Zone 2 Training Explained: Sorting the Trend From the Evidence

Hours of gentle, chatty cardio have been sold in popular media as the best way to build mitochondria, the tiny power plants in your muscles, and to raise your capacity to burn fat. That is the promise behind zone 2 training, and a 2025 review in Sports Medicine by researchers at Queen’s and McMaster universities in Canada found no good evidence that zone 2 is the best intensity for either. Its authors argued that harder efforts matter more for heart and metabolic health when weekly training time is short.¹

Easy cardio is still worth doing. It counts toward your weekly activity, it is gentle enough to repeat often, and it lets you build volume without wearing yourself out. Finding your own easy pace needs no lab: if you can speak in full sentences while breathing faster than at rest, you are probably there, and if your sentences break into short phrases, slow down. A heart-rate zone on a watch is a rougher guide than it looks, and the label “zone 2” means different efforts in different training plans.

What is zone 2 training?

Zone 2 training, in its popular sense, is continuous exercise at an easy, conversational intensity at or just below your first metabolic threshold: the point where blood lactate first rises above its resting level and breathing starts to quicken. The 2025 joint statement by the American College of Sports Medicine (ACSM) and Exercise and Sport Science Australia (ESSA) describes effort below that threshold as easy or low-intensity training.²

The threshold matters because the body’s response changes there: oxygen uptake, blood lactate and the causes of fatigue behave differently on either side of it, the same statement explains. Labs locate it with blood samples or breath-gas measurements during a test that gets harder in steps, which takes expensive equipment, time and expertise.²

On a bike ride with a friend, the threshold is roughly where easy chat on the flat turns into one-word answers on a steady climb. Below that point the ride feels comfortably steady; past it, your breathing climbs and talking takes effort.

The idea came from elite endurance sport. According to the 2025 review, the case for zone 2 rests largely on observational data from elite athletes, who do large amounts of easy training and have an unusually high capacity to use fat and oxygen.¹ Few office workers can match those training hours, so a mix that suits an athlete may not suit a few free hours a week.

How do I know if I’m in zone 2?

The simplest guide is the talk test: if you can speak in full sentences comfortably, you are probably at or below your first threshold. A 2014 review in Current Opinion in Cardiology concluded that comfortable speech is likely possible below the ventilatory or lactate threshold, and not likely possible above it, for healthy adults and for people with heart disease alike.³

Speech works as a gauge because the change in breathing near the threshold affects how easily you can talk. The ACSM and ESSA statement names its limits: comfortable talk can end before the first threshold in inactive people, and can continue past it in well-trained athletes. Used conservatively, it suits keeping exercise out of the hard range when no lab test is available, though the statement finds little evidence that it places most people accurately in a training zone.²

Heart-rate zones look more precise and often are not. The American Heart Association (US) estimates maximum heart rate as 220 minus your age, puts moderate effort at about 50 to 70 percent of that, and calls the figures averages to use as a general guide.⁴ For a 40-year-old, that works out to roughly 90 to 126 beats a minute, a wide band. A 2001 analysis pooling hundreds of studies found that the age formula underestimates maximum heart rate in older adults.⁵

Even a measured maximum does not settle it, because fixed percentages place different people at different metabolic intensities. In one older study cited by the ACSM and ESSA statement, at 80 percent of maximum heart rate about half the participants were above their first threshold and half below.² A watch zone can therefore sit on the wrong side of your threshold, and when it disagrees with your breathing, your breathing is the better guide.

Some medicines change the picture entirely. Beta blockers, prescribed for conditions such as angina, high blood pressure and atrial fibrillation, work partly by slowing the heartbeat.⁶ The American Heart Association notes that some medicines lower maximum heart rate and the target zone, and advises anyone with a heart condition or on medication to ask a health professional what their heart rate should be.⁴

Is this pace easy enough?

