Zone 2 Is Having a Moment. Here’s What the Evidence Actually Supports.

Searches for “zone 2 training” rose more than 850 percent between 2020 and 2023, pushed into the mainstream by podcasts featuring sports scientists and longevity figures. What was once a quiet foundation of elite endurance training became a wellness trend. It is now recommended to recreational exercisers as the single most important intensity to train. As with most things that move that fast, some of the enthusiasm is well founded and some has outrun the evidence.

This article covers what Zone 2 actually is, what it reliably does, where the 2025 research complicated the popular narrative, and how it fits into a training week for someone who is not a full-time athlete.

What Zone 2 Actually Is

Zone 2 is low-intensity aerobic exercise performed just below the first lactate threshold, LT1. In practice it is the effort at which you can still hold a conversation in full sentences. Breathing is controlled, and you could sustain the pace for a long time. Blood lactate sits around 1.5 to 2.0 mmol per litre. That is roughly a 3 to 4 out of 10 perceived effort, often near 60 to 70 percent of maximum heart rate, though the percentage varies widely between individuals.

The precision matters more than it sounds. A 2025 analysis found a problem with this. The common ways of defining Zone 2, by heart rate, perceived exertion, or ventilation, point to meaningfully different absolute intensities. Blood lactate below 2 mmol per litre is the most physiologically precise definition. But most people estimate the zone through heart rate formulas that can be well off. The familiar “220 minus age” is frequently inaccurate for a given individual, which means many people training “in Zone 2” by a formula are not actually there.

What It Reliably Does

The physiological case for aerobic base work is old and solid. Sustained low-intensity training improves several things at once: the heart’s stroke volume, capillary density in muscle, and the muscle’s capacity to use oxygen. It builds the aerobic engine that higher-intensity work sits on top of, and it does so at a low recovery cost. This is why endurance athletes have used high volumes of it for decades, under names like “base training” and “long slow distance.”

The concept predates the current branding. A German physician, Ernst van Aaken, built a low-intensity, high-volume training philosophy in the mid-20th century that he called Long Steady Distance. The physiologist Iñigo San Millán later elaborated on the specific intensity now called Zone 2. He argued it was an especially effective stimulus for mitochondrial function. The training worked long before it had a zone number, which is worth remembering when the number itself starts to feel like the point.

2025 Research Complicated the Story

The popular narrative made a stronger claim than the base-training tradition ever did: that Zone 2 is the optimal intensity for building mitochondria and improving metabolic health. A July 2025 narrative review in Sports Medicine, by Storoschuk and colleagues, examined that specific claim and reached a more skeptical conclusion.

The review found that current evidence does not support Zone 2 as the optimal intensity for mitochondrial or fat-oxidation capacity. Higher intensities appear to produce larger mitochondrial and metabolic responses, and this matters most at the training volumes most people realistically achieve. The mechanism is worth understanding: mitochondria adapt when they are stressed beyond their current capacity, not when they are working comfortably within it. Burning a high proportion of fat during easy exercise, often cited as a Zone 2 benefit, does not by itself drive the adaptation people are training for.

The review did not dismiss Zone 2. It challenged its promotion as the single best intensity. For people training fewer than about six hours a week, it noted, prioritizing some higher-intensity work is likely more effective than filling the week with easy aerobic sessions alone. The reviewers raised a separate point too. Much of the research cited in support of Zone 2 actually studied moderate-intensity work above true Zone 2, so its findings cannot be cleanly attributed to Zone 2 itself.

How It Fits for a Non-Elite Trainee

The resolution here is not “Zone 2 is overhyped” any more than it is “Zone 2 is the answer.” It is that Zone 2 is one effective component of a well-built week, not the whole thing, and its relative value depends heavily on how much someone trains.

For an elite endurance athlete training long hours, a large share of easy aerobic volume is genuinely optimal. They have the time to accumulate it, and they need to manage the recovery cost of their hard sessions. For a recreational trainee with three or four hours a week, spending nearly all of it in Zone 2 leaves adaptation on the table. Higher-intensity work would capture it more efficiently. A practical structure for the time-limited trainee is a mix: two to three easy aerobic sessions, one to two higher-intensity sessions, and two to three resistance sessions across the week.

The summary is simple. Zone 2 does real things, builds a real base, and belongs in most training weeks. It is just not a substitute for the harder work, and for people short on training time, it should not crowd that harder work out.

References

1. Storoschuk KL, Moran-MacDonald A, Gibala MJ, Gurd BJ. 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 Med. 2025;55(7):1611-1624. DOI: 10.1007/s40279-025-02261-y. PMID: 40560504
2. Inglis EC, et al. Heavy-intensity and high-intensity interval training produce superior adaptations to Zone 2 training in matched-work protocols. 2024.
3. San Millan I, Brooks GA. Assessment of metabolic flexibility by means of measuring blood lactate, fat, and carbohydrate oxidation responses to exercise in professional endurance athletes. Sports Med. 2018;48(2):467-479.
4. Coates AM, et al. Incongruent aerobic training intensity classification affects study interpretation. Sports Med. 2023. DOI: 10.1007/s40279-023-01938-6
5. Seiler S. What is best practice for training intensity and duration distribution in endurance athletes? Int J Sports Physiol Perform. 2010;5(3):276-291. PMID: 20861519

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