Is Zone 2 Training Overrated? What the Latest Research Actually Says

A 2025 narrative review hit PubMed with the title "Much Ado About Zone 2" and the running internet lost its collective mind. The paper argued that Zone 2 training does not actually increase mitochondrial capacity the way the endurance community has been claiming. Within days, the Reddit thread had 929 upvotes and 659 comments from runners, coaches, and physicians picking the study apart.

Some people read the headline and concluded that easy running is a waste of time. Others pointed out that the study examined a very narrow question and missed the actual reasons Zone 2 matters. Both sides had a point. But only one side was reading the full paper.

Here is what the research actually found, what it did not examine, and what it means for your training.

What the Study Actually Found

The 2025 review (PubMed ID: 40560504) specifically examined two claims about Zone 2 training: that it is optimal for mitochondrial biogenesis and that it is optimal for fat oxidation capacity. These are specific physiological adaptations, not a general assessment of whether easy running is useful.

The key findings were pointed:

• Mitochondrial capacity: The evidence argues against Zone 2 being uniquely effective at building mitochondrial density. Higher intensities appear to stimulate mitochondrial adaptations at least as well, and possibly better.
• Fat oxidation: Zone 2 does appear to improve fat oxidation capacity in untrained populations. However, higher exercise intensities may be more effective in untrained individuals and potentially required in trained athletes who have already adapted to low-intensity work.

This is a meaningful finding. But the scope matters enormously. The paper was a narrative review - not a systematic review or meta-analysis - focused on two specific cellular adaptations. It was not asking "is Zone 2 training useful?" It was asking "is Zone 2 uniquely magical for mitochondria and fat burning?"

The answer appears to be: probably not uniquely magical. But that is a very different claim than "Zone 2 is overrated."

What the Study Did NOT Examine

This is where the Reddit discussion got interesting. A physician posted a detailed rebuttal that earned over 700 upvotes, taking issue with the narrative review format and pointing out everything the paper left on the table.

Zone 2 training produces a wide range of adaptations that this study did not address:

• Cardiac adaptations. Stroke volume increases, cardiac output improves, and the heart becomes more efficient at pumping blood. These are central cardiovascular adaptations that happen primarily through volume accumulation at moderate intensities.
• Capillary development. More capillaries around muscle fibers means better oxygen delivery and waste removal. This capillarization responds strongly to sustained aerobic work.
• Musculoskeletal resilience. Tendons, ligaments, and bones adapt to load over time. Easy running builds this structural capacity without the tissue damage that comes from high-intensity work.
• Nervous system recovery. The autonomic nervous system needs low-stress training to balance the sympathetic load from hard sessions. Your HRV status reflects this balance directly.
• Injury prevention. This was the elephant in the room. One of the most upvoted responses - nearly 1,900 upvotes - cut straight to it: the main benefit of Zone 2 is that you can train longer and more often without a huge risk of injury.

The study asked whether Zone 2 is the best stimulus for mitochondrial biogenesis. It did not ask whether Zone 2 is the best way to structure a training program that keeps you healthy, consistent, and improving over months and years. Those are fundamentally different questions.

Why Zone 2 Actually Matters: Volume and Recovery

The physician's rebuttal made a point that resonated with hundreds of experienced runners: we already have proof of how people get better at running. The Kenyans are not following a high-intensity-only plan. Elite marathon programs are built on enormous volumes of easy running with targeted hard sessions layered on top.

Zone 2's real value has never been about mitochondrial magic. It is about two things:

First, volume accumulation. Training volume - total weekly time or distance - is the single strongest predictor of endurance improvement across nearly every study that has examined it. But you cannot accumulate high volume at high intensity without breaking down. Zone 2 is the intensity that lets you stack 5, 6, or 7 sessions per week without your body falling apart. The mitochondria are a side effect. The volume is the point.

Second, recovery compatibility. Hard sessions - intervals, tempo runs, race-pace work - are where acute fitness breakthroughs happen. But those sessions only work if you can recover between them. Easy Zone 2 runs generate a training stimulus while keeping systemic stress low enough that your body can actually absorb the hard work. If your Training Readiness is consistently tanking, you are probably not running easy enough on your easy days.

