The Metric Everyone Is Chasing May Be the Wrong One
Eric Topol, a cardiologist and researcher at Scripps Research, has watched two of his own patients spiral into anxiety over a number their smartwatches gave them: VO2 max. The metric has become a cultural fixation, promoted by longevity influencers and now baked into AI health platforms. Topol argues the entire craze rests on a fundamental conflation between two very different measurements.
The distinction matters. Cardiorespiratory fitness, measured in METs, captures real-world physical capacity through treadmill or bicycle tests. VO2 max, by contrast, requires a lab with a metabolic cart, trained technicians, a tightly sealed mask, and exercise to absolute exhaustion. Most people encounter VO2 max not through a lab but through a wrist-worn estimate that carries substantial error.
V02 max is only accurately determined as a performance lab test with a metabolic cart, trained technicians, a specialized tightly fit mask that captures every molecule of inhaled oxygen and exhaled C02 on a ramped treadmill or stationary bike exercise protocol until absolute exhaustion.
Topol is not splitting hairs over measurement techniques for sport. His core argument is about evidence. The massive datasets linking fitness to longevity—the Cleveland Clinic study of 122,007 patients, the Veterans Affairs study of over 750,000 participants—all measured cardiorespiratory fitness in METs, not VO2 max. The distinction gets lost in popular translation.
The Conflation Problem
Peter Attia, physician and host of The Drive podcast, has been the loudest voice championing VO2 max as a longevity predictor. Topol takes direct aim at the claim.
He has consistently asserted "V02 max is the singular most powerful marker for longevity." But the problem is conflation. He cites all of the studies of CRF without measuring V02 max and extrapolates to a V02 max result.
Topol traces this through Chapter 11 of Attia's book Outlive, where CRF data from the Kokkinos study is presented alongside VO2 max framing. The studies cited never actually measured VO2 max. They measured how many METs someone could sustain on a treadmill.
He writes: "this number [V02max] turns out to be highly correlated with longevity" citing all studies that did not measure V02 max.
The numbers themselves are striking. A 2024 meta-analysis found 234-fold more participants studied with exercise-based CRF than with direct VO2 max measurement. As Topol puts it:
Nearly all of the relevant data related to outcomes are based on exercise on a treadmill or bicycle with METS as the index of cardiorespiratory fitness.
Smartwatches Make It Worse
If the evidence base for VO2 max and longevity is thin, the smartwatch estimates that most people actually rely on are thinner still. Studies of Apple Watch, Garmin, and Fitbit devices show mean absolute percentage errors of 7 to 16 percent, with a consistent pattern: overestimating unfit users and underestimating fit ones. Optical heart rate sensors add further noise, particularly for people of color.
Topol recounts his own experience with characteristic candor:
My Apple Watch gave me encouraging high V02 max data over 6 months to suggest my CRF is well above people in my age group (70+) but I know the data is woefully unreliable.
The problem compounds when AI platforms ingest this unreliable data. Washington Post tech journalist Geoffrey Fowler submitted his Apple Health data to ChatGPT Health and received an F grade on cardiovascular health. After adding his electronic medical record, the grade rose only to a D, heavily penalized by a VO2 max reading of 34 ml/kg/min. Claude Health similarly gave him a D+ based on a VO2 max decline from 41 to 32 over a decade—despite maintaining over 7,500 steps per day throughout.
These outputs are indicative of the problem—the unreliable wearable V02 max data have become unduly emphasized by current AI platforms using smartwatch data!
What the Data Actually Support
The Cleveland Clinic data showed that the hazard ratio between above-average and below-average fitness was 1.41—comparable to the risk of smoking or diabetes. The gap between the lowest and elite fitness categories exceeded five-fold. The Veterans Affairs data across 750,000 participants confirmed the pattern. In both studies, there was no ceiling effect: the fittest groups consistently showed the lowest mortality, with no evidence that extreme fitness becomes harmful.
Topol highlights work from the UK Biobank analyzing over 70,000 participants that points to a more holistic approach:
Improving both CRF and muscle strength, as opposed to either of the two alone, may be the most effective behavioral strategy to reduce all-cause and cardiovascular mortality risk.
This is where the VO2 max fixation does real damage beyond bad metrics. By orienting people around a single aerobic ceiling number, it pulls attention away from strength training, balance work, and the diversity of exercise that the evidence actually supports.
A Fair Counterpoint
Topol's case is strong, but it is worth noting that VO2 max and METs are physiologically correlated—one MET is roughly 3.5 ml/kg/min of oxygen consumption. The studies measuring CRF in METs are, in a real sense, capturing a proxy for oxygen utilization. Attia's extrapolation is sloppy in its specifics, but the directional claim that aerobic capacity predicts longevity is not wrong. It is also fair to say that for motivated individuals who can access a proper lab test, serial VO2 max measurements offer a precision that treadmill METs cannot match. The problem is not the metric itself but the ecosystem of imprecise smartwatch estimates and inflated claims that has grown up around it.
Bottom Line
Topol never had his own VO2 max measured and sees no reason to start. For most people, the cost, discomfort, and limited evidence base make it unnecessary. METs are free to calculate, universally accessible, and backed by studies covering nearly a million participants.
If we're going to focus on a metric it ought to be METS, not V02 max. Not only is it free, simple and universally available, but it is the one best studied for health outcomes.
The AI health platforms now amplifying unreliable smartwatch VO2 max data risk deepening the confusion. Topol's prescription is simpler and better supported: be as physically active as possible, track what actually matters, and stop obsessing over a number your watch made up.