Zone 2 Isn't the Longevity Magic Bullet. The Honest Read.

A patient in his early forties came in last week with a Garmin watch, a spreadsheet, and a problem. He had been doing 45 minutes of Zone 2 cycling, four times a week, for nearly a year. His resting heart rate was lower. He felt fitter on the bike. His VO2max on a CPET in clinic had moved by less than 2 ml/kg/min. He looked at me and said, "Doctor, what am I doing wrong?"

The answer is that he is not doing anything wrong, exactly. He is doing one of the right things, by itself, for too long, and missing the other one entirely. Zone 2 has had an extraordinary run in the longevity world over the last three years. Every podcast, every Reels caption, every supplement company sponsoring those podcasts will tell you that easy, conversational, fat-burning, mitochondria-building Zone 2 is the secret to staying alive longer. It is not wrong. But it is not the whole picture, and a narrative review published in Sports Medicine in mid-2025 is the cleanest, calmest pushback we have so far. I want to walk you through what it actually says, and what to do with it on a Saturday morning in Singapore.

What Zone 2 Actually Is

Before we argue about it, we should agree on what we are arguing about. Zone 2, in physiology rather than Instagram, is the intensity at which you are still burning mostly fat, your blood lactate is sitting at roughly 1.7 to 2.0 mmol/L, and you can hold a conversation in full sentences but not sing. In a treadmill or cycling test, this usually corresponds to 60 to 70 per cent of your maximum heart rate, give or take. For most people aged 40 to 60, that lands somewhere around 110 to 135 beats per minute. The classic prescription, popularised by Iñigo San Millán and Peter Attia, is three to four sessions of 45 to 60 minutes per week.

The biological argument for Zone 2 is reasonable on paper. At this intensity you preferentially train type 1 muscle fibres, the slow-twitch, oxidative ones, which are densely packed with mitochondria. The theoretical hope is that, over months, you build mitochondrial volume, improve fat oxidation, lower fasting insulin, and improve metabolic flexibility. All of which would, theoretically, slow biological ageing.

The problem is that "theoretical" has been doing a lot of heavy lifting in this conversation. Most of the data behind the popular Zone 2 prescription is observational data from elite endurance athletes. Tour de France cyclists do enormous volumes of Zone 2 because they cannot recover from anything else. Saying you should do what they do is a bit like a pilot saying everyone should fly Cessnas because that is what flight school graduates train in.

The 2025 Sports Medicine Review

In June 2025, Kristi Storoschuk, Andres Moran-MacDonald, Brendon Gurd at Queen's University and Martin Gibala at McMaster, four exercise physiologists who collectively have spent careers measuring mitochondria in human muscle biopsies, published Much Ado About Zone 2 in Sports Medicine. It is a narrative review, not a meta-analysis, and they say so up front. But the question they asked is the right one. Is there evidence that Zone 2 is the optimal exercise intensity for the general public, the way the popular discussion implies?

Their answer, after working through the published controlled trials in untrained and recreationally active adults, is no. Three points stood out for me as a clinician.

First, when total work is matched, higher intensity matches or beats Zone 2 for mitochondrial adaptation. If you compare two training programmes that produce the same total energy expenditure, the higher-intensity programme produces equal or larger increases in mitochondrial content and oxidative enzymes. The volume-matched comparisons are the ones that matter, because volume by itself is a confounder. If Zone 2 looks better in a casual reading, it is often because the Zone 2 group simply spent more time exercising than the comparator group did.

Second, lower-volume, higher-intensity work produces a larger fitness response per minute. The clearest example is the Norwegian 4-by-4 protocol developed at NTNU by Jan Helgerud and colleagues. Four bouts of four minutes at 85 to 95 per cent of maximum heart rate, with three minutes of active recovery between them, performed three times a week, produced approximately a 7 to 9 per cent rise in VO2max in moderately trained adults in eight weeks. The matched continuous-training arm in the same trial produced a much smaller increase. A 2024 umbrella review in the Scandinavian Journal of Medicine and Science in Sports by Eric Poon and colleagues at the Education University of Hong Kong, pulling together systematic reviews of high-intensity interval training, came to the same place. HIIT produces consistently larger or equivalent VO2max gains compared with moderate-intensity continuous training, in roughly half the time on the bike or treadmill.

