VO2 Max: The Fitness Number That Predicts Longevity

Before any number: climb four flights of stairs at a steady pace, without stopping, and notice whether you could hold a conversation at the top.

That takes a minute and tells you more than most people learn from a year of wearing a watch. It is also roughly how VO2 max was assessed before laboratories existed — how much work you can do before oxygen delivery becomes the limit.

VO2 max is the formal version of that question, and it is one of the strongest predictors of how long someone lives that medicine has. It is also, for most people reading this, a number they have never actually measured.

Quick Fact: In 122,007 patients followed for a median of 8.4 years, the mortality risk associated with low cardiorespiratory fitness was comparable to or greater than the risk from smoking, diabetes or high blood pressure.

Last reviewed: October 2026

What VO2 Max Actually Measures

It is the maximum volume of oxygen your body can take in and use per minute, scaled to body weight — millilitres per kilogram per minute.

Three systems have to work for that number to be high: lungs moving air, heart and blood moving oxygen, and muscle mitochondria using it. VO2 max is the output of the whole chain, which is why it reflects so much and why no single intervention raises it quickly.

In untrained people, the heart’s stroke volume is usually the limiting step. That is why training that spends time near maximum cardiac output is what moves the number.

The Research, and What Kind of Research It Is

The headline figures come from a large retrospective cohort at a single medical centre — 122,007 consecutive patients who had exercise treadmill testing over 24 years.

Two things about that table are worth knowing before you repeat it at dinner.

It is not from a meta-analysis of millions of people, which is how it usually gets described. It is one cohort, and the people in it were referred for a treadmill test because a doctor had a reason to order one. That is not the general population.

And the comparison with smoking is a comparison of statistical associations, not a claim that being unfit damages you the way cigarettes do. Fitness is partly a consequence of health as well as a cause of it — someone with undiagnosed disease tests poorly.

The finding is still remarkable. It just deserves its actual label.

Fitness Beats Weight

The most useful row in that table is the one comparing a normal-weight unfit person with an obese fit person. The unfit person at a normal weight carried substantially higher risk.

This does not mean weight is irrelevant. It means fitness is doing more of the work than the bathroom scale suggests, and that a person who cannot change their weight can still change the thing that matters more.

Your Watch Is Not Measuring This

Here is the part that affects nearly every reader, because nearly every reader’s number comes from a wrist.

A validation study compared Apple Watch estimates against laboratory measurement. The findings:

That last row matters most. The estimate pulls everyone toward the middle — which means the people who most need to know they are in the low-fitness category are the ones most likely to be told they are fine.

An important caveat on the caveat: that study had 19 participants. It is small, it tested one device and one generation of it, and algorithms change between firmware versions. Treat it as a strong warning rather than a precise error figure.

The practical reading: a watch estimate is useful for watching your own trend on the same device, and close to useless as an absolute number or for comparing yourself to a table.

What the Numbers Mean

Approximate ranges in ml/kg/min. Treat as orientation, not diagnosis.

One threshold is worth more than the whole table: around 17.5 ml/kg/min — five METs — is roughly the level associated with maintaining independent living. For an older adult, the distance between their number and that one is the number that matters.

How to Actually Measure It

If you want to know your actual number, a laboratory test is the only way to get it. Many sports clinics and universities run them for a modest fee.

How to Improve It

Structured training improves VO2 max by roughly 15–30% in previously sedentary people, with measurable change usually appearing within six to eight weeks.

A standard protocol is four rounds of four minutes hard with three minutes of easy recovery. Our guide to HIIT workouts covers how to build toward that without overreaching, and why two sessions a week is the ceiling for most people.

One honest complication: the response to identical training varies enormously between individuals, and a substantial part of that is genetic. Two people on the same program for six months can see very different changes. If your number moves less than someone else’s, that is biology, not failure — and the mortality association tracks your fitness level, not your rate of improvement.

What People Actually Run Into

The most common pattern is treating the watch number as the measurement. People watch it fluctuate by two or three points across a month and draw conclusions, when the device’s own error bar is several times that. The trend over six months on one device means something. The reading on Tuesday does not.

The second is chasing the number instead of the training. VO2 max responds to a lot of hard aerobic work, and people who optimise directly for the metric tend to do too many hard sessions, accumulate fatigue, and stall. Our guide to rest days and recovery covers why the hard sessions only count if you recover from them.

Third, and the quietest: people who read the mortality findings, discover their number is below average, and conclude that the gap is too large to close. The dose-response curve here is steepest at the bottom. Going from poor to below-average is a larger change in risk than going from good to excellent — and it is also the easier of the two.

