Zone 2 Training Explained
Why low-intensity aerobic training builds the mitochondrial foundation that underpins metabolic health and longevity.
Zone 2 training refers to exercise performed at an intensity where you can sustain a conversation but are working steadily — roughly 60–70% of maximum heart rate, or below the first ventilatory threshold. It feels easy. That's the point.
At this intensity, the primary fuel source shifts almost entirely to fat oxidation, and mitochondrial biogenesis — the creation of new mitochondria — is maximally stimulated. Mitochondria are the energy-producing organelles in every cell. Their density and efficiency are among the most reliable predictors of metabolic health and longevity.
Dr. Iñigo San Millán, who works with professional cyclists and has published extensively on metabolic health, recommends 3–4 hours of Zone 2 per week as a minimum effective dose for significant mitochondrial adaptation. This can be accumulated in 45–60 minute sessions.
The key mistake most people make is training too hard. If you're slightly breathless and can't comfortably hold a conversation, you've drifted out of Zone 2 into Zone 3 — which provides different (and for these purposes, inferior) adaptations. A heart rate monitor and disciplined pacing are the tools.