If you could track only one number for the rest of your life and use it to predict how long that life would be, the evidence says it should be VO2 max — the peak rate at which your body can take in oxygen and turn it into work. Not your weight. Not your cholesterol. Not your step count.
Your watch probably already shows you a number for it. That number is useful, and it is also probably wrong by a few points. Here is what VO2 max actually measures, what the mortality data really shows, how much to trust the estimate on your wrist, and what the training research says about moving it.
What VO2 max is
VO2 max is the maximum volume of oxygen your body can use per minute, scaled to your body weight — reported in millilitres of oxygen per kilogram per minute (ml/kg/min). It is a whole-system score. To move oxygen from the air to a working muscle fibre, your lungs, heart, blood, blood vessels and mitochondria all have to do their jobs. A high VO2 max means that entire chain is in good shape.
That is why it behaves less like a fitness stat and more like a summary of how well your body is holding up. It is also why it is a common anchor for biological age: a 50-year-old with the aerobic capacity of an average 30-year-old is, in a real physiological sense, running an older body more slowly.
You will also see fitness expressed in METs. One MET is roughly 3.5 ml/kg/min — the oxygen cost of sitting still. Most of the large clinical studies report in METs, so it helps to be able to switch between the two.
What the mortality data actually shows
This is the part that gets oversold, so it is worth being precise about what was measured.
The largest single dataset comes from a 2018 JAMA Network Open analysis of 122,007 patients who completed a maximal treadmill test at the Cleveland Clinic and were then followed for mortality over more than a decade. Fitness tracked inversely with death from any cause across the whole range. The least-fit group had roughly five times the mortality risk of the elite-fit group — a gap larger than the one the same study found for smoking, diabetes or high blood pressure.
The dose-response side comes from a widely cited JAMA meta-analysis of healthy-cohort studies: each 1-MET increase in cardiorespiratory fitness was associated with roughly 13% lower all-cause mortality and about 15% lower cardiovascular mortality. One MET is about 3.5 ml/kg/min. So moving from 32 to 39 ml/kg/min is not a rounding error — it is two METs, and the associated risk reduction is in the same range as the biggest lifestyle levers we know about.
The long tail is striking too. In the Copenhagen Male Study, which followed men for 46 years, each unit of VO2 max was associated with about 45 extra days of life.
Now the honest caveats. Every one of these is observational. People with high VO2 max differ from people with low VO2 max in dozens of ways — they smoke less, sleep more, are less likely to be carrying undiagnosed disease. Some of the association almost certainly runs backwards: early illness lowers fitness before it kills you. And a meaningful chunk of VO2 max is genetic; twin and family studies put heritability of both baseline capacity and trainability somewhere around 50%.
What is proven: fitness is one of the strongest predictors of mortality we can measure, and training reliably raises fitness. What is inferred: that raising your own VO2 max by training moves your personal risk by the amount the cohort curves suggest. The mechanism is plausible and the direction is almost certainly right, but no one has run the 20-year randomised trial. Treat the curves as a strong signal, not a dosing chart.
What counts as a good number
The reference values most clinicians use come from the Cooper Institute's Aerobics Center Longitudinal Study, reproduced in the ACSM guidelines. Some rough anchors for the 50th percentile — genuinely average, not good:
- Men, 20–29: about 48 ml/kg/min. By 70–79 the median has fallen to about 24.
- Women, 20–29: about 38 ml/kg/min, with a similar decline curve.
- Men typically test 15–20% higher than women at the same age, driven by haemoglobin concentration, heart size relative to body mass and lean mass percentage. Compare yourself within your own sex and decade.
In the ACSM framework, the 60th–79th percentile is "good", 80th–94th is "excellent", and below the 20th is "poor". The most useful framing is not your absolute number but your percentile for your age and sex, and — more useful still — the direction it has moved over the last year.
Untrained adults lose roughly 10% of VO2 max per decade after about 30. That decline is not fixed. It steepens with inactivity and flattens considerably with training, which is why a well-trained 55-year-old can sit above the median 30-year-old.
How much to trust the number on your wrist
Start here: no consumer wearable measures VO2 max. A real test means a mask, a metabolic cart and someone pushing you to exhaustion. Your watch runs a regression — it watches your heart rate at a given pace or power, plus your age, sex and weight, and estimates where the curve would top out.