Third, Zone 2 may still earn a place in your week, but not as the only thing in it. The review does not say Zone 2 does not work. It says Zone 2 alone, at the doses people are doing it, is not the optimal way to build cardiorespiratory fitness or mitochondrial capacity in someone who is not already a competitive endurance athlete. The most efficient programme for the general public, the one with the largest fitness benefit per minute spent, includes some genuinely hard intervals.

Why VO2max Is the Number Worth Chasing

If you are going to choose a single number to chase from your exercise programme, it should not be your weekly Zone 2 minutes. It should be your VO2max, and there is one paper that explains why better than any other.

In 2018, Kyle Mandsager and colleagues at the Cleveland Clinic published the largest single-centre cardiorespiratory fitness mortality study to date in JAMA Network Open. They followed 122,007 adults who had undergone a clinical exercise treadmill test for a median of 8.4 years. They divided participants into five fitness categories, from low through elite, and tracked all-cause mortality. The relationship was steep, dose-dependent, and had no upper limit of benefit.

Moving from the lowest 25 per cent of fitness to merely below average roughly halved the ten-year mortality risk. Moving from below average to elite roughly halved it again. The hazard of low cardiorespiratory fitness was larger than the hazard of smoking, larger than the hazard of diabetes, larger than the hazard of end-stage renal disease in their cohort. There was no signal of harm at the highest fitness levels. The fitter you are, the longer you live, full stop.

VO2max is the maximum rate at which your body can take in oxygen, transport it through your blood, and use it in your mitochondria during exercise. It is reported in millilitres of oxygen per kilogram of body weight per minute. A sedentary 50-year-old man might be at 25 to 30 ml/kg/min. A reasonably active 50-year-old might be at 38 to 42. An athletic 50-year-old might be at 50 plus. Each step up the ladder is associated with a meaningful drop in mortality. Critically, you cannot move VO2max much without including intensity.

The Singapore Reality Check

Here is where the Sports Medicine review meets the local context. The Singapore Physical Activity Guidelines, jointly issued by the Health Promotion Board and Sport Singapore in 2022, recommend 150 to 300 minutes per week of moderate-intensity activity, or 75 to 150 minutes per week of vigorous-intensity activity, plus muscle-strengthening activity on at least two days. Vigorous activity counts double for the time floor. The Guidelines also recommend breaking up sedentary time across the day.

The honest reading of the local data is that most Singaporean adults are not yet meeting the guideline. The HPB's own surveys have estimated that around three in ten adults in Singapore are not sufficiently physically active by these criteria. For that majority, the question of Zone 2 versus high intensity is the wrong question. The first job is to move from "almost nothing" to "anything", reliably, on most days of the week. A daily 30-minute brisk walk is more useful, more sustainable and more impactful than an idealised but unrealistic interval programme that will be abandoned by week three.

For the second group, the people who already exercise three to five days a week and want their training to actually move their VO2max and their longevity numbers, the Sports Medicine review changes the prescription. It is not "stop doing Zone 2". It is "stop doing only Zone 2".

What I Now Tell Patients to Do

The answer that fits the evidence is boring, time-efficient, and looks roughly the same whether you live in Tampines or Tanglin.

1. Aim for the 80/20 split, not the 100/0

The classic polarised training model, the one used by most endurance athletes and increasingly by the cardiology rehab world, is roughly 80 per cent of weekly aerobic time at low to moderate intensity (which is your Zone 2 or a brisk walk), and 20 per cent of weekly time at genuinely high intensity. For someone who can give exercise four hours per week, that is roughly three sessions of 50 to 60 minutes of Zone 2 or brisk walking, plus one session of 20 to 30 minutes that contains real intervals. Not three sessions of Zone 2 and one session of "slightly faster Zone 2". The intensity has to be uncomfortable for it to count.

2. Do one real interval session a week

The simplest evidence-based interval session, and the one I recommend most often in clinic, is the Norwegian 4-by-4. Warm up for ten minutes. Then four bouts of four minutes at roughly 85 to 95 per cent of your maximum heart rate (or "I cannot speak in full sentences"), with three minutes of easy recovery in between. Cool down for five minutes. Total time roughly 35 to 40 minutes. Once a week is enough to start. If your knees do not love running, do this on a stationary bike, the elliptical, the rowing machine, or up a hill. The mode matters less than the intensity.

If 4-by-4 sounds intimidating, a gentler entry is a 1-minute hard, 1-minute easy interval set, repeated 8 to 10 times after a 10-minute warm-up. Same principle, smaller bites.

