Zone 2 cardio, the slow, steady, "conversational" pace beloved of longevity podcasters and fitness influencers, may not deserve its reputation as the optimal exercise for building healthier mitochondria and better metabolic health. That is the conclusion of a narrative review titled "Much Ado About Zone 2," published in the journal Sports Medicine in July 2025.
The authors, based at Queen's University and McMaster University in Canada, examined the evidence on Zone 2 training, defined as low intensity exercise below the lactate threshold, and found that it produces real aerobic benefits but is not uniquely powerful. When researchers compare it head to head with higher intensity work, harder efforts tend to win on the mitochondrial adaptations that Zone 2 is credited with.
What the review actually looked at
The team, led by first author Kristi L. Storoschuk with corresponding author Brendon J. Gurd of Queen's University, assessed whether Zone 2 uniquely improves mitochondrial capacity, the ability of muscle cells to burn fuel aerobically, and cardiorespiratory fitness in the general population. Their bottom line, drawn from the PubMed abstract of the review, is that current evidence does not support Zone 2 as the optimal intensity for improving either mitochondrial or fatty acid oxidative capacity.
For elite endurance athletes who log very high weekly volumes, low intensity work still has a clear role. For the rest of the population, especially people short on time, the authors argue that prioritizing higher intensities than Zone 2 is critical to maximize cardiometabolic health benefits.
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Why Zone 2 got oversold
Zone 2 became a viral fitness trend on the back of claims that easy aerobic training uniquely stimulates mitochondrial growth and fat burning. The review challenges that framing, noting that Zone 2 itself lacks a single, standardized definition and that different testing methods can put the target intensity in meaningfully different places for the same person.
The authors also flag a common misconception. Burning a larger share of fat during a slow workout does not automatically translate into a better ability to burn fat over 24 hours. Mitochondrial adaptations, according to the review, generally require an intensity high enough to challenge current aerobic capacity, and much of what people call Zone 2 sits below that threshold.
What the guidelines still say
None of this means low intensity movement is wasted. Walking, easy cycling and slow jogging still contribute to weekly activity totals and can be done more often with less recovery cost. The American Heart Association recommends at least 150 minutes per week of moderate intensity aerobic activity, 75 minutes of vigorous activity, or a combination of the two, plus muscle strengthening on two or more days.
The practical takeaway from the review is not that Zone 2 is useless. It is that for a healthy adult trying to build fitness on a limited schedule, sprinkling in harder efforts, intervals or vigorous continuous work, is likely to deliver more bang for the minute than staying exclusively in the easy lane. People with heart disease, high blood pressure or other medical conditions should talk with their doctor before ramping up exercise intensity.
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This article is made and published by Jesper Bengtson, who may have used AI in the preparation.