If you wear an Oura ring, a WHOOP band, an Apple Watch or a Garmin, you have probably seen a number labelled "VO2 max" or "cardio fitness" and wondered what it actually tells you. The short answer: more than almost any other single number your wearable produces. Among the metrics ordinary people can track, cardiorespiratory fitness is one of the most powerful predictors of how long — and how well — you are likely to live.
This article looks at what VO2 max is, why longevity scientists treat it as a window onto your ageing rate, how seriously to take the estimate on your wrist, and what the research says about the kinds of training associated with a higher number. It is educational, not medical advice — every substantive claim is cited, and the limitations are spelled out as we go.
What VO2 max actually measures
VO2 max is the maximum rate at which your body can take in, transport and use oxygen during hard exercise, usually expressed in millilitres of oxygen per kilogram of body weight per minute (ml/kg/min). It is an integrated test of your whole oxygen-delivery chain: lungs, heart, blood, blood vessels and the mitochondria inside your muscle cells. Because so many systems have to work well together to score highly, VO2 max behaves like a single composite read-out of cardiovascular and metabolic health rather than a measure of any one organ.
The gold-standard measurement is a graded exercise test in a lab, breathing through a mask connected to a metabolic gas analyser while the treadmill or bike gets progressively harder until you cannot continue. Clinicians often report the result in METs (metabolic equivalents), where one MET is resting metabolism and roughly 3.5 ml/kg/min of oxygen uptake.
Why longevity science cares: fitness and how long you live
The reason VO2 max keeps appearing in ageing research is the strength and consistency of its link to mortality. In a landmark retrospective study of 122,007 adults undergoing treadmill testing, all-cause mortality fell steadily as fitness rose, with no observed upper limit of benefit — the very fittest "elite" group had about one-fifth the death risk of the least-fit group over follow-up (Mandsager et al., 2018, JAMA Network Open). Strikingly, being in the lowest fitness category carried a larger mortality risk than living with coronary artery disease, diabetes or being a smoker in the same dataset.
This is not a single-study fluke. A meta-analysis of 33 cohorts (over 100,000 people) found that each one-MET increase in fitness was associated with roughly a 13% lower risk of all-cause mortality and a 15% lower risk of cardiovascular events (Kodama et al., 2009, JAMA). An updated meta-analysis pooling 37 cohorts and more than 2.25 million participants reached the same conclusion: every 1-MET increment in cardiorespiratory fitness was associated with an 11% lower risk of death from any cause, and the authors argued fitness should be part of standard risk panels (Laukkanen et al., 2022, Mayo Clinic Proceedings).
The most comprehensive view comes from an umbrella review of meta-analyses representing over 20.9 million observations from 199 cohort studies. It found high cardiorespiratory fitness was strongly and consistently associated with lower risk of death and of incident chronic disease: comparing high versus low fitness, the risk reduction was about 53% for all-cause mortality and as much as 69% for incident heart failure (Lang et al., 2024, British Journal of Sports Medicine). Importantly, fitness also appears to blunt the risks attached to a higher body weight — in a meta-analysis of 20 cohorts, "overweight-but-fit" and "obese-but-fit" individuals did not show a statistically higher mortality risk than normal-weight-fit people, whereas being unfit at any body size carried a two-to-threefold higher risk (Weeldreyer et al., 2025, British Journal of Sports Medicine).
None of these are randomised trials, so they cannot prove that fitness causes longer life — fitter people differ in many ways. But the dose-response pattern, the size of the effect, and its consistency across millions of people are why geroscientists treat VO2 max as a serious marker of biological resilience.
VO2 max as an ageing read-out
Here is what makes VO2 max feel like a biological-age dial rather than just a sports stat: it declines predictably as we get older — and the decline accelerates.
In the Baltimore Longitudinal Study of Aging, researchers repeatedly measured peak VO2 in 810 healthy adults aged 21 to 87. Cross-sectional snapshots had suggested a steady, gentle decline, but the longitudinal data told a harsher story: the rate of decline rose from around 3–6% per decade in people's twenties and thirties to more than 20% per decade from the seventies onward (Fleg et al., 2005, Circulation). The fall was steeper in men from the forties on, and — crucially — it happened across every level of self-reported physical activity. Ageing erodes aerobic capacity even in active people; the question is how high your starting point is and how slowly you let it fall.
