Zone 2

Research summary

• "Zone 2 training" (low-intensity aerobic exercise performed just below the first lactate/ventilatory threshold) has become a popular longevity concept, but the best current evidence is considerably more skeptical of the specific claim than the popular narrative suggests.
• A 2025 expert-consensus commentary in the International Journal of Sport Physiology and Performance, drawing on a panel of 14 applied sport scientists and coaches, found there isn't even clear agreement on how to define "Zone 2," and concluded the physiological adaptations attributed to it "might not be unique to zone 2" and could likely be induced at slightly higher or lower intensities too.
• A pointed 2025 narrative review in Sports Medicine goes further, directly challenging the claim that Zone 2 is the optimal intensity for improving mitochondrial and fatty-acid oxidative capacity in the general population: the authors conclude current evidence does not support that claim, and that prioritizing higher exercise intensities is actually more effective for maximizing cardiometabolic health benefits, particularly at lower training volumes.
• This is consistent with the broader training-intensity-distribution literature: meta-analyses of "polarized" training (which is dominated by low-intensity, Zone-2-range volume, paired with a smaller high-intensity component) find real but modest and context-dependent advantages over other distributions for competitive endurance athletes, while head-to-head meta-analyses comparing moderate-intensity continuous training against high-intensity interval training consistently find HIIT produces larger cardiorespiratory-fitness gains for a given time investment (one meta-analysis found nearly double the VO2peak improvement).
• A foundational descriptive study of elite cross-country skiers (Seiler & Kjerland, 2006) does confirm elite endurance athletes spend the large majority of their training volume in the low-intensity zone popularly labeled Zone 2 -- but this reflects a training-volume management strategy for people already training many hours a week, not evidence that this specific intensity is uniquely beneficial for a general population's health or longevity.