Common Mistakes

Comparing a watch estimate to a norms table. Different measurement entirely. The watch compresses everyone toward the middle.

Describing the hazard ratios as coming from a meta-analysis of millions. They are from one large cohort of patients referred for treadmill testing.

Reading “worse than smoking” literally. It is a comparison of statistical associations, not of biological damage.

Doing three or four hard interval sessions a week. The adaptation needs recovery. More hard days usually means less progress.

Retesting every two weeks. Meaningful change takes six to eight weeks minimum.

Ignoring the number because you cannot lose weight. Fitness carried more of the risk than weight did in the data above.

Treating low trainability as failure. Response to training varies substantially between people for genetic reasons.

Practical Tips

Do the stairs test today. It costs a minute and gives you a baseline you will remember.

If you use a watch estimate, use it only as a trend on that one device, and look at it every few months rather than every week.

Get one laboratory test if the number genuinely interests you. One real measurement is worth more than three years of estimates.

Build the week around one or two hard aerobic sessions and a larger volume of easy work. The easy work is not filler — it builds the peripheral half of the system.

Use intervals of two to four minutes near maximum rather than short sprints if VO2 max specifically is the target.

Retest no sooner than eight weeks after changing anything.

If you are over 60, treat the distance between your number and about 17.5 ml/kg/min as the one worth watching. Our guide to how much exercise you need covers the baseline activity that supports it.

When Should You See a Doctor?

Talk to a doctor before beginning maximal-intensity interval training if you have a known heart condition, uncontrolled hypertension, or have been sedentary for a long period. VO2 max training by definition involves working near your maximum.

Seek medical assessment for chest pain, unusual breathlessness, dizziness, palpitations or fainting during or after exertion. These are not things to train through or test around.

See a doctor about a marked, unexplained drop in exercise capacity — becoming breathless on stairs you used to manage comfortably. Declining cardiorespiratory fitness can be the first noticeable sign of cardiac, pulmonary or haematological problems.

Get medical clearance before a maximal laboratory test if you are over 45 with cardiovascular risk factors, or at any age with a family history of sudden cardiac death.

Frequently Asked Questions

What is a good VO2 max for my age? See the table above, but the more useful question is where you sit relative to the poor category, because that is where the risk difference is largest. For older adults, around 17.5 ml/kg/min is the level associated with maintaining independence.

Are smartwatch VO2 max estimates accurate? Not as absolute numbers. In validation work the error exceeded 10% in every fitness group, with poor to moderate reliability, and the estimate systematically flatters unfit users. Use it as a trend on your own device only.

How long does it take to improve? Measurable change in six to eight weeks of consistent training; 15–30% improvement is typical for previously sedentary people over several months. Individual response varies considerably.

Does VO2 max matter more than body weight? The data suggest fitness carries more of the mortality risk than weight does. A normal-weight unfit person showed nearly double the risk; an obese fit person showed almost none.

Does strength training raise VO2 max? Only modestly, and not in untrained-to-trained terms the way aerobic intervals do. Strength training is worth doing for other reasons — muscle mass, bone, function — but it is not the lever for this particular number.

The Bottom Line

The useful way to hold this is on three timescales at once.

In eight weeks, a consistent program will produce a measurable change, and that is the horizon on which you should judge whether you are actually training. In a year, someone starting from sedentary can move out of the poor category entirely — which is the single largest risk reduction available anywhere on this curve. And over decades, VO2 max declines by a few percent a year no matter what anyone does; training does not stop that, it shifts the whole line upward, so you cross the thresholds that matter later.

That last point is the real argument. Nobody trains their way out of ageing. What a higher number buys is the age at which climbing four flights of stairs stops being something you can do without thinking about it.

This article is for general informational purposes and is not a substitute for personalized medical advice. Consult a healthcare provider before beginning maximal-intensity exercise, particularly with a cardiac condition or after a long period of inactivity.

References

• Mandsager K et al. (2018). Association of Cardiorespiratory Fitness
  with Long-Term Mortality Among Adults Undergoing Exercise Treadmill
  Testing. JAMA Network Open 1(6):e183605
• Assessing the Accuracy of Smartwatch-Based Estimation of Maximum
  Oxygen Uptake Using the Apple Watch Series 7: A Validation Study.
  JMIR Biomedical Engineering (2024)
• Cardiorespiratory Fitness, Body Mass Index and Mortality: A Systematic
  Review and Meta-Analysis. British Journal of Sports Medicine
  59(5):339-346