A new review challenges Zone 2 cardio as the single best training intensity
Published Jul 1, 2025
Methods
Researchers reviewed existing evidence on Zone 2 training (low-intensity exercise below the lactate threshold) and its effects on mitochondrial capacity and cardiorespiratory fitness, comparing it with higher-intensity training approaches. This was a narrative synthesis of existing research rather than a systematic, protocol-driven search of every relevant study.
Findings
The review concluded that current evidence does not support Zone 2 as the optimal intensity for improving mitochondrial or fat-oxidation capacity, and that higher exercise intensities appear more effective for cardiometabolic fitness, particularly for people training at lower weekly volumes. It argues that popular Zone 2 recommendations rely mainly on observational data from elite endurance athletes rather than evidence from typical exercisers.
Caveats & Context
This is a narrative review, not a systematic review with a fixed search protocol, so it does not guarantee even coverage of every relevant study, and the interpretation offered is one perspective within an active scientific debate. It does not mean easy cardio has no value — it argues against treating Zone 2 specifically as the single best or most time-efficient intensity for most training goals.
Much Ado About Zone 2: A Narrative Review Assessing the Efficacy of Zone 2 Training for Improving Mitochondrial Capacity and Cardiorespiratory Fitness in the General Population
Sports Medicine · doi.org/10.1007/s40279-025-02261-y