Does Zone 2 Training Actually Build More Mitochondria?

Written by the Nuvirox Research Team

Short answer: yes, but probably less efficiently than harder training, and the popular claim has outrun the evidence. Zone 2 does drive aerobic adaptations. What it does not appear to do is beat higher-intensity work on a matched-effort basis for mitochondrial capacity. The reason zone 2 dominates elite endurance programmes is that it lets athletes accumulate enormous volume without wrecking themselves — which is a different argument from the one being made to the general public.

Schematic comparison of signalling response by intensity domain per matched unit of work. Bar heights are illustrative of the reported direction, not measured values.

What exactly is zone 2?

Steady aerobic work below your first lactate threshold. Practically, that means you can hold a conversation in full sentences, your breathing is controlled, and you could keep going considerably longer than you intend to. Blood lactate stays close to baseline rather than accumulating.

The physiological logic behind the enthusiasm is sound as far as it goes. This intensity relies heavily on fat oxidation, which happens inside mitochondria, and elite endurance athletes both spend most of their training hours here and possess extraordinary mitochondrial capacity. The leap — that the first fact causes the second — is where the argument gets shakier.

What human studies actually show

The 2025 review challenged the central claim directly. Storoschuk and colleagues, writing in Sports Medicine, examined the evidence for zone 2 training improving mitochondrial and fatty acid oxidative capacity in the general population. Their conclusion was blunt: the majority of available evidence argues against zone 2 being the optimal stimulus for mitochondrial capacity, and the popular recommendation contradicts substantial evidence supporting higher-intensity exercise for mitochondrial and cardiometabolic outcomes.

The signalling data favours intensity. Harder efforts generate greater energetic stress — phosphocreatine depletion, falling intracellular pH, rising lactate — and stronger activation of AMPK, p38 and CaMKII. Those are the upstream signals that converge on PGC-1α, the transcriptional coactivator that tells cells to build more mitochondria. Work below roughly 60% of maximum aerobic power has generally not been expected to produce meaningful gains in mitochondrial content in untrained people.

Low-volume interval training produced comparable outcomes to far larger endurance volumes. In a 12-week trial, sprint interval training improved indices of cardiometabolic health similarly to traditional endurance training despite a five-fold lower exercise volume and time commitment. That is not a claim that intervals are superior in every respect — it is a demonstration that time-matched comparisons and work-matched comparisons give different answers.

Intensity modulates signalling more than content markers. One study found that training intensity changed PGC-1α and p53 protein content and mitochondrial respiration, but did not change markers of mitochondrial content in the same way. This is an important nuance: mitochondrial quality and mitochondrial quantity are separate outcomes that respond differently, and studies measuring only one can appear to contradict each other when they do not.

The honest counterweight: zone 2 is not useless, and the review is a narrative review. Some zone 2 protocols, typically long-duration ones, do upregulate mitochondrial gene expression. Improvements in citrate synthase activity, mitochondrial respiration and phosphocreatine recovery have been reported after weeks of low-intensity training, including in people with type 2 diabetes. And a narrative review reflects the authors’ framing of a literature rather than a pre-registered quantitative synthesis. One of its authors also discloses equity in an exercise-related company. None of this makes the argument wrong; it means it should be read as a strong corrective rather than a final verdict.

Why do elite athletes do so much of it, then?

Because they need to survive their hard sessions. The 80/20 polarised model observed in world-class endurance athletes describes how they distribute an already enormous training load. Their 20% of high-intensity work represents a volume most people will never approach. The low-intensity 80% functions as the substrate that makes that high-intensity work repeatable week after week.

Read that way, zone 2 is a recovery and volume-accumulation tool in elite programmes rather than the primary driver of their mitochondrial phenotype. Transplanting the ratio to someone training four hours a week — whose 20% amounts to under an hour — discards most of the stimulus while keeping the label.

What zone 2 won’t do

It will not substitute for strength training, and it will not maximise mitochondrial adaptation per hour. Steady aerobic work does very little to preserve the fast-twitch muscle fibers that shrink fastest with age. If your training week is entirely zone 2, you have left the tissue most vulnerable to aging completely undefended.

