How do you know you are in Zone 2, and does the number matter?

Iñigo San MillánPhD · applied exercise physiologist, University of Colorado School of Medicine

A band, not a ceiling: blood lactate roughly 1.5 to 2.0 mmol/L, anchored at your first threshold, and clearly harder than an easy walk.

What it rests onHis own written definition, published on his own channel 13 Mar 2026: the zone is 'a biochemically-defined metabolic state created by three intersecting cellular mechanisms', occurring at 'a blood lactate concentration of roughly 1.5 to 2.0 mmol/L, corresponding to an exercise intensity that feels sustainable for hours but clearly more demanding than a casual walk or easy recovery effort'. His translation table places it at LT1 and VT1. He supplies the caveat himself: two cyclists at 300 W can sit at 2.0 and 8.0 mmol/L, so the number names a metabolic state and does not transfer between people. For anyone without a meter he endorses conversational strain instead.

Peter AttiaMD · longevity physician, founder of Early Medical

A ceiling: the highest output you can sustain with lactate below 2 mmol/L, and the dose is 3 to 4 hours a week.

What it rests onHis site states the definition in his own words, across the site: 'Zone 2 is the third of four pillars in my framework for exercise: aerobic exercise. Zone 2 is defined as the highest metabolic output/work that you can sustain while keeping your lactate level below two millimole per liter.' He recommends 3 to 4 hours a week, 2 for a beginner, and prefers steady-output modalities because an outdoor ride averaging zone 2 is not time spent in zone 2. He also endorses the talk test for people without a meter, and his own zone 2 sits at a lactate of 1.7 to 1.9 mmol.