The One Number Your Doctor Won’t Check That Predicts Your Death Better Than Smoking
In 2018, a team of researchers at the Cleveland Clinic in Ohio, led by Dr. Kyle Mandsager, finished tabulating the results of what would become one of the most consequential fitness studies ever conducted. They had tracked 122,007 patients, enrolled over a 24-year period from 1991 to 2014, each of whom had undergone treadmill stress testing, and stratified them into five tiers of cardiorespiratory fitness: low, below average, above average, high, and elite.1 The question was simple. Does being fitter actually make you live longer, and if so, by how much?
Individuals in the elite fitness category, the top 2.3%, had an 80% lower risk of dying from any cause compared to those in the bottom tier.1 Being in the lowest quartile of cardiorespiratory fitness carried an adjusted hazard ratio for all-cause mortality of approximately 5.04. For comparison, the hazard ratio for being a current smoker was 1.41. For having type 2 diabetes: 1.40. For coronary artery disease: 1.29.1 A 50-year-old sitting in the bottom quartile of fitness was, by the numbers, 2 to 3 times more likely to die prematurely than a regular smoker.1 And the data showed no ceiling. Even comparing the “high” fitness group to the “elite” group revealed a statistically significant survival advantage for the elites. More fitness meant more life, with no point of diminishing returns observed anywhere in the dataset.1
What Is VO2 Max?
The metric at the center of all of this is VO2 max, the quantitative measurement of what clinicians call cardiorespiratory fitness, the capacity of the body’s oxygen-transport-and-utilization system.
VO2 max, or maximal oxygen uptake, measures the maximum volume of oxygen your body can consume and utilize per minute during intense exercise. It is expressed in milliliters of oxygen per kilogram of body weight per minute.2 The number reflects far more than athletic capacity. Because VO2 max depends on the seamless integration of the pulmonary system (how well you take in oxygen), the cardiovascular system (how efficiently your heart and blood vessels deliver it), and the musculoskeletal system (how effectively your mitochondria, the cellular engines that generate ATP, burn it for energy), the measurement functions as a panoramic snapshot of total-system biological health.3 So much so that the American Heart Association has formally argued that cardiorespiratory fitness, quantified as VO2 max, belongs in clinical practice as a vital sign alongside blood pressure and pulse.4, 5 A high score means a strong heart with a large stroke volume, healthy vasculature, high hemoglobin concentrations, and a dense network of efficient mitochondria.3 A low score often signals dysfunction in one or more of those systems. Clinical symptoms may still be years away.6
Dr. Kyle Mandsager, lead author of the Cleveland Clinic study published in JAMA Network Open, found no observed upper limit of benefit. In both Mandsager’s cohort of 122,007 patients and a subsequent 750,000-veteran study by Kokkinos and colleagues, cardiorespiratory fitness outranked every traditional clinical risk factor examined as a predictor of all-cause mortality.1, 7
The same year, researchers in Denmark published the results of the Copenhagen Male Study, a 46-year longitudinal tracking of 5,107 men. Dr. Johan S.R. Clausen and his team found that each single-unit increase in VO2 max was associated with a 45-day increase in life expectancy.8 Compounded across the cohort, that translates to roughly 63,000 additional years of collective human life separating the fittest from the least fit, a figure equivalent to adding the entire adult population of a small city to the rolls of the living. Men in the top 5% of fitness lived an average of 4.9 years longer than those in the bottom 5%.8 The study’s design was particularly rigorous: even after excluding all participants who died within the first ten years (a standard method for ruling out reverse causation, where sicker people simply score lower on fitness tests because they are already dying), the results held.8 Midlife fitness was genuinely, independently predictive of how long you would live.
More Dangerous Than Smoking
Consider the hazard ratios side by side. Low VO2 max versus elite: 5.04. Current smoking: 1.41. Type 2 diabetes: 1.40. Coronary artery disease: 1.29.1 Obesity with a BMI above 35: 1.94.9 Low cardiorespiratory fitness is, by this accounting, more dangerous to your long-term survival than any of the conditions your doctor screens for at an annual physical.
