Zone 2 Training Was Meant to Fix Everything. A New Review Says It Might Be Overrated

For the last five years, Zone 2 has been sold as the magic pace: easy enough to hold a conversation, effective enough to build mitochondria, safe enough to do every day.

In 2025, a peer-reviewed narrative review published in Sports Medicine picked up the claim, tested it against 167 sources, and came to a different conclusion.

The paper is called Much Ado About Zone 2. And the running internet has not been the same since.

What the Review Actually Claimed

The lead author, Kristi Storoschuk, is a PhD researcher in mitochondrial biology.

Her team looked at two specific claims about Zone 2 training. The first was that it is the optimal intensity for improving mitochondrial capacity.

The second was that it is the optimal intensity for improving fat oxidation.

The review, hosted at PubMed via the NIH, concluded that current evidence does not support Zone 2 as the optimal intensity for either.

Higher intensity keeps winning the direct comparisons

When low-intensity and high-intensity training are compared head to head for mitochondrial adaptations in the studies the review examined, higher intensity consistently produces the bigger effect.

The threshold for reliably triggering mitochondrial adaptation appears to be roughly 65 percent of maximum work rate or higher. That is above what most Zone 2 definitions target.

Zone 2 is not even one zone

A separate 2025 methods paper tested multiple common definitions of Zone 2 and found they point to meaningfully different absolute intensities.

Two runners with the same maximum heart rate can have ventilatory or lactate thresholds that differ by 15 to 20 beats per minute.

That is enough to put one runner into a productive aerobic effort and the other into an unproductive slog.

What Zone 2 Advocates Say Back

The response from the coaching side has been fair, not dismissive.

Zone 2 does not fail. It just does something narrower than the loudest content made it sound.

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The volume argument still holds

Zone 2 lets you accumulate high aerobic volume at low fatigue and low injury cost. That volume is what makes the 80/20 polarized training model work.

You cannot do that many hard sessions in a week without breaking down.

Cardiac adaptations do not care about mitochondria

Even the Cleveland Clinic's explainer on Zone 2 calls out benefits that were not measured by the Storoschuk review: heart-chamber remodelling, improved stroke volume, and generally better cardiac efficiency.

Those adaptations come from time spent at moderate intensity, not from short high-intensity spikes.

Injury risk is a training variable too

A study that maximises the mitochondrial signal per unit of time does not care whether you got injured on Tuesday. Coaches do.

Zone 2 pays a lower fatigue price for the aerobic base it builds.

What This Means For Your Training

The nuanced answer is not: stop running easy. The nuanced answer is: stop running easy at the same effort every single day.

Here is a practical translation from the science to your week.

1. Keep the aerobic base, especially if you run a lot

If your weekly mileage is high, most of it still needs to be genuinely easy. That is the entire point of easy days.

If you are new to that idea, our guide on why slow running actually makes you faster explains the mechanism.

2. Add one or two genuinely hard sessions a week

If every run in your week is at the same middling effort, you are getting the worst of both worlds.

One tempo or interval session, and one long steady run, will usually deliver more adaptation than five identical Zone 2 outings.

3. Stop chasing a specific heart-rate number

Zone 2 was never a single heart rate. It is an intensity defined by your own physiology.

A talk test still beats a formula. If you can speak in complete sentences but not sing, you are in the useful part of Zone 2.

Our companion piece on the only way to know if you are doing Zone 2 right walks through this.

4. Do not throw the volume out

The review does not say easy running is useless. It says easy running is not uniquely magical.

Keep the base. Just do not treat it as a substitute for hard efforts.

The Bigger Picture

Zone 2 is not broken. The story around Zone 2 was oversold.

The single best endurance-training predictor in the peer-reviewed literature is not one intensity or one heart-rate zone.

It is total consistent volume, appropriately distributed.

That is not a headline. That is the truth.

If you already run six days a week at three different efforts and follow a structured plan, none of this changes your week. If you have been plodding at the same easy pace every single day for a year, it changes almost everything.

The fix is not to run harder every day. The fix is to add contrast.

One hard, one long, the rest genuinely easy. That is the framework the actual data supports.