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

Abstract

Popular media has recently positioned Zone 2 training-defined as low-intensity exercise below the lactate threshold-as the optimal intensity for improving mitochondrial and fatty acid oxidative capacity, thereby supporting cardiometabolic health and chronic disease prevention. These recommendations largely stem from observational data of elite endurance athletes who engage in large volumes of Zone 2 training and possess high mitochondrial and fatty acid oxidative capacity. However, we challenge the broad endorsement of Zone 2 training for members of the general public, as it contradicts substantial evidence supporting the use of high-intensity exercise for improving mitochondrial capacity and cardiometabolic health. This narrative review critically examines the current evidence on Zone 2 training and mitochondrial and fatty acid oxidative capacity outcomes to assess the appropriateness for a public recommendation. We conclude that current evidence does not support Zone 2 training as the optimal intensity for improving mitochondrial or fatty acid oxidative capacity. Further, evidence suggests prioritizing higher exercise intensities (> Zone 2) is critical to maximize cardiometabolic health benefits, particularly in the context of lower training volumes.