Almost no physicians ever test it.
Why Your Doctor Does Not Check It
Your doctor will check your blood pressure, your cholesterol, your fasting glucose, and your thyroid. But he almost definitely will not check the one number that predicts your death better than all of them combined.
Poor results on those routine markers can point to a 20% to 50% increased risk of dying.1, 9 A low VO2 max, though, can multiply it by five. It outpredicts smoking. It outpredicts diabetes. It outpredicts heart disease itself.1
Out in the field, where most of us interact with day-to-day doctors, medicine is perennially behind the times. Ask your average MD how much of their four-year medical school curriculum was devoted to diet, for example, and you will hear most reply with answers on the order of one afternoon or one day if they were lucky.10, 11 VO2 max is simply not a measure that traditional medicine has bought into yet.
To be charitable to doctors, though, VO2 max is difficult to measure. The gold standard is a treadmill test with a mask strapped to your face, a cart of gas analyzers, and a technician watching you run yourself to exhaustion. A blood pressure cuff takes thirty seconds. A finger stick gives you glucose in 30 seconds. The hardest number to measure is the one that matters most.
The Decline Is Not Fixed
The biology behind the decline is sobering but not hopeless. For most people, VO2 max peaks in the late 20s or early 30s.12 From there, it drops by roughly 10% per decade in sedentary or recreationally active adults, about 1% per year.13 After 70, the rate accelerates to 15 or even 20% per decade.12 The drivers are a familiar catalog of aging: maximal heart rate falls, left ventricular compliance decreases (reducing stroke volume), sarcopenia strips away metabolically active muscle tissue, capillary density thins, and mitochondrial efficiency erodes.13–15
Behavior drives about half of the decline.
Frank Booth, a physiologist at the University of Missouri, showed in 2011 that a lifetime of physical inactivity accelerates what he called secondary aging. Secondary aging is the part of decline that tracks with disuse. The calendar has less to do with it than most people think.16
Cardiovascular efficiency drops. Stairs feel steeper. You take them less often. Efficiency drops further. The cycle tightens with each skipped trip.12
The cycle runs in both directions.
How Much Can You Gain Back?
How much can a 50-year-old realistically gain back? What about a 60-year-old? What about someone who hasn’t exercised seriously in decades?
The numbers are better than they sound. Zoran Milanović and colleagues at the University of Niš pooled 28 controlled trials in 2015 and found that endurance training raised VO2 max by about 12% in healthy young and middle-aged adults.17 High-intensity interval training raised it closer to 14%.17 Guoyuan Huang of the University of Southern Indiana ran a separate meta-analysis in 2005 of 41 trials in adults over 60, with an average age of 67, and found a pooled improvement of 16.3%.18 Adriano Oliveira and colleagues at Rio de Janeiro State University confirmed in 2024 that interval training produces gains in older adults that match or slightly exceed continuous moderate training.19
A 16% bump in a 67-year-old may sound like no big deal. Think again. It is the difference between climbing three flights to your granddaughter’s apartment without stopping, and having to stop at the second landing to catch your breath. It’s making it to the top of the trail with your grandson without him offering to wait for you. It’s the difference between recovering from hip surgery in six weeks and recovering in six months.
The cellular machinery for adaptation never fully shuts down. You can always get better.
The single most impactful move, according to the Cleveland Clinic data, is also the most modest. Study participants, followed for a median of 8.4 years each, were sorted into five fitness bands based on their peak fitness treadmill MET score. A MET is a unit of oxygen consumption. Multiply it by 3.5 and you have VO2 max in the standard units. Over the course of the study, 23.7% of the low-fit patients had died. In the next tier up, merely the 25th to 50th percentile, just 10.6% had.1 Roughly a 50% cut in all-cause mortality from a single step up the fitness ladder. Look at it this way. In the same study, smoking bumped a patient’s risk of dying about 40%. Moving from the bottom fitness tier to the next tier up cut risk by nearly half. The step up in fitness mattered more. For a 50-year-old who has not trained in a decade, that step is a brisk walk most days of the week for three or four months. It is taking the stairs instead of the elevator. It is parking at the far end of the lot. The bar is lower than it looks.
