Is Zone 2 training overrated?
As a magic stimulus, yes. A 2025 review found no evidence that Zone 2 beats higher intensities for building mitochondria or fat burning, though critics note it leans on short studies, so we grade it B rather than A. Easy running still matters for marathoners, because it is how you carry a lot of volume.
What to do
- Keep most of your running easy, but don't obsess over staying inside a narrow Zone 2 band.
- If you have little time to train, don't make every session Zone 2. Some harder running is more efficient for fitness.
- Ignore a fixed '2 mmol/L' lactate target unless it comes from your own tested lactate curve.
- Use easy running to build volume. That is where its value for marathoners lies.
What Zone 2 means
“Zone 2” means different things in different systems, which is part of the confusion.
In the three-zone model used in most research, zone 2 is the band between the two thresholds: steady, comfortably hard running. In the five-zone systems used on many watches and in podcasts, Zone 2 is the top of easy running.
A 2025 paper proposed a clear definition for the popular version: Zone 2 is the intensity just below the first threshold, often called LT1 (Sitko and colleagues, 2025). LT1 is the point where lactate, a by-product of burning carbohydrate, first starts to rise above resting levels. Below it, you can hold a full conversation.
This page is about that popular meaning.
The claims
Zone 2 became famous mainly through longevity content. The biggest source is the book Outlive (2023) and podcasts featuring San Millán. The core claim is that Zone 2 is a uniquely good stimulus for mitochondria (the parts of your muscle cells that use oxygen to make energy) and for fat burning.
A related claim, common in the same discussion, is that Zone 2 alone is the best way for time-limited amateurs to raise VO2max. VO2max is the most oxygen your body can use per minute, a key marker of aerobic fitness.
What the evidence says
A 2025 review in Sports Medicine looked at whether Zone 2 beats higher intensities for mitochondrial or fat-burning adaptation. It found no evidence that it does, especially for people who don’t train many hours (Storoschuk and colleagues, 2025).
That puts the first claim in the overstated category. The second claim, that Zone 2 alone maximizes VO2max for amateurs short on time, is contradicted by the same review.
The counterpoint
The author and podcaster Steve Magness pushed back (Magness, 2026). He argued that the review leans on short studies and on signalling markers rather than longer-term changes. That is a fair point about the quality of the evidence, and it is why we grade this page B rather than A.
But Magness also agreed with the practical conclusion: lots of easy running plus some well-placed intensity. On what to actually do, the two sides land in the same place.
Why easy running still matters for marathoners
None of this means easy running is overrated. It means the reason for it is often misstated.
In logs from 119,452 marathoners, the fastest runners ran more than three times the volume of slower runners. Their hard running was about the same as everyone else’s. The extra was almost all easy (Muniz-Pumares and colleagues, 2025). See how much of your running should be easy.
Easy running is valuable because it lets you run a lot without breaking down. It is the carrier of volume, not a special switch that harder running can’t flip.
The 2 mmol/L problem
Some Zone 2 advice sets the upper limit at a fixed blood lactate of 2 mmol/L. That number doesn’t transfer well from person to person, or from meter to meter.
- Meters are imprecise. Portable lactate analysers have a typical error of 0.4–1.0 mmol/L (Tanner and colleagues, 2010). That is a large share of a 2.0 reading.
- The same nominal pace gives different values. When runners ran at a speed meant to give 2 mmol/L, actual readings ranged from 1.9 to 3.8 mmol/L (Fleckenstein and colleagues, 2023).
- Thresholds vary widely between people. Across 100 people, the lactate threshold fell anywhere from 45% to 74% of VO2max (Iannetta and colleagues, 2020).
If you use a lactate meter, base targets on your own tested curve, not on a universal number. For simpler ways to find your easy zone, see how to set your heart-rate zones.
What we don’t know
- Long-term trials are missing. Most studies in the review are short, which is exactly the critics’ point. Longer studies could change the picture.
- No study links Zone 2 specifically to marathon performance. The question has been studied with lab markers, not race times.
- The longevity claims are a separate question. This page looks at training adaptations, not at lifespan.
- “Zone 2” is defined differently across studies and devices, which makes the evidence hard to pool.
Sources
- Sitko et al. (2025). Proposed definition of Zone 2 training.
- Storoschuk et al. (2025). Review of Zone 2 versus higher intensities for mitochondrial and fat-oxidation adaptation.
- Magness (2026). Commentary on the Zone 2 debate.
- Muniz-Pumares et al. (2025). Training characteristics of 119,452 marathon runners.
- Tanner et al. (2010). Accuracy of portable lactate analysers.
- Fleckenstein et al. (2023). Lactate values when running at a fixed 2 mmol/L speed.
- Iannetta et al. (2020). Individual variation in the lactate threshold as a share of VO2max.
Last reviewed 13 November 2026.