Much Ado About Zone 2 Training

Everywhere I turn, I’m seeing the recommendation to do “zone 2” training. If you aren’t familiar, Zone 2 is low-intensity cardio where you can still maintain a conversation, but it’s slightly challenging—roughly 60-70% of your max heart rate. Think of it as a brisk walk.

Where did the enthusiasm for zone 2 training come from?

First, Zone 2 is accessible. Brisk walking feels a lot more sustainable than a high-intensity Peloton, Soul Cycle, or Orange Theory workout.

Second, influential voices have moved the needle. Dr. Peter Attia hosted Inigo San-Millán, an exercise physiologist who trains elite endurance athletes like professional cyclists. These athletes spend 80% of their time in Zone 2 and possess some of the highest VO2 max numbers on the planet. The logic seems sound: If it helps them achieve excellence, shouldn’t we all be doing it?

What does the evidence show?

In June 2025, Storoschuk et al. published a review in the journal Sports Medicine that evaluated the evidence for zone 2 training. The authors point out that recommendations for Zone 2 training as optimal are based largely on observational data from elite endurance athletes. Elite athletes train 15–30+ hours per week. For them, Zone 2 is a necessity to build volume without burning out. But for the rest of us—who might have 3 to 5 hours a week for exercise—focusing on Zone 2 may not be the best use of limited time.

The authors conclude:

Our review failed to uncover substantial evidence supporting claims that zone 2 is superior to higher exercise intensities for improving mitochondrial and fat oxidative capacity, a result possibly driven by the lack of studies explicitly examining zone 2 training as it is commonly characterized.

It wasn’t that they found evidence against Zone 2; rather, they found an absence of evidence for its "magic" status, especially when compared to the mountain of high-quality data supporting High-Intensity Interval Training (HIIT).

The Power of Intensity

If we look at the clinical data, the results for high intensity are staggering. Edwards et al published a systematic review analyzing 97 randomized controlled clinical trials (RCT’s) with 3,000+ participants and found that HIIT significantly improved 14 clinically relevant cardiometabolic parameters compared to non-exercise controls:

Cardiovascular function: Peak VO₂ ⬆️, left ventricular ejection fraction ⬆️,Stroke volume ⬆️, Resting heart rate ⬇️

Blood pressure: Systolic BP ⬇️ , Diastolic BP ⬇️

Body composition: BMI ⬇️ , Waist circumference ⬇️, Body fat percentage ⬇️

Metabolic markers: Fasting insulin ⬇️ , triglycerides ⬇️ , HDL cholesterol ⬆️, LDL cholesterol ⬇️ high-sensitivity CRP ⬇️

HIIT vs. moderate intensity continuous exercise

While we lack direct “head-to-head” RCTs between Zone 2 and HIIT, we have plenty comparing HIIT to Moderate-Intensity Continuous Exercise (MICT) which is similar to Zone 2 but probably a little more intense. HIIT demonstrates clear superiority for cardiorespiratory fitness. When directly compared to MICT, HIIT produces significantly greater improvements in peak VO₂. For most other cardiometabolic risk factors, HIIT and MICT produce comparable effects when matched for total energy expenditure. However, HIIT achieves these benefits with substantially less time. In fact, one review found that just 15 minutes per session of HIIT provided comparable benefits.

My take home message

If you are currently inactive, zone 2 is a fantastic place to start. It is sustainable, has a low injury risk, and the adherence rate is high. It isn’t “magical” because of its heart rate zone; it’s helpful because you will actually do it.

If you are already active and looking to optimize your health, higher intensity is better. HIIT can get you better results in less time. The current evidence does not support Zone 2 as the “optimal” intensity for mitochondrial or cardiovascular health when training volume is limited.

If you only have 3 hours a week, don’t spend them all walking briskly. Pushing your heart rate higher—even for just 15 minutes a few times a week—will likely yield significantly better results for your heart, your waistline, and your insulin levels.