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The LetterAbout NOV 2026
Movement · 9 min read · November 5, 2026

Zone 2 vs Zone 3 vs HIIT: What a 2025 Review Found

Is zone 2 training overrated? What a 2025 review says about zone 2 vs higher intensities, zone 2 vs zone 3, and how to balance easy and hard sessions.

Rooted Glow Editorial
By ROOTED GLOW EDITORIAL
A person cycling along a straight country road

Photo: Mehmet Turgut Kirkgoz / Pexels. Contextual stock photograph, not a product test or endorsement. License.

Where the Zone 2 Hype Came From

Zone 2 means low-intensity aerobic work, usually described as effort below the lactate threshold. You can talk in full sentences. Recreational runners, cyclists and podcast guests have treated it as the “best” zone for building mitochondria and burning fat.

A lot of that story comes from watching elite endurance athletes. Pros often log large volumes of easy training. That is observational. It describes what highly trained people do when they can train many hours a week. It does not prove the same recipe is optimal for someone who trains a few hours a week, or that zone 2 alone is best for mitochondrial capacity.

Podcast culture amplified the claim. Easy miles feel safe to recommend. The science is more mixed once you look past elite training diaries. For zone definitions, see heart rate zones explained and our zone 2 training overview. This page stays on what the research says, not how to set your watch.

What the 2025 Review Found

In July 2025, Storoschuk, Moran-MacDonald, Gibala and Gurd published “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” in Sports Medicine (DOI 10.1007/s40279-025-02261-y, PMID 40560504).

It is a narrative review, not a new trial. The authors summarize existing evidence and challenge a popular claim.

Their main points, from the abstract:

  • Popular media positioned zone 2 (low intensity below the lactate threshold) as optimal for mitochondrial and fatty acid oxidative capacity.
  • Those recommendations largely stem from observational data on elite endurance athletes who do large zone 2 volumes.
  • Broad endorsement for the general public contradicts substantial evidence that supports high-intensity exercise for mitochondrial capacity and cardiometabolic health.
  • Current evidence does not support zone 2 as the optimal intensity for improving mitochondrial or fatty acid oxidative capacity.
  • Evidence suggests prioritizing higher intensities (above zone 2) is critical to maximize cardiometabolic health benefits, particularly at lower training volumes.

In plain terms: easy work still has a place, especially for adding volume you can recover from. The review argues that calling zone 2 “optimal” for mitochondria and fat oxidation goes beyond what the evidence shows for most people.

A secondary discussion of the same debate is at Physiologically Speaking. Treat it as context, not primary evidence.

Zone 2 vs Zone 3: the “Grey Zone” Debate

Coaches often warn runners away from zone 3, the so-called grey zone: harder than easy, easier than true hard work. The fear is that you get stuck in medium effort that is too hard to recover from quickly and too easy to drive big fitness gains.

The Storoschuk review pushes back on the idea that the body flips a switch at neat zone lines. Physiology is continuous. Labels such as “zone 2” and “zone 3” are useful coaching tools. They are not sharp biological borders.

For zone 2 vs zone 3, the same logic applies. If training time is limited, spending almost everything just below threshold may not raise mitochondrial capacity or cardiorespiratory fitness as well as a mix that includes harder work. That does not mean every session should be hard. It means the “never touch zone 3” rule is weaker than social media often claims.

If you want practical zone ranges and talk-test cues, use zone 2 cardio for beginners and heart rate zones explained. This article does not prescribe heart-rate formulas.

Zone 2 vs HIIT: Time-Efficiency vs Recoverability

High-intensity interval training (HIIT) and sprint interval training (SIT) ask more of you per minute. Easy zone 2 asks less per minute and is easier to stack across a week.

