What the evidence actually says about Zone 2

A 2025 narrative review in Sports Medicine went looking for evidence that Zone 2 training improves mitochondrial capacity, and concluded that most of the available evidence argues against it. The authors described their finding as a refutation of the popular media narrative. The training itself is not the problem. The claims attached to it are.

Does Zone 2 training build mitochondria?

The review, by Storoschuk, Moran-MacDonald, Gibala and Gurd, published in Sports Medicine in 2025, assessed Zone 2 specifically for mitochondrial capacity and cardiorespiratory fitness in the general population.

They found small and inconsistent activation of mitochondrial biogenic signalling after Zone 2 exercise, with some studies showing improvement, others showing no effect, and some showing decreases. A meta-analysis cited in the review indicates that exercise below roughly 60% of maximum work rate is not expected to increase mitochondrial content, and Zone 2 sits below that line for most people.

The proposed mechanism explains why. Mitochondrial biogenesis is driven substantially by metabolic stress, via the AMP to ATP ratio and AMPK signalling. Zone 2 produces minimal disturbance to that ratio. Higher intensities produce more.

Why is Zone 2 so widely recommended then?

Three reasons, and the review names all of them.

The first is an attribution error. Elite endurance athletes do spend roughly 80% of training in low intensity, and they have exceptional mitochondrial capacity. But their remaining 20% of high-intensity work can exceed the total weekly training volume of a recreational athlete. Crediting the adaptation to the 80% ignores the 20%.

The second is definitional. Zone 2’s upper boundary ranges from about 24% to 80% of VO2 max depending on fitness level, which means two people following “Zone 2” can be doing physiologically different things. A 2025 paper testing multiple definitions found they pointed to meaningfully different absolute intensities.

The third is that much of the cited research studied moderate-intensity continuous training sitting above Zone 2, not Zone 2 itself.

None of this makes easy aerobic work useless. It is sustainable, it accumulates volume, and it is low risk, and polarised training models still allocate most volume to low intensity for good reason. The review’s conclusion is narrower and more specific: prioritising higher intensities is unlikely to jeopardise mitochondrial benefits. Where an individual’s own zones actually fall is a question about their physiology, not the label.

The bottom line

• A 2025 Sports Medicine review concluded that most available evidence argues against Zone 2 training as a driver of increased mitochondrial capacity.
• Mitochondrial adaptation appears to require metabolic stress that Zone 2, by definition, largely avoids.
• Zone 2’s upper boundary spans roughly 24% to 80% of VO2 max across fitness levels, so the label describes very different efforts in different people.