That approach works better than you might expect and worse than the display implies:
- A 2025 PLOS ONE validation study put Apple Watch estimates against indirect calorimetry in 30 participants. The watch underestimated VO2 max by an average of 6.07 ml/kg/min, with a mean absolute percentage error of about 13%.
- Garmin has the strongest independent record, with errors commonly reported around 5–7% in general users — but that is under good conditions: an outdoor run, flat ground, and ideally a chest strap rather than a wrist optical sensor.
- Comparisons across WHOOP, Garmin and Polar in trained athletes find reasonable correlation with lab values but systematic bias in different directions — WHOOP tends to run high, Garmin slightly low.
The practical rule: your wearable's VO2 max is a decent trend line and a poor absolute value. A five-point move over three months on the same device, same shoes, same route means something real. A five-point gap between your watch and your friend's different watch means nothing. And the estimate degrades badly if you mostly do activities the algorithm cannot read — lifting, cycling on a device that only sees wrist motion, hills, heat, treadmills with wrong incline calibration.
If you want the true number, a lab CPET is the only answer. Between tests, a 12-minute Cooper run or a validated submaximal step test will give you a more stable estimate than a watch that has only seen easy jogs.
What actually raises it
This is one of the better-evidenced areas in exercise science, and the answer is unglamorous: mostly easy volume, plus a small amount of genuinely hard work.
The polarized 80/20 structure
The distribution that repeatedly outperforms in endurance research is roughly 80% of training time at low intensity — conversational, zone 2, nose-breathing pace — and about 20% at high intensity, with little time in the moderate middle. The middle zone is tiring enough to blunt your hard sessions without being hard enough to drive the adaptations those sessions produce.
Long intervals for the ceiling
Meta-analyses of high-intensity interval training find that longer intervals of three to five minutes produce the largest VO2 max gains — bigger than short sprint work. The Norwegian 4×4 is the best-studied version: four minutes at roughly 90–95% of max heart rate, three minutes easy between, four times through, once or twice a week. Reported gains in the region of 10–13% over eight weeks are common in previously untrained people. If you are already trained, expect much less; trainability is limited and partly genetic.
The rest of it
- Consistency beats intensity. Three sessions a week for a year beats six sessions a week for six weeks.
- Recovery is not optional. 4×4 sessions on tired legs become moderate-zone junk. Watch your resting heart rate and HRV trend; if both are drifting the wrong way, do the easy session instead.
- Strength training does not raise VO2 max much — but it protects the lean mass and movement quality that let you keep training aerobically into your sixties and beyond. Keep it.
- Anaemia, untreated thyroid problems and poor iron status cap your oxygen delivery no matter how well you train. If your VO2 max is stuck despite consistent work, that is a reason to look at bloodwork, not to train harder.
What to ignore
No supplement raises VO2 max meaningfully. Beetroot juice and beta-alanine have modest, real effects on performance at a given intensity; neither moves the ceiling by much. Altitude tents, breathing-restriction masks and most "mitochondrial support" stacks have thin or absent evidence for VO2 max in trained humans. The peptide market makes claims here with essentially no controlled human data behind them. Save the money for a lab test and shoes.
Putting it together
Find your percentile for your age and sex rather than fixating on the raw number. Track the trend on one device, not the absolute value across several. Get a real test once a year if you can. Build a base of easy aerobic work, add one or two long-interval sessions a week, and give it six months — VO2 max moves on a timescale of months, not days.
And read it alongside the rest of your data. VO2 max is a powerful summary of cardiovascular capacity, but it says nothing about your lipids, your glucose control or your inflammation. Someone with excellent fitness and an ApoB of 130 still has a problem.
This article is educational content and not medical advice; talk to a qualified clinician before making decisions about your health, your training or your medication, particularly if you have known heart disease or have been sedentary for a long time.
VO2 max is one input into how old your body actually is. VitalNexa pulls your wearable data alongside your bloodwork — lipids, glucose, inflammation, hormones — and turns the whole picture into a single VitalNexa Biological Age you can watch move as you train. Get started free and see where your fitness sits against the rest of your biology.