• I can speak in full sentences without gasping
• My breathing is faster than at rest but steady
• I could keep this pace for much longer than I plan to
• My watch zone agrees with the talk test, or I trust the talk test

In practice, start each session at a pace where talking is easy, and test it every few minutes by saying a full sentence aloud or chatting with a training partner. When sentences start breaking into phrases, ease off; on a hilly run, that may mean walking the climbs.

Why your zone 2 and your coach’s may differ

Zone numbers are labels, and different systems use them for different efforts. The three-zone model common in sport science, described in a 2024 meta-analysismeta-analysis: A study that combines the results of earlier studies on the same question into one overall estimate. Pooling makes the estimate more precise, but it cannot repair the studies it pools: a meta-analysis of surveys is still survey evidence.Full entry in the glossary of athlete training studies, puts the first zone below the first threshold and the second zone between the first and second thresholds.⁷ The popular version, as the 2025 review defines it, is low-intensity exercise below the lactate threshold, which the three-zone model would call its first zone.¹

The ACSM and ESSA statement makes the same point in general terms: training zones are often numbered, but many different schemes exist, and inconsistent intensity terms hamper research and confuse recommendations. Its authors propose five plain labels instead, running from very low intensity up to very high.²

A polarized plan that keeps the middle zone to a small slice of the week and a podcast urging hours of easy zone work can both be right, because they mean different efforts. Before you follow any zone-based plan, check which model it uses and where each zone sits relative to the first threshold.

Is zone 2 training better than high-intensity training?

For building mitochondria, the best current reviews find no advantage for easy training over harder work, and harder work gets more done per hour. The zone 2 claim itself was examined head-on in the 2025 narrative review by Kristi Storoschuk and colleagues.¹

The study

Moderate evidence

Much ado about zone 2: Storoschuk and colleagues, Sports Medicine 2025

Four researchers in Canada reviewed the studies behind the claim that zone 2 is the best intensity for the general public. They concluded that current evidence does not support it as the optimal intensity for improving mitochondrial or fatty acid oxidative capacity, and that prioritizing intensities above zone 2 matters for heart and metabolic health, particularly when training volume is low. It is a narrative reviewnarrative review: A review in which an expert summarizes a body of research in prose, choosing which studies to discuss. It can be a good map of a field, but without a stated search and pooling method it is easier for selection and opinion to shape the conclusions than in a systematic review.Full entry in the glossary, a less formal design than a systematic review, and one author, Martin Gibala, advises and holds equity in an exercise-related company, Longevity League.¹

A larger pooled analysis points the same way. A 2025 systematic reviewsystematic review: A review that fixes its question and its rules for including studies in advance, then searches out every study that fits and weighs them together. Some systematic reviews pool the results into a meta-analysis; others describe what the studies found without combining the numbers.Full entry in the glossary and meta-regression by Knut Sindre Mølmen and colleagues combined training studies with about 6,000 participants in all. Mitochondrial content rose by a similar amount with low- or moderate-intensity continuous training and with high-intensity intervals, and per hour of exercise the hardest interval formats were linked to the largest gains. These are comparisons across separate studies, so they show associations. The authors call total training load, how long multiplied by how hard, the most suitable predictor of change, conclude that higher intensities can compensate for lower volumes and the reverse, and found that the least fit improved most.⁸

So the trade-off is time against effort. A parent with three 20-minute gaps in the week may get more change from making one or two of them hard, if their health allows. A retiree who enjoys long, gentle bike rides, or anyone who cannot or should not train hard, may build similar adaptations with more easy hours.

Does zone 2 training burn more fat?

During a session, the rate at which you burn fat does peak around easy to moderate effort, but fat loss over weeks depends far less on intensity. A 2004 review by Juul Achten and Asker Jeukendrup describes fat-burning rates rising from low to moderate intensity and falling again as effort becomes high.⁹

That pattern is the grain of truth behind the “fat-burning zone” on gym machines and watch screens. The fuel mix during one workout says little about body fat weeks later. A 2017 meta-analysis of trials in younger adults with overweight or obesity found that high-intensity intervals and moderate continuous training both brought modest reductions in body fat, with neither clearly ahead, while the interval programs took about 40 percent less training time.¹⁰

Myth

You have to stay in zone 2 to lose fat.

