Zone 2 and Mitochondria: What the Sports Medicine 2025 Narrative Review Says About Low-Intensity Training

Where did the idea that zone 2 is optimal for mitochondria come from?

Interest in "zone 2" training surged in 2022–2024 through popular podcasts and books, which presented this intensity as the primary tool for mitochondrial biogenesis and metabolic health. The argument was built on observational data: elite endurance athletes spend 70–80% of their training time in low zones (the "polarized" model).

The problem is that "elite athletes train a lot in zone 2" to "zone 2 is optimal for mitochondrial adaptations in ordinary people" is a large logical leap. Professionals accumulate 20–30+ hours per week; low intensity is the only way to sustain such volume without overtraining. For a person with 4–6 hours per week, the equation looks different.

What did the Sports Medicine 2025 narrative review show?

Storoschuk and colleagues (Sports Medicine, 2025, DOI: 10.1007/s40279-025-02261-y, PMID 40560504) conducted a systematic narrative review to assess the validity of broad zone 2 recommendations for the general population. The authors focused on two questions: (1) does zone 2 improve mitochondrial capacity and fat oxidation ability, and (2) does it outperform other intensities in these effects.

Key conclusions of the review:

• Zone 2 induces mitochondrial adaptations in untrained individuals and patients with metabolic disorders — this is confirmed by a number of studies.
• However, to achieve meaningful mitochondrial biogenesis, the intensity must exceed the current aerobic potential. In sufficiently trained individuals, zone 2 may fall below this threshold.
• Direct comparative studies (zone 2 versus higher intensities at equal volume) do not support the superiority of zone 2 for gains in mitochondrial capacity and VO2max.
• The authors conclude that for the general population with limited training time, higher intensities yield greater returns for the same time invested.

How do you accurately identify zone 2 for a specific person?

A common reference point is 70–80% of maximum HR or "can talk but it's hard." Meixner and colleagues (Translational Sports Medicine, 2025, DOI: 10.1155/tsm2/2008291) tested 50 experienced cyclists (30 men, 20 women) using multiple zone 2 markers simultaneously and uncovered a serious problem.

The coefficient of variation (CV) between markers across participants:

• HR 72–82% of maximum: CV 6–7% — smallest spread, but still significant.
• Power at lactate 1.5 mmol/L: CV 29% — a threefold spread between participants.
• The ventilatory threshold (VT1) and the point of maximum fat oxidation (FatMax) showed the greatest agreement with each other and with metabolic states.

Practical takeaway: prescribing training intensity based on a fixed HR percentage means that different people end up in completely different metabolic zones, even if their heart rates match. The most accurate way to identify zone 2 is a graded exercise test with lactate or ventilatory threshold measurement.

What does zone 2 training actually deliver?

It would be wrong to conclude that zone 2 is useless. The available data points to the following evidence-based effects:

• Aerobic base. Regular low-intensity training develops capillary networks, increases mitochondrial density and fat oxidation capacity — especially in untrained individuals and those with metabolic disorders.
• Recovery and volume. Zone 2 allows accumulating a large weekly volume without excessive stress — hence its value in endurance athlete programs with high volumes.
• Low injury and overtraining risk. For people starting a training program, zone 2 is a safe entry point.
• Fat oxidation. At low intensity, the body predominantly uses fat as a substrate — this does not mean fat burning is more effective than at high intensity, but it is a physiological fact.

What does this mean for choosing training intensity?

Scientific consensus (2025 narrative review, HIIT and SIT meta-analyses) indicates: for a person with 4–6 hours of training per week, a rational program includes a mix of intensities, not exclusively zone 2. High-intensity intervals (2–3 sessions per week) and moderately intense work (above LT1 but below VO2max) yield greater gains in VO2max and mitochondrial capacity for the same time.

Zone 2 remains a valuable part of the program — but as a complement to higher intensities, not a replacement for them.