The study's finding that higher intensity may be better for mitochondrial adaptations is probably correct at the cellular level. But no one runs intervals six days a week. The body cannot handle it. Zone 2 exists to fill the space between hard sessions with productive work that does not compromise recovery.

The Garmin Zone Problem: Your Zone 2 Might Be Wrong

One of the most popular responses in the thread - over 2,000 upvotes - was a runner joking that they cannot even get into Zone 2. Their easy pace puts them in Zone 3, and staying in Zone 2 would require walking.

This is not a fitness problem. It is a calibration problem.

Garmin's default heart rate zones use the 220-minus-age formula, which is wildly inaccurate for many people. If your max heart rate is significantly higher than the formula predicts (common), or if you have a low resting heart rate (common in trained runners), your default zones will be compressed and shifted. What Garmin labels "Zone 3" on your watch might physiologically be Zone 2 if your zones were set correctly.

We wrote an entire guide on this: How to Set Up Garmin Heart Rate Zones Correctly. The short version is that switching from %MaxHR to %HRR (the Karvonen method) with your actual max heart rate and resting heart rate fixes the problem for most people. For a trained runner with a resting heart rate of 50, the difference between methods can shift zone boundaries by 15-20 bpm.

If you have been struggling to stay in Zone 2, or if Zone 2 pace feels absurdly slow, the fix might not be "run slower." The fix might be "recalibrate your zones."

This also has downstream effects on everything else your Garmin calculates. Training Effect scores, training load distribution, and Training Status all depend on accurate zone data. Wrong zones in, wrong analysis out.

Who Actually Needs Zone 2 Training

The Reddit thread revealed something important: Zone 2 does not matter equally to everyone. Its value depends entirely on how much you train.

Beginners Running 2-3 Times Per Week

If you are new to running and running three times a week, obsessing over Zone 2 is premature. At low weekly volume, the distinction between Zone 2 and Zone 3 barely matters. You are not running enough for recovery between sessions to be a limiting factor. Just run at a comfortable effort - easy enough to hold a conversation - and focus on consistency. If the only pace that keeps you in Zone 2 is a walk, that is a sign your zones need recalibrating, not that you need to walk.

At this stage, the study's findings are actually somewhat relevant: a couple of harder efforts per week might stimulate more fitness improvement than purely easy running, simply because you have so few sessions that each one needs to count.

Intermediate Runners at 3-4 Times Per Week

This is the transition zone. You are running enough that recovery starts to matter, but not so much that easy days are non-negotiable. A reasonable split is 1-2 harder sessions (intervals, tempo, or race-pace work) and 2 truly easy sessions. Whether those easy sessions are Zone 2 or low Zone 3 is less important than making sure they are genuinely easy - low enough that they do not compromise your next hard session.

Monitor your Training Readiness and HRV trends to gauge whether your easy days are actually easy enough. If readiness is consistently low, your easy runs are too hard.

Serious Runners at 5+ Times Per Week

This is where Zone 2 becomes essential. At high volume, the margin between productive training and overtraining is thin. You physically cannot run five or more times per week at moderate-to-hard intensity without breaking down - through injury, illness, or chronic fatigue that tanks your VO2 max and leaves your watch stuck on Unproductive.

The 80/20 rule exists for this group: roughly 80% of training at Zone 1-2 intensity, 20% at Zone 4-5 intensity. Not because Zone 2 is a mitochondrial miracle, but because it is the only sustainable way to run 50, 60, or 70+ miles per week while absorbing hard sessions. The physician commenter put it simply: run conversationally easy and it is an easy run. No need to overthink the zone number.

A Practical Framework for Easy/Hard Balance

Instead of fixating on Zone 2 specifically, focus on the principle: make easy days easy and hard days hard. Here is a simple framework based on weekly frequency.

