"Much Ado About Zone 2" — a narrative review from the Gurd and Gibala groups: current evidence does not support Zone 2 as the optimal intensity for improving mitochondrial or fatty-acid oxidative capacity; the popular claim rests largely on observational data from elite endurance athletes, and higher intensities matter more at lower training volumes
The authors examine the popular positioning of Zone 2 (low intensity below the lactate threshold) as the optimal intensity for mitochondrial and fatty-acid oxidative capacity, and whether it justifies a public recommendation. Content from the abstract only.
Key Results
Popular recommendations for Zone 2 largely stem from observational data of elite endurance athletes who do large volumes of Zone 2 and have high mitochondrial and fatty-acid oxidative capacity.
Conclusion: current evidence does not support Zone 2 training as the optimal intensity for improving mitochondrial or fatty-acid oxidative capacity.
Evidence suggests prioritising higher intensities (> Zone 2) is critical to maximise cardiometabolic health benefits, particularly in the context of lower training volumes.
Practical Application
"Is Zone 2 the magic zone for mitochondria and fat burning?" A 2025 narrative review by Storoschuk, Gibala and Gurd concludes that current evidence does not support Zone 2 as the optimal intensity for mitochondrial or fat-oxidative capacity; the popular claim leans on what elite athletes do, which is an association, not a test.
At lower training volumes the authors judge higher intensities (> Zone 2) critical to maximise cardiometabolic benefit; the abstract gives no hours threshold.
Not from this paper; the base's own practice (SYNTH-51): for endurance athletes easy work is still most of the week — because it is cheap and adds volume, not because it is a special fat-burning stimulus.
Limitations
Narrative, not systematic; the abstract gives no studies, numbers or pooled effects.
Scope is the general population; it does not test an athlete's mix of easy volume and intensity.
Declared conflict: one author holds equity in an exercise-related company.
How this entry was checked and changed
Why this level
authoritative narrative review from a leading group (rubric B+); not systematic, no pooled estimate. One author (Gibala) declares equity in an exercise-related company