VO2max has hit the headlines because a lot of people have noticed a mismatch between what metric the strongest long term datasets linking cardiorespiratory fitness and mortality actually measured, METs, and what the public conversation thinks they measured, VO2max.

Much of the data for these big studies assessing cardiorespiratory fitness comes from treadmill tests reported in METs, not VO2max from breath by breath gas analysis. At the same time, consumer devices have started pushing VO2max estimates as if they were a clinical grade assessment. That combination has created a predictable backlash.

I understand the frustration. If you turn VO2max into a longevity badge, people will chase it, at the detriment of other similarly important aspects of health and fitness such as resistance training, mobility and stability. If a watch can estimate it (with a significant meaningful error), people will treat small week to week changes as real. If clinicians then respond to a shaky estimate with more testing, you end up optimising for a number that has drifted several steps away from the underlying physiology. The argument should not be anti VO2max. It is anti abstraction.

The problem is not VO2max as a physiological concept. The problem is the method used to generate the number, and what we try to do with it. Directly measured VO2 from a properly run cardiopulmonary exercise test tells you far more than a headline score. It is diagnostic, prognostic and prescriptive. It gives you a map of limitations and a basis for personalised training prescription.

Start with the thing we actually care about: cardiorespiratory fitness

Cardiorespiratory fitness (CRF) is the underlying physiology that we are interested in and is what may often be referred to as athletic fitness. It is a measure of how well our cardiovascular, respiratory and metabolic systems interact with each other, one of the few metrics that represent a whole-systems assessment of how well these systems are functioning. It is your ability to deliver oxygen to working muscle and use it to produce energy.

CRF matters because it is strongly linked to health outcomes. The American Heart Association has argued that CRF deserves "vital sign" status in clinical practice because low CRF consistently associates with higher risk of cardiovascular disease and all cause mortality. One of the most recent meta-analyses, spanning 20.9 million observations, found that every 1 MET higher level of CRF was associated with an 11-17% reduction in all-cause mortality.

It is one of the best predictors of overall risk of death, cardiovascular mortality, cancer mortality, and the incidence of diabetes, high blood pressure, stroke, dementia, kidney disease and depression. CRF has been demonstrated to be a more powerful predictor of risk of death than the more traditional risk factors such as hypertension, diabetes, obesity, hyperlipidaemia and smoking.

VO2max versus METs

VO2max is the most direct expression of cardiorespiratory fitness. It measures the highest rate that your tissues and muscles take up oxygen during a progressive intensive treadmill or cycling test, captured in a lab using breath by breath gas analysis in a cardiopulmonary exercise test.

METs are not a different biological process. A MET, or metabolic equivalent, expresses oxygen cost as a multiple of rest, with 1 MET conventionally defined as 3.5 mL O2 per kg per minute, roughly the oxygen consumption of sitting quietly. You can convert a directly measured VO2 value into METs by dividing by 3.5. But when METs come from treadmill speed and incline tables, they are only an estimate of oxygen uptake, not a direct measurement, and they carry more individual level error.

That convenience is exactly why METs show up everywhere in research and clinical reports. They let you compare workloads across protocols, they are cheaper and simpler to measure, and they let you standardise outcomes across large cohorts. But the key point is that METs are not a different physiological construct from VO2. They are a way of expressing oxygen uptake, or estimated oxygen uptake, in a standardised unit.

1 MET is not "one size fits all"

The 3.5 value is a convention, not your personal resting metabolic rate. Resting oxygen consumption varies with sex, age, body composition, pregnancy status, and training status. A paper that directly tested the 1 MET assumption across different groups showed meaningful mismatch between the standard value and measured resting metabolic rate. This does not mean METs are wrong, but it does mean you should treat them as useful approximations, not precision assessments for individuals.

What went wrong reporting the big studies

In the longevity field, several papers are commonly cited, such as Mandsager, Kokkinos and Kodama. These analyses standardise cardiorespiratory fitness expressed as METs, though some of the primary papers included VO2 based testing. Some have used this literature to claim that VO2max is the single most powerful metric that predicts how long you will live, when much of the underpinning evidence actually reports fitness in MET units.

The core issue is method, not meaning. Many of the largest cohorts did not measure oxygen uptake directly. They estimated exercise capacity from treadmill workload and expressed it in METs. That is still cardiorespiratory fitness. It is still aerobic capacity. But it is not technically the same as a lab measured VO2max from gas exchange.

Importantly, the signal does not vanish when you compare methods. A more recent systematic review and meta analysis of 42 studies and 3.8 million observations compared objectively measured fitness, exercise estimated fitness, and non exercise estimated fitness, and found similar risk reductions per stepped increment in cardiorespiratory fitness. So the problem is not that MET based evidence is weaker. The problem is that people collapse distinct measurement approaches into one headline term, then build further abstraction on top of that.

