Zone 2 cardio is the internet's favorite training concept of the last two years. Rogan talks about it. Peter Attia talks about it. Every fitness YouTuber talks about it.
Here's what the actual endurance science literature says — and what most of the popular takes get wrong.
What Zone 2 actually is
Zone 2 refers to a specific intensity of aerobic exercise defined by physiological markers, not just heart rate.
The strict definition: the highest intensity you can sustain while producing energy almost entirely from fat oxidation, with blood lactate under ~2 mmol/L.