3 runs per week:

• 1 hard session (intervals or tempo)
• 2 easy runs (conversational pace, whatever zone that lands in)

4 runs per week:

• 2 hard sessions (with at least one easy day between them)
• 2 easy runs

5-6 runs per week:

• 2 hard sessions
• 3-4 easy runs (Zone 2 or low Zone 3)
• Consider making one easy run a true Zone 1 recovery run

7 runs per week:

• 2 hard sessions
• 1 medium-long run (moderate effort)
• 4 easy runs, at least one at Zone 1

The actual zone numbers matter less than the effort distribution. If your hard days feel hard and your easy days feel genuinely easy, you are doing it right. If every run feels "medium" - that is the moderate-intensity trap that leads to stagnation.

Use your Garmin's Training Effect data to verify: hard sessions should produce aerobic Training Effect of 3.5+ while easy sessions should stay below 2.5. If your easy runs are consistently scoring 3.0+, they are not easy.

The Bottom Line

Is Zone 2 training overrated? The 2025 study makes a reasonable case that Zone 2 is not uniquely special for mitochondrial biogenesis. That is a fair scientific point. But the endurance community did not adopt Zone 2 because of mitochondria. It adopted Zone 2 because it works as a training strategy - it lets you run more, recover better, stay healthy, and absorb hard sessions that actually push your fitness forward.

The study examined the mechanism and found it wanting. The Reddit thread of 659 runners examined the outcome and found it still works. Both can be true at the same time.

If you are running fewer than four times per week, do not stress about zones. Run easy on easy days, push hard on hard days, and build consistency. If you are running five or more times per week, Zone 2 is not optional - it is the foundation that makes everything else sustainable.

And if your Garmin zones feel wrong, fix them first. No amount of training philosophy matters if your watch is telling you the wrong numbers.

FAQ

Is Zone 2 training a waste of time?

No. The 2025 review found that Zone 2 may not be uniquely optimal for mitochondrial biogenesis, but it did not examine the broader benefits: cardiac adaptations, injury prevention, volume accumulation, and recovery between hard sessions. Zone 2 remains the most sustainable intensity for building an aerobic base, especially at higher training volumes.

Should I stop doing Zone 2 runs after this study?

No. The study's findings are about a specific cellular mechanism, not a training program recommendation. Zone 2 running is still the best way to accumulate weekly volume without accumulating injury risk. If you are running four or more days per week, easy runs at Zone 2 intensity are essential for recovery and long-term progress.

Is zone 2 training worth it for beginners?

For beginners running 2-3 times per week, strict Zone 2 adherence is unnecessary. At low volume, just running at a comfortable conversational pace is sufficient. The distinction between Zone 2 and Zone 3 matters most for runners doing 5+ sessions per week where recovery between sessions becomes the limiting factor.

What should I do instead of Zone 2?

This is the wrong framing. Zone 2 is not a replacement for hard training - it is the complement to it. A well-structured program includes both easy running (Zone 1-2) and hard running (Zone 4-5). The key is separating them clearly rather than defaulting to moderate Zone 3 effort for every session.

How do I know if my Garmin Zone 2 is accurate?

If staying in Zone 2 requires walking or an absurdly slow jog, your zones are likely miscalibrated. Garmin's default zones use the 220-minus-age formula, which is inaccurate for many people. Switch to %HRR (Karvonen method) in your zone settings and enter your actual max heart rate and resting heart rate. See our complete guide to Garmin heart rate zones for step-by-step instructions.

Does the 80/20 rule still apply after this study?

Yes. The 80/20 principle - 80% easy, 20% hard - exists because of injury prevention and recovery logistics, not because of mitochondrial optimization. Even if higher intensity is better for cellular adaptations per minute of exercise, you cannot sustain high intensity for 80% of your weekly volume. The 80/20 split is a practical constraint, not a physiological claim about Zone 2 superiority.

Train Smarter With Your Actual Data

The Zone 2 debate is ultimately about training structure - how to balance easy and hard days for maximum improvement. But the right balance depends on your individual recovery, fitness level, and training history. What works for a 25-mile-per-week runner is wrong for a 60-mile-per-week runner.

At shoulditrain.com, we connect directly to your Garmin data and analyze your Training Readiness, HRV status, training load, and recovery trends together. Instead of guessing whether today should be easy or hard, our AI coach tells you exactly what intensity your body can handle based on how you have actually been recovering.

Try it free and take the guesswork out of your easy/hard balance.