That accelerating curve is exactly why fitness matters more, not less, with age. Because everyone loses aerobic capacity over time, the absolute level you hold determines when you cross thresholds below which everyday tasks — climbing stairs, carrying shopping, recovering from illness — become hard. Maintaining a higher VO2 max effectively pushes those thresholds further into the future, which is what "healthspan" means in practice.
The link extends to the ageing brain, too. Higher midlife cardiovascular fitness has been associated with better-preserved white-matter integrity on brain imaging (d'Arbeloff et al., 2021, Frontiers in Aging Neuroscience). In the large Norwegian HUNT study, adults who followed a trajectory of higher fitness and lower blood pressure through adulthood had a markedly lower risk of dementia decades later (Lerfald et al., 2024, Journals of Gerontology Series A), and a related HUNT analysis found that maintaining or increasing a heart-rate-based activity score was associated with fewer cases of dementia and additional dementia-free years (Tari et al., 2022, EClinicalMedicine).
The number on your wrist: how much to trust it
Most people will never do a lab VO2 max test, so the practical question is whether the estimate from a watch or ring is good enough to be useful. The honest answer: it is a reasonable ballpark for tracking your own trend over time, but not a precise clinical value, and it tends to be least accurate at the extremes of fitness.
In a validation study comparing the Apple Watch Series 7 against laboratory gas analysis, the watch's estimate had a mean absolute percentage error of around 16% and showed only "poor" reliability by the intraclass correlation, systematically overestimating in less-fit people and underestimating in very fit people (Caserman et al., 2024, JMIR Biomedical Engineering). Accuracy improved in the fitter participants, but the takeaway is clear: treat the absolute figure with humility.
What wearables do reasonably well is detect change. If you train consistently and your device's estimate climbs by a couple of ml/kg/min over a few months, that direction of travel is meaningful even if the exact number is off. The estimate is most trustworthy when you compare it against your own past readings on the same device, under similar conditions, rather than against a friend's different watch or a population chart.
What the evidence says about raising VO2 max
The encouraging part is that VO2 max is one of the more modifiable markers of ageing — and improving it is associated with lower risk, not just having a high baseline.
Improving fitness is linked to lower risk. In the Kuopio Ischaemic Heart Disease cohort, each 1 ml/kg/min improvement in directly measured VO2 max over an 11-year period was associated with a 10% lower risk of developing heart failure (Khan et al., 2018, American Journal of Cardiology). In cardiac-rehabilitation patients, increasing fitness by as little as 0.5 METs from baseline to one-year follow-up was associated with improved survival, regardless of how many other conditions a person had (Ozemek et al., 2022, Journal of Cardiopulmonary Rehabilitation and Prevention). The trend, in other words, is something you can move.
Both steady aerobic work and intervals help. A large evidence base supports two complementary approaches. Lower-intensity, sustainable aerobic exercise — the kind that keeps you in a conversational "zone 2" effort — builds the aerobic base, while higher-intensity intervals tend to drive larger gains in peak capacity. In a meta-analysis of patients with lifestyle-related cardiometabolic disease, high-intensity interval training produced a roughly 9% greater improvement in VO2 peak than moderate-intensity continuous training — nearly double the gain (Weston et al., 2014, British Journal of Sports Medicine). For most people the practical implication is a mix: a foundation of easy aerobic minutes most days, with occasional harder efforts. Crucially, even the act of moving from "unfit" toward "fit" captures the largest slice of the mortality benefit seen in the cohort studies above; the steepest part of the curve is at the bottom.
This is general, population-level evidence about exercise, not a prescription for any individual. How hard to push, how often, and where to start depend on your current health, and anyone with a heart or lung condition, or who has been sedentary for a long time, should get individual medical guidance before ramping up intensity.
The bottom line for your ageing
VO2 max earns its place in longevity science because it ties together so much of what determines how we age: cardiovascular health, metabolic function, muscle quality and, indirectly, brain health. It declines as we get older, that decline speeds up, and the level you hold is one of the strongest predictors of mortality that anyone can measure — with no clear ceiling on the benefit of being fitter.
Your wearable's estimate is a useful, if imperfect, way to keep an eye on that dial. Watch the trend rather than the decimal, pair it with the occasional objective check, and remember that the research consistently points the same way: the direction you move your fitness is associated with the direction you move your long-term health.
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VO2 max is one signal among many. omniwo combines wearable data with blood biomarkers to build a fuller picture of how your body is ageing — turning scattered numbers into something you can actually act on. Explore omniwo tests in the shop →
This article is educational and is not medical advice. See our medical disclaimer.
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