How to Train VO2 Max: The 80/20 Split
So, what’s the best way to improve VO2 max? The evidence shows that elite endurance athletes have converged on a training cycle split of 80% low intensity workouts and 20% high intensity workouts. Stephen Seiler of the University of Agder in Norway documented this pattern across athletes in country after country, sport after sport: four out of five sessions at a pace that can be sustained for an hour with the fifth session at high intensity.20
The Norwegian 4×4 Protocol
Within a single high-intensity session, the most research-validated protocol for VO2 max improvement is the Norwegian 4×4, developed at the Norwegian University of Science and Technology in Trondheim: a 10-minute warm-up, followed by four intervals of 4 minutes at 90 to 95% of maximal heart rate (heavy breathing, only able to speak in single words), with 3-minute active recovery periods between intervals, and a 5-minute cool-down. In 2007, Dr. Jan Helgerud and his team ran the head-to-head trial that established the protocol. Forty moderately trained men, four matched-work training regimens, eight weeks. The 4×4 group gained 7.2% in VO2 max. The long slow distance and lactate-threshold groups gained nothing measurable.21 The Norwegian 4×4 is a classic HIIT (high-intensity interval training) workout. For adults training 4 to 6 times per week, a roughly 80/20 distribution translates to one to two HIIT sessions layered on top of three to four Zone 2 sessions.20
What About Smartwatches?
A note on the technology that has put VO2 max on millions of wrists. Apple, Garmin, Polar, Fitbit, whatever. Learn to think of them as trend indicators, not laboratory results. In 2024, Dr. Polona Caserman and her team at the Technical University of Darmstadt tested the Apple Watch Series 7 against a metabolic gas analyzer in 19 adults. The watch was off by about 16% on average, and the error grew wider, not tighter, in the people who trained the hardest. Fit users had their VO2 max lowballed by as much as 12 mL/kg/min, which is the difference between “average for your age” and “top 10%.” The watch was reading a trained 45-year-old as if he were an untrained one. Unfit users, on the other hand, had theirs overstated. The watch tells the couch potato he is fine and tells the marathon runner she is slowing down.22
Garmin’s running watches tend to do better than their competitors. In a 2019 Technical University of Munich study of 24 adults, the Garmin Forerunner 920XT came in at 7.3% average error, though it still underestimated the lab value by a statistically significant margin.23 The catch with Garmin is that its estimate leans on GPS-tracked outdoor runs. No run, no number worth trusting. If you treat these devices as trend-indicators, their information can be valuable. But don’t confuse it with your exact VO2 max.
Does the Number Itself Extend Life?
Even setting aside the question of whether a watch can measure it accurately, there is a deeper question about what VO2 max itself represents. In 2024, a research team led by Dr. Alisa Kjaergaard at the Steno Diabetes Center in Aarhus, Denmark used a technique called Mendelian randomization, which uses inherited genetic variation to test whether a trait plays a causal role in an outcome. Their finding, published in the Journal of Clinical Endocrinology and Metabolism: while VO2 max is an extraordinary predictor of mortality, a genetic predisposition for high VO2 max did not directly correlate with longer life.24 The study suggested that the factors that produce a high VO2 max (low body fat, high muscle mass, consistent physical activity) may be the actual causal drivers, with VO2 max itself serving as a kind of receipt for the metabolic work those factors have performed. Whether the number itself extends life or merely reflects a body that is extending its own life through composition and activity remains an active debate. Dr. Daniel Lieberman, a professor of human evolutionary biology at Harvard, has argued from an evolutionary framework that the distinction may be largely academic: human healthspans and lifespans are, in his analysis, both a cause and an effect of habitual physical activity, making the two inseparable.25 The prescription is the same either way. Train. Get fitter. Live better. Live longer.
