Zone 2 Exercise
The optimal aerobic intensity for building mitochondrial capacity
Definition
Zone 2 exercise is the baseline aerobic intensity — usually 60-70% of maximum heart rate, or the threshold at which you can still hold a conversation but can no longer sing. Biochemically it corresponds to lactate threshold 1 (blood lactate ≤2 mmol/L). It is the intensity that maximises fat oxidation as fuel and most specifically stimulates mitochondrial biogenesis and metabolic flexibility improvement.
Detailed explanation
Iñigo San Millán, UAE Team Emirates physiologist (Tadej Pogačar's coach), and Peter Attia popularised the concept in longevity medicine. The recommended protocol: 3-4 weekly sessions of 45-90 min in strict Zone 2, complemented by 1-2 weekly sessions of short HIIT (4×4 min Tabata or Norwegian 4×4 protocol) to improve VO2 max.
Cellular mechanisms: Zone 2 preferentially activates type I muscle fibres (oxidative, rich in mitochondria), stimulates mitochondrial biogenesis via PGC-1α, improves fatty acid oxidation capacity (intramuscular lipolysis), elevates the lactate shuttle between muscle fibres, and reduces baseline insulinaemia. The result is better 'metabolic flexibility' — capacity to alternate between glucose and fat oxidation as needed.
Monitoring: chest-strap heart rate monitor (more accurate than wrist), capillary lactate to validate zone in initial sessions, and CGM to observe glycaemic response to exercise. Key progress metric: for the same heart rate, greater speed/power should be achieved with training — that is improvement of the 'aerobic threshold'.
Scientific sources
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