Zone 2 training
Easy, steady cardio clearly improves fitness and health, but the popular claim that "Zone 2" is a special, optimal intensity for building mitochondria and burning fat is not supported. A 2025 review concluded the evidence does not show Zone 2 is optimal and that higher intensities matter, especially when time is limited. Do easy cardio because it is easy to recover from, not because it is magic.
What the science says
- Zone 2 is the optimal intensity for mitochondria and fat burning Limited evidence
A 2025 narrative review in Sports Medicine (Storoschuk et al.) found the case for Zone 2 rests largely on observations of elite endurance athletes and concluded that current evidence does not support Zone 2 as the optimal intensity for improving mitochondrial or fat-oxidation capacity.
- Moderate continuous training does improve fitness Strong evidence
Generic continuous endurance training produced large VO2max gains versus no exercise in a 28-trial meta-analysis (Milanović et al., 2015), but interval training produced slightly larger gains. Steady cardio works; it is just not uniquely effective.
- Fat loss is similar to intervals Strong evidence
In overweight adults, moderate continuous training reduced fat mass about as much as HIIT (Wewege et al., 2017), though it took about 40% more training time.
- With little time, intensity matters more Moderate evidence
Storoschuk et al. (2025) concluded that prioritizing intensities above Zone 2 is important for cardiometabolic benefits, particularly at lower training volumes.
How to apply it
Make most of your cardio easy and conversational, and add some harder work if fitness is a goal.
Who should be careful
Easy cardio is low-risk for healthy adults. If you have a heart condition or other medical issue, ask a health professional which intensities are safe for you; heart-rate zones from formulas are estimates and can be off for many people.
Frequently asked questions
Is Zone 2 the best intensity for burning fat?
No. A 2025 review concluded the evidence does not support Zone 2 as the optimal intensity for fat-oxidation or mitochondrial capacity.
Is easy cardio a waste of time, then?
No. Continuous endurance training produces large VO2max gains and similar fat loss to HIIT; it is effective, just not uniquely so.
If I only have a little time, what should I do?
Include harder intervals. The 2025 review concluded higher intensities matter most for health benefits when training volume is low.
Scientific references
- Much Ado About Zone 2: A Narrative Review Assessing the Efficacy of Zone 2 Training for Improving Mitochondrial Capacity and Cardiorespiratory Fitness in the General Population
- Effectiveness of High-Intensity Interval Training (HIT) and Continuous Endurance Training for VO2max Improvements: A Systematic Review and Meta-Analysis of Controlled Trials
- The effects of high-intensity interval training vs. moderate-intensity continuous training on body composition in overweight and obese adults: a systematic review and meta-analysis
More training methods
- Drop sets Moderate evidence
A drop set takes a set to failure, then immediately cuts the load and keeps going.
- Rest-pause sets (including myo-reps) Limited evidence
Rest-pause means taking a set close to failure, resting 15–30 seconds, then squeezing out a few more reps, repeated a few times.
- Supersets Moderate evidence
A superset pairs two exercises back to back with little or no rest.
- Fasted training (fasted cardio) Limited evidence
Training on an empty stomach burns more fat during the session, but that does not translate into more fat lost over weeks.
- Static stretching before training Limited evidence
Holding long static stretches right before lifting or sprinting slightly lowers strength and power, and it has not been shown to prevent injuries.
- Periodization Moderate evidence
Periodization means planning how load and volume change over weeks instead of repeating the same workout.