A Direct Line to the Aging Brain
New research from 2024 has also identified a direct neurobiological link that should change how we think about exercise and the aging brain. High cardiorespiratory fitness is associated with greater signal intensity in the locus coeruleus, a small cluster of neurons in the brainstem that produces norepinephrine, the neurotransmitter governing alertness, attention, and the stress response. A 2024 study from Trinity College Dublin documented the association in 41 healthy older adults. The researchers used a form of high-resolution brain MRI that measures the density and volume of brain tissue one tiny three-dimensional cube at a time, allowing them to isolate a structure as small as the locus coeruleus, which is only about 15 millimeters long. The fitter the participant, the brighter and denser that small brainstem cluster appeared on the scan, and the effect was specific to the locus coeruleus rather than to other neurotransmitter centers in the brain.26
The significance of that finding rests on earlier work by Dr. Heiko Braak and his team at the University of Ulm, who in 2011 examined 2,332 brains spanning ages 1 to 100 and concluded that the toxic protein tangles that define Alzheimer’s disease, called tau tangles, may first appear in the locus coeruleus, years before they reach the regions of the cortex responsible for memory.27 Tau is a protein that normally helps stabilize the scaffolding inside neurons, keeping them structurally intact. In Alzheimer’s, tau misfolds and clumps into tangles that choke the cells from the inside until they die. Braak’s work suggests that this process starts in the brainstem and works its way outward over decades. What the Trinity College data suggests is that cardiorespiratory fitness may physically fortify this structure, building what neurologists call “cognitive reserve,” a buffer of neural capacity that delays the onset of clinical symptoms even as pathology accumulates.26 The implications are unsettling in the best possible way. A 55-year-old who trains consistently may be reinforcing a brainstem structure whose degradation would otherwise begin silently eroding their cognitive function a decade later. The locus coeruleus does not announce its decline. By the time you notice, the erosion is well advanced. Fitness, it appears, is one of the few interventions that reaches it early enough to matter.
London’s Bus Workers: Where This Began
In the early 1950s, a British epidemiologist named Jerry Morris noticed something peculiar about London’s bus workers. The drivers, who sat all day behind the wheel, suffered heart attacks at nearly twice the rate of the conductors, who spent their shifts climbing up and down the stairs of double-decker buses collecting fares.28 Same employer, same routes, same socioeconomic background. The only difference was that one group moved and the other didn’t. Morris published his findings in The Lancet in 1953, and the paper is now regarded as the founding document of exercise epidemiology.28 It took the medical establishment decades to fully absorb what Morris had found: that physical activity was not a lifestyle preference but a biological requirement, and that its absence carried a cost measured in heart attacks, in years, in lives. Seventy years later, the Cleveland Clinic data says the same thing Morris said, with 122,007 data points and a precision he could not have imagined. The conductors outlived the drivers. The fit outlive the unfit. The mechanism is better understood now, the numbers are sharper, the protocols more refined. The fundamental truth has not changed.
A man climbed the stairs. He lived longer.
Frequently Asked Questions
What is a good VO2 max for my age?
In the FRIEND reference database, the median VO2 max for a man in his 40s is about 39 mL/kg/min. For a woman in her 40s, about 30. Those numbers fall roughly 10% per decade without training. With consistent aerobic work, the decline stalls or reverses. Where you land is largely a choice.
Can you actually improve VO2 max after 50?
Yes. A 2005 meta-analysis by Dr. Guoyuan Huang pooled 41 controlled trials in adults over 60, average age 67, and found an average VO2 max gain of 16.3% from structured endurance training. The cellular machinery for adaptation never fully shuts down. You can always get better.
How accurate is the VO2 max reading on my Apple Watch?
Not very. A 2024 Technical University of Darmstadt validation study found the Apple Watch Series 7 had an average error of 16% against lab testing. Fit users can be lowballed by 12 mL/kg/min, roughly the gap between average and top 10% for their age. Trend indicator, not lab result.
What is the Norwegian 4×4 workout?
The Norwegian 4×4 is the most research-validated VO2 max workout in the literature. A ten-minute warmup. Then four intervals of four minutes at 90 to 95% of maximal heart rate, with three minutes of active recovery between each. A five-minute cooldown. Developed in Norway in 2007.