Wearable VO2 estimates: useful direction, limited precision

Wearables can estimate VO2max using rest data, exercise data, or both. At the population level, some of the data look acceptable. At the individual level, the error can be significant. The INTERLIVE systematic review and meta analysis found wide limits of agreement, even when average bias looked small. That is the scenario where people overinterpret their figures month to month and make unwarranted adjustments to their training.

Use wearables for behaviour feedback and broad direction. Do not use them to diagnose problems or to panic over minor dips.

Why CPET becomes more valuable at the individual level

Population research asks: "Who is at higher risk?" Individual care asks: "Why is this person limited, and what should they do next?"

VO2peak or VO2max is only one output from a cardiorespiratory fitness test. It is a whole-systems measure of lung, heart and muscular function, which is precisely why a CPET is so useful. One CPET gives you VO2max plus 20 to 30 additional, clinically useful metrics for diagnosis, prognosis and prescription of a personalised training programme. It can separate common limiting physiological issues, be that heart function, dysfunctional breathing, the exchange of gas from the lungs to the blood, or issues in the muscle. It also lets you prescribe training from actual physiological heart rate or power thresholds rather than population-based formulas.

So which is more useful: METs or VO2max?

It depends on the decision.

For public and population health, estimated MET capacity is practical and scalable. It allows large health systems to stratify risk and track trends, and it performs well because random individual error washes out in large samples.

For an individual who wants personalised decisions, direct measurement becomes more valuable when you need to answer specific questions: Why does exercise feel disproportionately hard? Is breathlessness cardiac, pulmonary, or conditioning? What training zones match your physiology today?

Practical takeaways

1. Stop treating METs and VO2max as competing constructs. They are different ways of expressing the same underlying capacity, with very different measurement error.

2. Use estimated MET capacity when you want a cheap, rough risk stratification or a fitness ranking.

3. Use CPET when you need precision, explanation, or personalised prescription.

4. If you track VO2 estimates from a watch, focus on multi month direction, not weekly variation.

5. If you want one clean question to guide measurement choice: "Am I trying to predict risk, or am I trying to understand limitations and prescribe training?"

FAQs

Can CPET results be reported in METs? Yes. If VO2 is measured, METs are a unit conversion: VO2 in mL per kg per minute divided by 3.5 gives METs.

If most longevity studies use METs, should I ignore VO2peak? No. Outcome studies often rely on estimates because they scale. That does not make direct measurement pointless. It becomes more valuable when you need individual level accuracy or diagnostic insight.

What is the simplest way to think about the difference? CRF is the attribute. VO2peak is a direct measurement of oxygen uptake at peak effort. METs are a unit that can express measured, or estimated, oxygen uptake.

Further reading

1. R. Ross et al., 'Importance of Assessing Cardiorespiratory Fitness in Clinical Practice: A Case for Fitness as a Clinical Vital Sign', Circulation, vol. 134, no. 24, pp. e653-e699, Dec. 2016, doi: 10.1161/CIR.0000000000000461.

2. J. J. Lang et al., 'Cardiorespiratory fitness is a strong and consistent predictor of morbidity and mortality among adults: an overview of meta-analyses representing over 20.9 million observations from 199 unique cohort studies', 2024, doi: 10.1136/bjsports-2023-107849.

3. B. Singh et al., 'Comparison of objectively measured and estimated cardiorespiratory fitness to predict all-cause and cardiovascular disease mortality in adults', J. Sport Health Sci., vol. 14, p. 100986, Dec. 2025, doi: 10.1016/j.jshs.2024.100986.

4. S. Wang et al., 'Association of cardiorespiratory fitness with dementia risk across different levels of genetic predisposition', Br. J. Sports Med., vol. 59, no. 3, pp. 150-158, Feb. 2025, doi: 10.1136/bjsports-2023-108048.

5. K. Mandsager et al., 'Association of Cardiorespiratory Fitness With Long-term Mortality Among Adults Undergoing Exercise Treadmill Testing', JAMA Netw. Open, vol. 1, no. 6, p. e183605, Oct. 2018, doi: 10.1001/jamanetworkopen.2018.3605.

6. P. Kokkinos et al., 'Cardiorespiratory Fitness and Mortality Risk Across the Spectra of Age, Race, and Sex', J. Am. Coll. Cardiol., vol. 80, no. 6, pp. 598-609, Aug. 2022, doi: 10.1016/j.jacc.2022.05.031.

7. S. Kodama et al., 'Cardiorespiratory Fitness as a Quantitative Predictor of All-Cause Mortality and Cardiovascular Events in Healthy Men and Women: A Meta-analysis', JAMA, vol. 301, no. 19, pp. 2024-2035, May 2009, doi: 10.1001/jama.2009.681.