It also will not fix fatigue that has a medical cause. Persistent exercise intolerance, breathlessness disproportionate to effort, chest discomfort, or a heart rate that behaves oddly at low intensities are reasons to talk to a doctor before adjusting a training plan. Building an aerobic base is not a substitute for finding out why the base collapsed.

So how should you actually train?

Mixed intensities, with the ratio driven by your total available time. If you have ten or more hours a week, a mostly-easy distribution makes sense because you need to survive the volume. If you have three or four, weighting more of that time toward efforts that actually stress the system is the more defensible reading of the evidence.

None of this argues against walking, which is doing a different job entirely — see our look at how many steps a day you actually need for the mortality data on movement volume. Zone 2 and step count are complements, not competitors. And if you want the supplement side of the mitochondrial question examined with the same scepticism, we went through it in which mitochondria supplements have real evidence.

Frequently asked questions

How do I know if I’m actually in zone 2?

The talk test is more reliable than a heart rate formula. If you can speak in full sentences but would rather not sing, you are in the right neighbourhood. Age-predicted heart rate zones carry substantial individual error.

Is zone 2 a waste of time?

No. It builds aerobic capacity, is sustainable, has a low injury and recovery cost, and is genuinely pleasant. The claim being challenged is narrower: that it is the optimal stimulus for mitochondrial adaptation. That claim does not hold up well.

How long do zone 2 sessions need to be?

The protocols that did show mitochondrial gene expression changes tended to be long-duration. That is part of the practical problem — the low-intensity route to a given adaptation generally requires substantially more time than the high-intensity one.

Should beginners start with zone 2?

Starting easy is sensible for tissue tolerance, technique and adherence, none of which are mitochondrial arguments. Those are still good reasons. Just don't confuse them with the claim about optimal adaptation.

Does zone 2 burn more fat?

It oxidises a higher proportion of fat during the session, which is not the same as producing more fat loss over weeks. Total energy expenditure and overall dietary intake dominate that outcome.

From Nuvirox

Why we formulated NAD+ Restore

Training is the strongest lever anyone has on mitochondrial capacity, and we would rather say that plainly than pretend a capsule competes with it. What a supplement can plausibly do is support the cofactor side of that machinery — which is why our formula is built around a precursor with published human trial data.

• 500 mg Nicotinamide Riboside Chloride (NR) — one of the two most-researched NAD+ precursors, within the dose range used in published human trials.
• 150 mg trans-resveratrol (Japanese Knotweed) and 50 mg quercetin (Sophora japonica) — polyphenols studied alongside NAD+ pathways for cellular health support.
• 10 mg galactomannans from fenugreek — to support absorption.
• 60-day money-back guarantee — long enough to actually evaluate it the way the research says you should.

Learn more about NAD+ Restore →

The bottom line

Zone 2 became a longevity talking point by borrowing the training distribution of elite endurance athletes and attributing their mitochondrial capacity to the easy portion of it. The comparative evidence does not support that attribution. When work is matched, harder efforts generate the stronger signal. The fair reading is that zone 2 is a genuinely useful, sustainable, low-cost component of a training week — and that if your training time is limited, spending all of it there is leaving adaptation on the table.

References

1. Storoschuk KL, Moran-MacDonald A, Gibala MJ, Gurd BJ. 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. Sports Medicine. 2025;55(7):1611–1624. PMID: 40560504. DOI: 10.1007/s40279-025-02261-y.
2. Gillen JB, Martin BJ, MacInnis MJ, et al. Twelve weeks of sprint interval training improves indices of cardiometabolic health similar to traditional endurance training despite a five-fold lower exercise volume and time commitment. PLOS ONE. 2016;11(4):e0154075. PMID: 27115137.
3. Granata C, Oliveira RSF, Little JP, Renner K, Bishop DJ. Training intensity modulates changes in PGC-1α and p53 protein content and mitochondrial respiration, but not markers of mitochondrial content in human skeletal muscle. FASEB Journal. 2016;30(2):959–970. PMID: 26572168.

*These statements have not been evaluated by the Food and Drug Administration. This product is not intended to diagnose, treat, cure, or prevent any disease. This article is for informational purposes only and is not a substitute for professional medical advice.