Is Zone 2 Training Overrated? Reviewing the Evidence

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Everybody thinks they know what Zone 2 training is; it’s the most talked-about training intensity in fitness. It’s easy to incorporate, and the benefits are vast. The claim repeated most is that low-intensity work below the first lactate threshold is the optimal way to build mitochondria, improve fat oxidation, and reduce chronic disease risk.

A narrative review published in Sports Medicine in June 2025 set out to test that claim against the evidence. The authors, from Queen's University and McMaster University, concluded that the case has not been made.

The paper addressed two specific claims: that Zone 2 is the optimal intensity for improving mitochondrial capacity, and that it is the optimal intensity for improving fatty acid oxidative capacity. It then asked a third question: whether Zone 2 is optimal for improving health and fitness in the general population, defined as people who are not endurance trained.

The authors did not run a systematic search. They gathered studies using terms including low-intensity exercise, continuous training, lactate threshold, ventilatory threshold and Fatmax, then added papers from reference lists, from social media, and from literature already known to them.

Very few studies have explicitly prescribed Zone 2 as popular media defines it. To assemble an evidence base at all, the authors had to include exercise performed below the first lactate threshold, or exercise showing responses consistent with the moderate-intensity domain: blood lactate under 2.0 mmol/L, below the first ventilatory threshold, below Fatmax, or under 45% of VO2max.

Zone 2 sits just below the first lactate threshold, which is the highest intensity at which lactate production and clearance stay roughly matched. That threshold is highly individual.

The review reports the boundary between the moderate and heavy intensity domains falling anywhere between roughly 24% and 80% of VO2max depending on fitness and training status. In the authors' own laboratory data, three participants reached their first lactate threshold at 23%, 45% and 57% of peak work rate. In absolute terms, Zone 2 can mean walking at a normal pace for a sedentary person and cycling at around 300 watts for an endurance athlete.

The acute signalling evidence was weak and inconsistent. Two hundred minutes of Zone 2 exercise did not change ATP, ADP or AMP in the muscle of untrained adults. AMPK activity, a primary trigger for mitochondrial biogenesis, was unchanged in endurance-trained men after two hours of Zone 2 and only rose at exhaustion at around three and a half hours. PGC-1α gene expression responses were mixed, and appeared to depend on duration, increasing after 60 and 90 minutes but not after 30.

The training studies were also mixed. In support, 12 weeks of low-intensity cycling improved phosphocreatine recovery rates in healthy men and men with type 2 diabetes. Against it, four weeks of Zone 2 training produced no increase in citrate synthase activity or mitochondrial respiration in recreationally active men. Five months of training that was 86% Zone 2, performed seven days a week, failed to improve citrate synthase or succinate dehydrogenase activity in elite endurance athletes. A 42-day ski expedition averaging six hours a day at about 60% of maximum heart rate reduced citrate synthase activity and mitochondrial respiration.

The review also cites a meta-analysis of 56 training studies indicating that exercise performed below 60% of maximum work rate is not expected to improve mitochondrial content or respiratory capacity.

The authors found a single study that measured fat oxidation following training confirmed as Zone 2 by blood lactate. In it, one year of Zone 2 training increased both maximal fat oxidation and the intensity at which it occurred in previously sedentary adults.

Taking the wider literature together, the review concludes Zone 2 does improve fat oxidation capacity, with improvements likely limited to sedentary and untrained populations. Training at intensities above Zone 2 produces the same effect. An older meta-analysis found no effect of low-intensity training on the first lactate threshold, suggesting trained people require higher intensities to move it.

Cardiorespiratory fitness is a stronger and better established predictor of cardiometabolic disease and all-cause mortality than mitochondrial capacity. On that outcome, the comparisons split between no difference and a clear advantage for higher intensities. In healthy active people and trained athletes, fitness improved only at intensities above Zone 2. For a fixed amount of exercise, higher intensities produced greater improvements in fitness and in markers such as glucose tolerance.

The review makes a practical point about guidelines. Public health recommendations classify intensity as light, moderate and vigorous rather than by zones, and the American College of Sports Medicine recommends a minimum of 150 minutes a week of moderate activity while acknowledging that higher intensities may be required to improve fitness. Because Zone 2 covers such a wide absolute range, in an unfit person it can fall below the minimum intensity those guidelines recommend.

Two things worth a look:

The first is the evidence gap the authors point to themselves. The finding is that the claim for Zone 2 being optimal is unsupported, which is a different statement from Zone 2 having been tested and found ineffective. Much of the case against rests on studies the authors judged to be Zone 2 rather than studies designed to test it.

The second is the authors' position in the field. Martin Gibala is among the most published researchers on high-intensity interval training and discloses equity in a company whose services relate to exercise. The review argues a position consistent with that body of work. It is disclosed in the paper and does not undo the evidence, though it is context worth having.

Low-intensity aerobic work is still valuable. It is sustainable, recoverable, and builds a base that supports everything else.

What the review challenges is narrower. There is no good evidence that staying under Zone 2 unlocks adaptations that higher intensities cannot produce, and the instruction to avoid exceeding Zone 2 has no support here. When there's limited time to train, it's better to include more intense workouts instead of avoiding them.

For prescription, the practical definition holds up. The highest aerobic intensity you can sustain while holding a conversation, without fatigue building across the session, gets most people close enough. A specific heart rate or a colour on a watch not so much.

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