Fact

Fat burning peaks at easy to moderate effort during a session, but in trials of adults with overweight or obesity, interval training reduced body fat about as much as steady moderate training.

If fat loss is your aim, choose the format you will keep doing week after week; in these trials, steady moderate sessions and intervals did about equally well.

Why athletes do most of their training easy

Easy training earns its place because it lets people train often without wearing themselves out. Stephen Seiler, a physiologist who studies elite endurance athletes, summarized descriptive studies in a 2010 review: competitive endurance athletes who train many times a week typically do about 80 percent of their sessions at low intensity, with the rest dominated by hard intervals.¹¹ He suggests the mix balances training signals against an acceptable level of stress.

Controlled trials in athletes support the mix more cautiously. The 2024 meta-analysis of athlete studies found slightly larger gains in peak oxygen uptake with polarized training, mostly easy work plus some very hard work, than with other distributions, pooling randomized and non-randomized studies. The advantage appeared mainly in shorter programs and in highly trained athletes, and time-trial results and other performance measures improved about equally.⁷

For a desk worker, the logic that transfers from these athlete studies is recovery: easy sessions are cheap to recover from, so they let you move on more days of the week. Regular moderate exercise is also linked to feeling less tired, as trials pooled in the guide to exercise and everyday energy levels show. If an easy session leaves you too tired to train the next day, slow the pace next time.

How many minutes of zone 2 per week?

The WHO sets no target for zone 2 as such. Its 2020 guidelines recommend that adults build up 150-300 minutes of moderate-intensity activity each week, or half as much vigorous activity (75-150 minutes), or a mix of the two, plus strength exercises for the major muscles twice a week or more.¹²

Easy, conversational cardio usually falls within the moderate band: the UK’s NHS describes moderate activity as raising your heart rate and breathing, with talking still possible but singing not.¹³ Talking-pace minutes therefore count toward the weekly total, and the guide to weekly exercise guidelines and the talk test covers what else counts. The NHS also notes that vigorous activity brings similar benefits in about half the time, which is the time argument for adding some harder work.¹³

Adapt the shape to your body and diary: someone with knee trouble might swap jogging for cycling or swimming, and a person short of time might shorten two easy sessions and keep the harder one.

Heart conditions, beta blockers, pregnancy and warning signs

Under the WHO’s 2020 guidelines, people without contraindications generally need no medical clearance to take up light or moderate activity no more demanding than brisk walking, and the guidelines advise anyone who develops new symptoms as they increase activity to consult a healthcare provider.¹² The UK’s NHS adds that a GP visit makes sense beforehand for anyone coming back after a long spell without exercise, or living with a medical condition or worry.¹³ In other countries, a doctor or nurse can answer the same questions.

• Now: ring your local emergency number (911 is the US example) for chest pain that is sudden and crushing, squeezing or tight, that spreads to your jaw, left arm or the area between your shoulder blades, or that comes with nausea, sweating, dizziness, a racing heart or shortness of breath, as MedlinePlus advises.¹⁴
• Soon: see a healthcare provider if new symptoms appear as you build up your training, as the WHO advises.¹²
• Before you start: with a heart condition, or if you take medicines that slow the heart such as beta blockers, ask your doctor what effort and heart rate suit you, since standard zones may not apply.⁴ In pregnancy, the UK’s NHS says exercise is safe, recommends keeping to moderate effort where talking is possible but singing is not, advises against starting new vigorous activity such as running, and suggests speaking to your midwife if you have a known health condition.¹⁵

The bottom line

Zone 2 is a sensible way to spend the easy hours of your week, and the 2025 review found no good evidence that it beats harder work for mitochondria or fat-burning capacity. Keep easy sessions at a pace where you can talk in full sentences, count them toward the WHO’s weekly minutes, and if your time is short and your health allows, swap one easy session for a few harder efforts.