If you only have a few hours a week for cardio, should most of it be easy Zone 2 work or should it be harder?

Martin J. GibalaPhD · exercise physiologist, McMaster University

Harder. At three to six hours a week, Zone 2 is not a hard enough stimulus to accrue benefits efficiently, and putting 80 percent of a small weekly budget there leaves you less fit than you could be.

What it rests onA narrative review he co-authored argues the Zone 2 recommendation rests on observational data from elite endurance athletes doing very large volumes, that it contradicts substantial evidence for high-intensity exercise on mitochondrial capacity and cardiometabolic health, and that prioritising intensities above Zone 2 matters particularly at lower training volumes. He adds that Zone 2 has no uniform definition and that consumer wearables bound it differently from one another.

Peter AttiaMD · longevity physician, founder of Early Medical

Mostly easy. Two hours a week of zone 2 is a good place to start, three to four hours is the target, and you probably cannot do too much of it.

What it rests onHe defines zone 2 as the highest level of exertion that is effectively pure mitochondrial oxidative phosphorylation before lactate begins to net accumulate, crediting Iñigo San Millán for the definition, and treats mitochondrial function as the lever behind metabolic health. He reports doing it four times a week himself with a lactate meter at a target of 1.8 to 1.9 millimolar.