Zone 2 helps, but higher intensity delivers more.

Zone 2 training, widely promoted as the sweet spot for mitochondrial health and fat metabolism, is defined as exercise just below the first lactate threshold—where you can comfortably converse while working at roughly 67–82% of your maximum heart rate. Advocates claim that it boosts mitochondrial function, insulin sensitivity, glucose control, and metabolic flexibility while being approachable and safe. However, evidence supporting its superiority is mixed. While some studies in populations like type 2 diabetics show improvements in mitochondrial enzyme activity, many meta-analyses and comparative studies reveal that higher-intensity exercise produces far stronger adaptations. Biochemically, Zone 2 creates relatively little metabolic stress, whereas higher intensities generate the signals (via AMP, ADP, AMPK, and calcium pathways) that more robustly stimulate mitochondrial biogenesis, cardiovascular gains, and improvements in VO₂max and lactate thresholds.

This doesn’t mean Zone 2 is useless—it can be a valuable tool for beginners, those returning to exercise, or as a low-recovery option on days when higher-intensity training isn’t feasible. It promotes fat oxidation, encourages regular movement, and can extend total training volume without taxing the body heavily. But the narrative that Zone 2 is uniquely optimal for longevity or metabolic health is misleading. For time efficiency and maximal mitochondrial and cardiovascular benefits, higher-intensity exercise (including intervals and continuous efforts above Zone 2) should take priority. Zone 2 works best as a supplement—helpful for recovery and consistency—but it should not replace higher-effort sessions if the goal is to maximize performance, health, and adaptation.

https://brokenscience.org/do-we-really-need-zone-2-exercise-for-mitochondrial-and-metabolic-health/