A few studies help frame the tradeoff:

  • Inglis et al., 2024 (Medicine & Science in Sports & Exercise, PMID 38376995). Eighty-four young healthy adults trained for 6 weeks in work-matched groups (except SIT). Heavy, severe and extreme intensities raised VO2max and thresholds. Moderate intensity (zone-2-like) did not significantly increase VO2max compared with control. HIIT produced the largest VO2max increase. The study did not measure mitochondria directly.
  • Granata et al., 2018 (Sports Medicine, PMID 29934848). Training volume mattered for mitochondrial content. Relative intensity mattered for mitochondrial respiratory function. HIIT at or above about 90% of maximal power produced the greatest absolute increase in mass-specific mitochondrial respiration. SIT was the most efficient per unit of time.
  • Mølmen et al., 2025 (Sports Medicine, PMID 39390310). In a meta-regression, after covariates, endurance training, HIT and SIT produced similar percentage gains in mitochondrial content. Per hour of training, SIT was about 2.3× more efficient than HIT and about 3.9× more efficient than endurance training.

Taken with Storoschuk 2025, the pattern is clear enough for everyday choices. Higher intensities tend to deliver more fitness per minute. Easy volume still helps you accumulate work you can recover from and keep joints happier across a season.

Zone 2 often fits better when you are tired, new to running or building consistency. HIIT often fits better when time is short and you can recover between hard days. Neither cancels the other out.

Zone 2: Useful, Not Optimal

For the claim that zone 2 is the best intensity for mitochondria and fat oxidation in the general population: yes, that claim looks overrated next to Storoschuk 2025.

For zone 2 as a useful, recoverable intensity: no. Easy aerobic work still earns its place.

Beginners and deconditioned adults. Easy sessions help you show up more days than you skip. Walking and slow jogging build the habit without stacking fatigue. Our how to start running guide and couch to 5K plan stay in that lane. Comparing walking vs running can help you pick a starting point.

Trained people with limited hours. If you only have a few sessions a week, the review and the secondary studies above argue that some higher-intensity work matters more than filling every slot with pure zone 2.

People with lots of available volume. Elites can afford huge easy mileage because hard sessions still happen and recovery capacity is high. Copying only the easy percentage without the hard work is a common recreational mistake.

If you have heart disease, are returning from injury or feel chest pain, dizziness or unusual shortness of breath with exercise, see a doctor before you change training intensity.

A Simple Weekly Mix

Keep this high-level. No heart-rate formulas and no medical prescriptions live here.

A balanced week for many recreational runners looks like mostly easy movement, plus one or two harder efforts when you feel recovered. Use easy days to add time on feet. Use harder days for short efforts above your all-day pace. Protect at least one lighter day.

For planning tools and deeper how-tos, use:

Progress by consistency first. Add intensity only when easy days still feel easy.

Frequently Asked Questions

Is zone 2 training overrated?

The claim that zone 2 is optimal for mitochondrial and fat-oxidation gains in the general population is overrated relative to current evidence. Storoschuk et al. (2025) say the evidence does not support that claim. Easy zone 2 remains useful for volume and recovery.

Is zone 2 or zone 3 better for fitness?

There is no single winner for every goal. Zone lines are coaching labels, not hard biological switches. If training time is limited, some work above zone 2 appears important for cardiorespiratory and mitochondrial adaptations. Zone 3 is not automatically “junk” mileage.

Is HIIT better than zone 2?

HIIT and related high-intensity work often produce larger fitness gains per minute. Zone 2 is easier to recover from and easier to accumulate. “Better” depends on your time, recovery and experience. Many solid weeks use both.

What did the 2025 “Much Ado About Zone 2” review conclude?

Storoschuk and colleagues concluded that current evidence does not support zone 2 as the optimal intensity for improving mitochondrial or fatty acid oxidative capacity. They argue that prioritizing higher intensities is critical for cardiometabolic benefits, especially at lower training volumes. The paper is a narrative review, not a new experiment.

Should beginners still do zone 2?

Yes, as a practical starting intensity. Easy work helps beginners stay consistent. That does not require the claim that zone 2 is uniquely best for mitochondria.

How much of my training should be easy?

Elite plans often keep most volume easy, with a smaller share hard. Recreational athletes with less total time usually need more quality work than a pro’s calendar suggests. Default to easy days, and keep hard days scarce enough to recover. For a week layout, see the weekly movement planning worksheet.

Sources

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zone 2zone 3HIITmitochondrialtrainingrunning
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