Article breakdown

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 (Auckland, N. Z.) • 2025 Jul • Vol 55, 1611-1624. PMID 40560504.

This review argues that current evidence does not show Zone 2 exercise is the best intensity for improving the body’s energy-producing capacity. The authors say the popular push for Zone 2 in the general public leans too much on observations from elite endurance athletes. Based on the research they discuss, higher-intensity exercise may be more important for maximizing heart, lung, and metabolic benefits, especially when people do not exercise for long durations.

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In short

What this paper found

This review argues that current evidence does not show Zone 2 exercise is the best intensity for improving the body’s energy-producing capacity.

The authors say the popular push for Zone 2 in the general public leans too much on observations from elite endurance athletes.

Based on the research they discuss, higher-intensity exercise may be more important for maximizing heart, lung, and metabolic benefits, especially when people do not exercise for long durations.

Deep dive

What the paper is actually saying

Why This Paper Exists

Zone 2 training has been widely promoted as the best exercise intensity for building mitochondrial function and supporting long-term metabolic health, but the authors say this message may be overstated for everyday adults.

What The Authors Wanted To Answer

The authors wanted to assess whether existing research really supports recommending Zone 2 training as the optimal exercise intensity for improving mitochondrial capacity, fat oxidation capacity, and cardiometabolic health in the general population.

How The Study Was Done

This is a narrative review, meaning it summarizes and interprets earlier research rather than reporting one new experiment. The abstract says the authors critically examined available evidence on Zone 2 training and outcomes related to mitochondrial and fatty acid oxidative capacity.

What The Study Found

The review concludes that current evidence does not support Zone 2 training as the best intensity for improving mitochondrial or fatty acid oxidative capacity. The authors also state that evidence favors giving greater priority to exercise above Zone 2 to maximize cardiometabolic benefits, especially when total exercise volume is low.

Takeaway

For the general public, this review does not support treating Zone 2 as the single best exercise intensity. If exercise time is limited, higher-intensity training may offer stronger health gains according to the evidence the authors reviewed.

Limits

What this abstract does not fully answer

This paper is a narrative review, so it summarizes prior research rather than presenting one new controlled experiment.

The abstract does not describe exactly how studies were selected or evaluated, so the review methods are not clear from the abstract alone.

The authors’ argument against broad public recommendations appears to rely partly on the point that much enthusiasm for Zone 2 comes from observational data in elite athletes, which may not directly apply to the general population.

Key numbers

Numbers the abstract makes important

> Zone 2

The authors specifically argue that exercise intensities above Zone 2 may be important for maximizing cardiometabolic health benefits.

Source abstract

Original abstract sections

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.