Zone 2: the “ideal” intensity? Myth or scientific reality

Popularized over 15 years ago by Seiler, Zone 2 training has become one of the most widely used terms in discussions about training and health in recent years. Promoted by influential figures in the media and podcasts, this exercise intensity is presented as THE method of choice for improving mitochondrial capacity, optimizing fat oxidation, and, as a result, preventing chronic diseases. The arguments put forward are often based on observations of the practices of high-level endurance athletes: considerable amounts of moderate-intensity work, accompanied by exceptional performance and enviable metabolic health.

But can this logic really be applied to the general population? The reasoning has its limits. First, athletes combine this low-intensity work with high-intensity sessions for a fairly high total training volume, often exceeding 20 hours per week, which far exceeds the standard physical activity recommendations for the general public. Second, the physiological benefits they achieve may depend more on the combination of intensities than on the amount of time spent in Zone 2 alone.

At the same time, exercise physiology research has extensively documented the superior effects of high-intensity training on mitochondrial adaptations, cardiorespiratory fitness (VO₂max), and numerous cardiometabolic markers. In this case, is Zone 2 really the ideal training intensity for boosting mitochondrial health and burning more fat? Or are its effects overestimated compared to other more intense methods?

The study

In a narrative review, Canadian researchers analyzed the literature to determine whether this intensity was truly optimal for improving mitochondrial capacity, fat oxidation, and overall health. Zone 2 corresponds to exercise below the first lactate threshold, i.e., a blood lactate concentration below ~2.0 mmol/L, often close to the maximum fat consumption intensity (Fatmax), and corresponding to moderate effort that allows for easy conversation.

Not to be confused with zone 2 of the 3-zone model based on ventilatory thresholds. In this case, the zone we are discussing here would correspond to zone 1, below the first ventilatory threshold.

The authors included studies conducted on untrained or simply active individuals who were not endurance specialists and who met these intensity criteria. The parameters studied were:

• Intracellular signals and signaling pathways involved in mitochondrial biogenesis (AMPK, CaMKII, PGC-1α, etc.).
• Structural and functional adaptations: mitochondrial enzyme activity, mitochondrial respiration, phosphocreatine recovery, capillary density, enzymes involved in lipid metabolism.
• Effects on fat oxidation capacity.
• Impacts on cardiorespiratory fitness and cardiometabolic health markers.

In the absence of numerous studies explicitly designed to test Zone 2, the authors broadened their analysis to include equivalent protocols, while excluding significantly higher or lower intensities.

Results

Effects on mitochondrial signaling

Available data show that Zone 2 exercise generally induces modest or no changes in the AMP/ADP:ATP ratio, an indicator of muscle energy stress and a key trigger for AMPK activation. This low energy stimulation results in limited or no activation of signaling pathways leading to mitochondrial biogenesis. The observed increases in PGC-1α (the master regulator of this biogenesis) are inconsistent and appear to depend on duration. Sessions lasting 60 to 90 minutes can induce a response, whereas 30 minutes do not…

Regarding the calcium signaling pathway (CaMKII), often put forward as a mechanism specific to low intensities, evidence is scarce and contradictory. Some studies show no activation, while others report effects on downstream targets. Overall, the signals triggered by Zone 2 appear weaker and less consistent than those observed at higher intensities.

Measured mitochondrial adaptations

The results are mixed. A few studies indicate improvements in mitochondrial enzyme activity (citrate synthase, mitochondrial respiration) or phosphocreatine recovery after several weeks of Zone 2 training, including in people with type 2 diabetes. Others report no changes, even after several months or very high volumes. A major finding in the literature is that, according to previous meta-analyses, intensities below approximately 60% of maximum aerobic power are not expected to induce significant gains in mitochondrial content or capacity in untrained subjects.

In contrast, high-intensity training activates mitochondrial signals more strongly, increases the expression of genes such as PGC-1α more rapidly, and improves enzyme activity more markedly, even in high-level athletes.

Effects on fat oxidation capacity

Zone 2 is frequently associated with the stimulation of lipid oxidation, as it often coincides with the intensity of Fatmax. In sedentary, obese, or diabetic individuals, several studies show an increase in this fat oxidation capacity after Zone 2 programs: improvement in Fatmax, increase in maximum fat oxidation rate, and decrease in respiratory quotient during exercise.

However, these effects are not specific to Zone 2: higher-intensity training also improves FAO, sometimes even more, particularly by increasing the mitochondrial and enzymatic capacities related to lipid oxidation. Direct comparisons between intensities yield mixed results: some favor Zone 2, others favor higher intensities, and many show no clear difference. And it often comes down to personal preference when it comes to the intensity of workouts.

Impact on cardiorespiratory fitness

On this point, the findings are clear: gains in VO₂max/peak and aerobic performance are greater when intensity exceeds Zone 2, at equal volume. In untrained individuals, Zone 2 can improve CRF, but the effects are equivalent to or less than those of higher intensities. In people who are already active or trained, Zone 2 alone is generally not sufficient to increase VO₂max.

Analysis

The authors highlight a significant discrepancy between popular discourse around Zone 2 and current scientific evidence. While there is no doubt that this intensity is beneficial, particularly for developing basic endurance, promoting lipid regulation, and allowing for high training volume with a low risk of excessive fatigue, there is no evidence that it is superior to higher intensities for improving mitochondrial capacity, fat oxidation capacity, or cardiorespiratory fitness.

The essential nuance is contextual: for endurance athletes, Zone 2 is a strategic tool for accumulating considerable volume while preserving recovery, but always combined with more intense sessions. In sedentary or inactive individuals, it can be an accessible and effective starting point, but it appears that gradually adding higher intensities maximizes the benefits.

Another key point is the lack of research directly targeting Zone 2 as defined in popular discourse. Much of the work cited uses similar intensities or protocols that have not been verified by lactate measurements. This lack of clarity makes it difficult to definitively validate the claims.

Practical applications

For the general public, Zone 2 is a comfortable, accessible intensity that is conducive to establishing regularity. It allows you to increase your weekly activity volume, work on technique or posture, and support metabolic health, especially in people who are overweight, diabetic, or unfit.

However, to improve mitochondrial capacity, fat oxidation capacity, and especially cardiorespiratory fitness (VO₂max/peak), it is recommended to incorporate efforts above Zone 2: moderate-high intensity sessions, high-intensity intervals, or even short sprints for individuals who are able to do so. These more intense stimuli activate molecular pathways and structural adaptations that Zone 2 alone does not fully trigger.

In summary, Zone 2 is useful, but not the ultimate solution. It should be seen as one component of a varied program, not the only intensity to aim for. The benefits are maximized by combining several intensities, depending on the goals, level, and constraints of each individual.

Reference

Storoschuk, K. L., Moran-MacDonald, A., Gibala, M. J., & Gurd, B. J. (2025). 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, 55(7), 1611–1624.