes, perfect for rehydration. Their Brazil Queen Manor offers a nutty, chocolatey balance that pairs well with recovery nutrition. Both are single origins roasted to highlight clarity and smooth body—ideal for refreshing your palate post-sweat. Freshly roasted within 5 days · Orders placed before 17:00 ship the same day · Next-day delivery across most of Guangdong Province.