VO2 max: what it measures and what it predicts

What the number describes

VO2 max is the ceiling on how fast your body can take in oxygen, deliver it, and use it in working muscle. It is usually expressed relative to body weight, in milliliters of oxygen per kilogram per minute, which is why body composition changes the number without any change in cardiac capacity.

It is a whole-system measurement. Lung ventilation, cardiac output, hemoglobin, capillary density and mitochondrial function all contribute, and in most healthy people the limiting factor is cardiac output rather than the lungs. That breadth is why it correlates with so much: it is a summary of how well the oxygen delivery system works.

For context, one MET is defined as 3.5 mL/kg/min, which is why fitness is sometimes reported in METs from a treadmill test instead.

How it is actually measured

A proper test means a graded exercise protocol to volitional exhaustion on a treadmill or cycle ergometer while expired gas is analyzed breath by breath. Done in a lab, this is the measurement everything else is validated against.

Everything else is an estimate. Submaximal step and walk tests estimate from heart rate response. Watches and rings estimate from the relationship between pace and heart rate during outdoor activity, which means they are sensitive to heat, hills, sleep, caffeine, stress and GPS accuracy. They are reasonable at showing whether you are trending up over months, and they should not be read as a precise value.

If you want a real number, ask for a cardiopulmonary exercise test. If you want to track your own progress, a consistent estimate from the same device under similar conditions is genuinely useful.

What the outcome data shows

This is one of the better-evidenced associations in preventive medicine. In a retrospective cohort of 122,007 patients who underwent treadmill testing, reported by Mandsager and colleagues in JAMA Network Open in 2018, higher cardiorespiratory fitness was associated with lower all-cause mortality, and the benefit continued into the elite fitness category without an observed plateau.

Related cohort work has found the mortality difference between the least fit group and the next group up to be larger than the differences attributed to several conventional risk factors. The steepest gain, in other words, is at the bottom of the distribution.

The honest caveat: these are observational data. Fitness travels with a lot of other things, and no trial has randomized people to a VO2 max. What is clear is that low fitness is a strong marker of risk and that fitness is one of the more modifiable things on the list.

Reading your own value

Compare against age and sex norms rather than against a friend or an athlete. Published normative tables, including the ACSM reference data, place values into percentiles by age band, and that percentile is more informative than the raw number.

A useful frame for later life is functional threshold. Ordinary independent activity has an oxygen cost, so a VO2 max that is comfortable at 45 can sit uncomfortably close to the cost of climbing stairs at 80. Training capacity now is partly a decision about what your ninth decade looks like.

What improves it

The training literature is fairly consistent, and the combination matters more than the specific protocol.

• A base of easy aerobic work, commonly called Zone 2, builds mitochondrial density and capillary supply. Volume is what makes this work.
• High-intensity intervals near maximal effort drive improvements in cardiac output and generally raise VO2 max fastest per hour spent.
• Resistance training supports the muscle mass that makes the rest possible, particularly with age.
• Time frame: meaningful change is measured over months. The less trained you are, the faster the early improvement.
• Before starting intense exercise, particularly with existing cardiovascular disease or symptoms, clearance from your physician comes first. That is not boilerplate.

How Baseline uses it

We suggest tracking VO2 max alongside grip strength, resting heart rate, HRV and body composition, because those track function rather than diagnosis.

It is also a check on progress that the scale cannot provide. Weight loss with fitness improving and weight loss with fitness declining are different outcomes, and only one of them is what anyone actually wanted.

References

1. Mandsager et al., association of cardiorespiratory fitness with long-term mortality among 122,007 patients, JAMA Network Open, 2018
2. Ross et al., importance of assessing cardiorespiratory fitness in clinical practice, AHA scientific statement, Circulation, 2016
3. ACSM's Guidelines for Exercise Testing and Prescription, normative cardiorespiratory fitness data
4. Biomarker hub: Biometric biomarkers