Cardiorespiratory fitness and longevity: what VO₂max predicts and how to raise it

Cardiorespiratory fitness is the ability of the heart, lungs, blood and muscles to take up and use oxygen during sustained exercise. Across large cohort studies, it is one of the strongest single predictors of how long adults live: people in the lowest fitness group die earlier than fitter people of the same age, and in one large cohort the difference was comparable to or larger than that linked to smoking or diabetes. The largest drop in risk comes from moving out of the least-fit group, not from reaching athletic levels, and both steady aerobic training and intervals raise fitness. Most of this evidence is observational, so it shows a strong association rather than a proven cause.

How fitness is measured

The reference measure is VO₂max, the highest rate at which the body can consume oxygen, usually expressed in millilitres per kilogram of body weight per minute (mL/kg/min). It is measured by analysing breathing gases during an exercise test that ends at exhaustion.

Medical studies often report fitness in METs (metabolic equivalents). One MET is roughly the oxygen used at rest, set at 3.5 mL/kg/min. A VO₂max of 35 mL/kg/min therefore equals 10 METs. In many large studies, METs are not measured directly but estimated from the speed and incline reached on a treadmill test.

The 2016 American Heart Association scientific statement names gas-analysis testing as the gold standard and lists submaximal tests, the six-minute walk and questionnaire-based equations as less precise alternatives. VO₂max values shown by sports watches belong in the same estimated category: useful for following a trend, not an exact measurement.

What the evidence shows about mortality

A 2024 umbrella review in the British Journal of Sports Medicine pooled 26 systematic reviews covering 199 cohort studies and more than 20.9 million observations. In adults without known disease, high fitness was associated with a 41–53% lower risk of death from any cause than low fitness, and each additional MET with an 11–17% lower risk. The authors rated the certainty of the evidence as very low to moderate, mainly because most participants in the underlying studies were men and the results varied between studies.

A 2018 Cleveland Clinic cohort in JAMA Network Open followed 122,007 patients referred for treadmill testing for a median of 8.4 years. Fitness was graded against people of the same age and sex. The least fit group had about five times the mortality risk of the fittest 2.3%, after adjustment for other risk factors. The risk linked to low fitness was comparable to or greater than the risk linked to smoking, diabetes or coronary artery disease in the same data. The study population consisted of patients sent for testing, not the general public, and the design cannot prove that fitness itself caused the difference.

Where the largest gain lies

The relationship is curved. According to the AHA statement, more than half of the reduction in all-cause mortality occurs between the least fit group, below about 5 METs, and the next group, at 5–7 METs. Fitness above roughly 8–10 METs is associated with better survival. Each 1-MET increase is associated with a 10–20% lower mortality risk, and the statement suggests an increase of at least 10% as a first goal for lowering cardiovascular risk.

The Cleveland data found no upper limit: elite performers still had lower mortality than those with high fitness. That extra difference, however, was statistically significant only in patients aged 70 or older and in those with high blood pressure. For a sedentary adult, the practical message is to leave the bottom category. For a trained athlete, further gains have a smaller expected effect on mortality.

A worked example

Illustration, not a prescription: an adult with a measured VO₂max of 25 mL/kg/min.

A 1-MET gain here is a 14% rise in VO₂max. In the cohort data, a difference of that size corresponds to roughly 10–20% lower mortality risk. This is a population association; it does not predict what will happen to one person.

How training raises VO₂max

A 2015 meta-analysis in Sports Medicine analysed 28 controlled trials with 723 healthy adults aged 18–45. Compared with no exercise, continuous endurance training raised VO₂max by about 4.9 mL/kg/min and high-intensity interval training (HIIT), repeated hard efforts separated by recovery, by about 5.5 mL/kg/min. Both gains are about 1.5 METs. Intervals gave a small additional benefit of about 1.2 mL/kg/min. The trials involved young, healthy adults, so the results may not transfer directly to older or less healthy people, and the AHA statement describes comparisons between the two methods as inconsistent.

Responses also vary between individuals. A 24-week trial in Mayo Clinic Proceedings randomised 121 sedentary middle-aged adults with abdominal obesity to five sessions a week at different amounts and intensities. With a small amount at low intensity, 38.5% showed no meaningful fitness gain. Doubling the amount at the same intensity reduced that share to 17.6%, and the same larger amount at higher intensity reduced it to zero. In that sample, a lack of improvement reflected the dose of training more than a fixed inability to respond.

Turning this into a training week

A heart-rate zone system makes the dose visible. In Pulz8, zones are percentages of maximum heart rate: zone 2 is 60–69%, zone 3 is 70–79%, and zone 4 is 80–89%. For a stored maximum of 180 beats per minute, zone 2 starts at 108 and zone 4 at 144. These boundaries come from a stored maximum, not from a laboratory test, so they approximate each person's intensity.

A week that follows the evidence above combines regular moderate aerobic work with a smaller amount of interval work. On a shared class board, the useful check is whether the hard intervals reach high zones and the steady work stays moderate, not whether everyone shows the same colour.

Key points

1. Measure or estimate a baseline, and treat watch or equation estimates as trends rather than exact values.
2. Prioritise leaving the lowest fitness group, because the largest reduction in risk lies there.
3. Aim first for about a 10% increase in VO₂max, the first goal suggested by the American Heart Association.
4. Combine steady aerobic sessions with some intervals, since both raise VO₂max in controlled trials.
5. Increase the amount or intensity when fitness does not improve, before concluding that someone does not respond to training.

Sources

• Lang JJ 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. British Journal of Sports Medicine, 2024.
• Mandsager K et al. Association of cardiorespiratory fitness with long-term mortality among adults undergoing exercise treadmill testing. JAMA Network Open, 2018.
• Ross R et al. Importance of assessing cardiorespiratory fitness in clinical practice: a case for fitness as a clinical vital sign. American Heart Association, Circulation, 2016.
• Milanović Z, Sporiš G, Weston M. Effectiveness of high-intensity interval training (HIT) and continuous endurance training for VO2max improvements. Sports Medicine, 2015.
• Ross R, de Lannoy L, Stotz PJ. Separate effects of intensity and amount of exercise on interindividual cardiorespiratory fitness response. Mayo Clinic Proceedings, 2015.

Heart rate is a training signal, not medical advice. Anyone with a heart condition, or who is starting hard training after a long sedentary period, should discuss exercise testing and intensity with a doctor.