3. Do not abandon Zone 2 (or brisk walking)

Lower-intensity aerobic work still earns its place. It is what allows you to recover from your hard sessions, what builds the volume needed to drive long-term mitochondrial adaptation, and what fits realistically into a working week. Two or three sessions a week of brisk walking, easy cycling, slow running, or a swim at a chatty pace gets you there. The hawker stall walk after dinner counts as a fine starter dose.

4. Keep two strength sessions in the week

This is the part of the longevity prescription that no amount of cardio replaces. Two full-body resistance sessions per week, each 30 to 45 minutes, covering legs, hips, back, chest, shoulders and core, will protect your muscle, bone density and falls risk through your sixties, seventies and eighties. The Asian Working Group for Sarcopenia 2019 Consensus Update, and its 2025 revision, both put resistance training at the centre of midlife longevity for exactly this reason. Cardio extends your lifespan. Strength extends your healthspan.

5. Measure once a year, not once a week

Your VO2max in May is the same as your VO2max in April. Daily ring or watch estimates are interesting trend data but the absolute numbers are noisy. If you want a real number to track, do a proper cardiopulmonary exercise test (CPET) once a year. Ten minutes on a bike or treadmill with a mask, measuring your actual oxygen consumption at peak effort, gives you an absolute number with a confidence interval and lets you set a target you can actually train towards. It is also one of the few clinical tests that directly maps to your future risk of dying.

What This Does Not Mean

Three quick things to head off the inevitable misreadings of this piece.

First, the Sports Medicine review is a narrative synthesis, not a meta-analysis. It is one careful argument by one group of experienced researchers, and a fair-minded reading of it should sit alongside the broader literature, not replace it. Some Zone 2 advocates will argue, with reason, that the review under-weighs the long-term metabolic and cardiovascular benefits of higher training volumes. They are not wrong about that.

Second, "do high intensity" does not mean "ignore your knees, your blood pressure, your stress test, your shoes". If you have known cardiac disease, uncontrolled hypertension, recent surgery, severe osteoporosis, or significant joint problems, an interval session designed for a healthy 40-year-old is not the right starting point for you. Talk to your doctor first. A baseline CPET, and an exercise prescription written for your physiology, is what serious cardiac rehab does for a reason.

Third, the worst exercise programme is the one you do not do. If 60 minutes of Zone 2 four days a week is the only thing you will do reliably for the next five years, it will absolutely beat a perfectly polarised 80/20 plan that lasts six weeks before life takes it apart. Adherence is the highest-yield variable.

The Bottom Line

The Zone 2 cult of the past three years has been useful in one important way. It has nudged a generation of inactive desk workers into structured, regular, achievable cardio. That is not nothing. But the 2025 Sports Medicine narrative review, by some of the people who have spent the most time looking at human mitochondria, is a clear signal that the popular advice has overshot. The optimal cardiorespiratory training programme for the average healthy adult is not 200 minutes a week of easy cycling. It is roughly 150 to 200 minutes a week split mostly into easy-to-moderate aerobic work, with one short, hard interval session, plus two short strength sessions. Total time, if you are honest, around four hours a week. Not crushing, but not lazy either.

My patient with the Garmin and the spreadsheet went home with a revised plan. Three of his Zone 2 rides a week stayed. The fourth was replaced with a 4-by-4 interval session on the bike. Two strength sessions were added on the days he had previously taken off. We are doing a follow-up CPET in October. I will let you know how it goes.

The honest read of Zone 2, in a sentence, is that it is one of the right things, but it is not the whole answer. The whole answer always was, and still is, a sensibly mixed week.

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 Medicine. 2025;55(7):1611-1624. DOI: 10.1007/s40279-025-02261-y
2. Mandsager K, Harb S, Cremer P, Phelan D, Nissen SE, Jaber W. Association of Cardiorespiratory Fitness With Long-term Mortality Among Adults Undergoing Exercise Treadmill Testing. JAMA Network Open. 2018;1(6):e183605. DOI: 10.1001/jamanetworkopen.2018.3605
3. Helgerud J, Høydal K, Wang E, Karlsen T, Berg P, Bjerkaas M, et al. Aerobic high-intensity intervals improve VO2max more than moderate training. Medicine & Science in Sports & Exercise. 2007;39(4):665-671. DOI: 10.1249/mss.0b013e3180304570
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