Zone 2 Hype Challenged: Higher Intensity Exercise May Beat Low-Intensity Training
A new narrative review questions whether Zone 2 training is truly optimal for mitochondrial health and cardiometabolic fitness in everyday people.
Summary
Zone 2 training — low-intensity exercise below the lactate threshold — has become a popular longevity recommendation, but a 2025 narrative review from Queen's and McMaster Universities challenges this consensus. The authors argue that Zone 2's reputation is largely built on observational data from elite endurance athletes, not controlled trials in general populations. After critically examining the evidence on mitochondrial capacity and fatty acid oxidation, they conclude that higher-intensity exercise (above Zone 2) produces equal or superior improvements in these markers, especially when total training volume is limited. For most adults who cannot dedicate large hours to exercise, prioritizing moderate-to-high intensity training may deliver greater cardiometabolic benefits per minute invested.
Detailed Summary
Zone 2 training has exploded in popularity across longevity and wellness communities, championed as the gold-standard intensity for building mitochondrial capacity, enhancing fat oxidation, and protecting long-term cardiometabolic health. But is the science as solid as the social media consensus suggests?
This 2025 narrative review, published in Sports Medicine by researchers from Queen's University and McMaster University, takes a critical look at the evidence base underpinning Zone 2 recommendations for the general public. The authors define Zone 2 as low-intensity aerobic exercise performed below the lactate threshold — roughly a conversational pace — and examine whether it truly outperforms other intensities for improving mitochondrial and fatty acid oxidative capacity.
The review concludes that the case for Zone 2 as the optimal training intensity rests heavily on observational data from elite endurance athletes. These athletes train at enormous volumes, and their high mitochondrial capacity may reflect total training load rather than any unique benefit of low-intensity work. When controlled studies in non-elite populations are examined, higher-intensity exercise consistently produces comparable or superior improvements in mitochondrial markers and cardiorespiratory fitness.
Critically, the authors emphasize that for individuals with limited time — the reality for most adults — higher exercise intensities above Zone 2 appear to maximize cardiometabolic health benefits per unit of time invested. This has direct implications for public health guidance, where recommending large volumes of low-intensity exercise may be impractical and potentially suboptimal.
Caveats apply: this is a narrative rather than systematic review, meaning study selection was not exhaustive or pre-registered. The authors also acknowledge that Zone 2 training is not without value — it remains a viable and accessible option — but argue it should not be positioned as uniquely superior for the general population.
Key Findings
- Zone 2 training's reputation is largely based on observational data from elite athletes, not controlled trials in general populations.
- Current evidence does not support Zone 2 as the optimal intensity for improving mitochondrial or fatty acid oxidative capacity.
- Higher-intensity exercise (above Zone 2) produces equal or superior cardiometabolic and mitochondrial adaptations in most studies.
- For time-limited individuals, prioritizing higher exercise intensities may maximize health benefits per training session.
- Broad public recommendations for Zone 2 training are not well-supported by the available controlled evidence.
Methodology
This is a narrative review published in Sports Medicine (2025), critically examining existing literature on Zone 2 training and its effects on mitochondrial capacity, fatty acid oxidation, and cardiorespiratory fitness. As a narrative review, study selection was not conducted via a pre-registered systematic protocol, which introduces potential selection bias. The authors draw on mechanistic studies, training intervention trials, and observational data from athletic populations.
Study Limitations
As a narrative rather than systematic review, the paper is subject to author selection bias in which studies are emphasized or omitted. The review focuses on mitochondrial and fatty acid oxidative outcomes and may not fully capture other benefits of Zone 2 training such as recovery, injury risk reduction, or adherence. Findings may not apply equally across all populations, including older adults, those with chronic disease, or highly trained individuals.
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