Is VO2 max really more important than cholesterol or blood pressure?
By the numbers, yes. In the Cleveland Clinic’s 122,007-patient study, low VO2 max carried a hazard ratio of about 5.0 for all-cause mortality. Smoking: 1.4. Coronary artery disease: 1.3. Diabetes: 1.4. Low fitness outpredicted every traditional risk factor screened in that cohort.
Keep Reading
- Unlock the Cellular Power of Intense Exercise
- The Centenarian Code: Simple Secrets to a Longer, Healthier Life
- Exercise Is the Most Powerful Drug, and It’s Free
- Hack Your Fitness Motivation With This One Simple Trick
The Receipts
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2. American College of Sports Medicine, Gary Liguori, Yuri Feito, Charles Fountaine, and Brad Roy. ACSM’s Guidelines for Exercise Testing and Prescription. Eleventh edition. Philadelphia: Wolters Kluwer, 2021.
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5. Kaminsky, Leonard A., Ross Arena, Øyvind Ellingsen, Matthew P. Harber, Jonathan Myers, Cemal Ozemek, and Robert Ross. “Cardiorespiratory Fitness and Cardiovascular Disease: The Past, Present, and Future.” Progress in Cardiovascular Diseases 62, no. 2 (2019): 86-93. https://doi.org/10.1016/j.pcad.2019.01.002
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7. Kokkinos, Peter, Charles Faselis, Immanuel Babu Henry Samuel, Andreas Pittaras, Michael Doumas, Rayelynn Murphy, Michael S. Heimall, et al. “Cardiorespiratory Fitness and Mortality Risk across the Spectra of Age, Race, and Sex.” Journal of the American College of Cardiology 80, no. 6 (2022): 598-609. https://doi.org/10.1016/j.jacc.2022.05.031
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12. Fleg, Jerome L., Christopher H. Morrell, Angelo G. Bos, Larry J. Brant, Laura A. Talbot, Jeanette G. Wright, and Edward G. Lakatta. “Accelerated Longitudinal Decline of Aerobic Capacity in Healthy Older Adults.” Circulation 112, no. 5 (2005): 674-82. https://pubmed.ncbi.nlm.nih.gov/16043637/
13. Hawkins, Steven A., and Robert A. Wiswell. “Rate and Mechanism of Maximal Oxygen Consumption Decline with Aging: Implications for Exercise Training.” Sports Medicine 33, no. 12 (2003): 877-88. https://pubmed.ncbi.nlm.nih.gov/12974656/
14. Short, Kevin R., Maureen L. Bigelow, Jane Kahl, Ravinder Singh, Jill Coenen-Schimke, Sreekumar Raghavakaimal, and K. Sreekumaran Nair. “Decline in Skeletal Muscle Mitochondrial Function with Aging in Humans.” Proceedings of the National Academy of Sciences 102, no. 15 (2005): 5618-23. https://doi.org/10.1073/pnas.0501559102
15. Verdijk, Lex B., Tim Snijders, Tanya M. Holloway, Janneau Van Kranenburg, and Luc J. C. Van Loon. “Resistance Training Increases Skeletal Muscle Capillarization in Healthy Older Men.” Medicine & Science in Sports & Exercise 48, no. 11 (2016): 2157-64. https://doi.org/10.1249/mss.0000000000001019
16. Booth, Frank W., Matthew J. Laye, and Michael D. Roberts. “Lifetime Sedentary Living Accelerates Some Aspects of Secondary Aging.” Journal of Applied Physiology 111, no. 5 (2011): 1497-504. https://pubmed.ncbi.nlm.nih.gov/21836048/
17. Milanović, Zoran, Goran Sporiš, and Matthew Weston. “Effectiveness of High-Intensity Interval Training (HIT) and Continuous Endurance Training for VO2max Improvements: A Systematic Review and Meta-Analysis of Controlled Trials.” Sports Medicine 45, no. 10 (2015): 1469-81. https://pubmed.ncbi.nlm.nih.gov/26243014/
18. Huang, Guoyuan, Cheryl A. Gibson, Zung V. Tran, and Wayne H. Osness. “Controlled Endurance Exercise Training and VO2max Changes in Older Adults: A Meta-Analysis.” Preventive Cardiology 8, no. 4 (2005): 217-25. https://pubmed.ncbi.nlm.nih.gov/16230876/
19. Oliveira, Adriano, Andressa Fidalgo, Paulo Farinatti, and Walace Monteiro. “Effects of High-Intensity Interval and Continuous Moderate Aerobic Training on Fitness and Health Markers of Older Adults: A Systematic Review and Meta-Analysis.” Archives of Gerontology and Geriatrics 124 (2024): 105451. https://pubmed.ncbi.nlm.nih.gov/38728872/
20. Seiler, Stephen. “What Is Best Practice for Training Intensity and Duration Distribution in Endurance Athletes?” International Journal of Sports Physiology and Performance 5, no. 3 (2010): 276-91. https://doi.org/10.1123/ijspp.5.3.276
21. Helgerud, Jan, Kjetill Høydal, Eivind Wang, Trine Karlsen, Pål Berg, Marius Bjerkaas, Thomas Simonsen, et al. “Aerobic High-Intensity Intervals Improve VO2max More Than Moderate Training.” Medicine & Science in Sports & Exercise 39, no. 4 (2007): 665-71. https://doi.org/10.1249/mss.0b013e3180304570
22. Caserman, Polona, Sungsoo Yum, Stefan Göbel, Andreas Reif, and Silke Matura. “Assessing the Accuracy of Smartwatch-Based Estimation of Maximum Oxygen Uptake Using the Apple Watch Series 7: Validation Study.” JMIR Biomedical Engineering 9 (2024): e59459. https://doi.org/10.2196/59459
23. Passler, Stefanie, Julian Bohrer, Lukas Blöchinger, and Veit Senner. “Validity of Wrist-Worn Activity Trackers for Estimating VO2max and Energy Expenditure.” International Journal of Environmental Research and Public Health 16, no. 17 (2019): 3037. https://doi.org/10.3390/ijerph16173037
24. Kjaergaard, Alisa D., Christina Ellervik, Niels Jessen, and Sarah J. Lessard. “Cardiorespiratory Fitness, Body Composition, Diabetes, and Longevity: A 2-Sample Mendelian Randomization Study.” The Journal of Clinical Endocrinology & Metabolism 110, no. 5 (2025): 1451-59. https://doi.org/10.1210/clinem/dgae393
25. Lieberman, Daniel E., Timothy M. Kistner, Daniel Richard, I-Min Lee, and Aaron L. Baggish. “The Active Grandparent Hypothesis: Physical Activity and the Evolution of Extended Human Healthspans and Lifespans.” Proceedings of the National Academy of Sciences 118, no. 50 (2021): e2107621118. https://doi.org/10.1073/pnas.2107621118
26. Plini, Emanuele R. G., Michael C. Melnychuk, Ralph Andrews, Rory Boyle, Robert Whelan, Jeffrey S. Spence, Sandra B. Chapman, Ian H. Robertson, and Paul M. Dockree. “Greater Physical Fitness (VO2max) in Healthy Older Adults Associated with Increased Integrity of the Locus Coeruleus-Noradrenergic System.” Acta Physiologica 240, no. 8 (2024): e14191. https://doi.org/10.1111/apha.14191
27. Braak, Heiko, Dietmar R. Thal, Estifanos Ghebremedhin, and Kelly Del Tredici. “Stages of the Pathologic Process in Alzheimer Disease: Age Categories from 1 to 100 Years.” Journal of Neuropathology & Experimental Neurology 70, no. 11 (2011): 960-69. https://doi.org/10.1097/NEN.0b013e318232a379
28. Morris, Jeremiah N., J. A. Heady, P. A. B. Raffle, C. G. Roberts, and J. W. Parks. “Coronary Heart-Disease and Physical Activity of Work.” The Lancet 262, no. 6796 (1953): 1111-20. https://doi.org/10.1016/s0140-6736(53)91495-0
