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Cardiorespiratory fitness

Cardiorespiratory fitness, usually measured as maximal aerobic capacity in an exercise test, is one of the most consistent predictors of health in large cohort studies: adults with higher fitness have lower rates of death from all causes and of cardiovascular events, with a dose-response pattern per 1-MET difference. Higher fitness is also associated with lower rates of cancer death, type 2 diabetes and depression, though that evidence is thinner. Controlled trials show that endurance and interval training both raise VO2max in healthy young to middle-aged adults. The fitness-outcome evidence is observational, so it shows association rather than proof that raising fitness causes the lower risk.

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Evidence retrieved up to

What the research shows

In cohort studies of adults, higher cardiorespiratory fitness is associated with lower all-cause mortality, with a dose-response pattern in which each 1-MET higher level of fitness goes with roughly 11-17% lower risk of death.

Moderate certaintyFurther research may change the size of the effect.
No known concerns at usual amountsBenefit

Who this is about

adults in the general population

Practical note

Graded moderate: the evidence is observational (GRADE starts at low) but very large and consistent across meta-analyses of cohort studies, with a large effect (hazard ratio 0.47 for high versus low fitness in the 2024 overview) and a clear dose-response gradient, which supports upgrading. Residual confounding and reverse causation keep it below high. Studies measured fitness as maximal aerobic capacity in metabolic equivalents (METs).

Sources

In cohort studies of healthy adults, higher cardiorespiratory fitness is associated with lower cancer mortality, though the association per unit of fitness is weaker than for cardiovascular outcomes.

Low certaintyFurther research is likely to change this.
No known concerns at usual amountsBenefit

Who this is about

healthy adults

Practical note

Graded low: observational cohort evidence with a dose-response gradient (risk ratio 0.93 per 1-MET higher fitness and 0.57 for highest versus lowest fitness across 34 cohorts), not upgraded because the per-MET effect is modest and confounding by smoking, body weight and other behaviours is plausible.

Sources

In cohort studies of adults, higher cardiorespiratory fitness is associated with fewer cardiovascular events and less heart failure; as part of a healthy lifestyle, better fitness may help reduce the risk of cardiovascular disease.

Moderate certaintyFurther research may change the size of the effect.
No known concerns at usual amountsBenefit

Who this is about

healthy adults

Practical note

Graded moderate: observational cohort evidence upgraded for a large, consistent effect and a dose-response gradient (per 1-MET higher fitness, risk ratio 0.85 for coronary or cardiovascular events in the 2009 meta-analysis of 33 cohorts and 0.87 for cardiovascular death in the 2022 meta-analysis of 34 cohorts; hazard ratio 0.82 for heart failure in the 2024 overview). Confounding by other health behaviours cannot be excluded.

Sources

In cohort studies of adults, higher cardiorespiratory fitness is associated with a lower rate of type 2 diabetes in a linear dose-response pattern; as part of a healthy lifestyle, better fitness may help reduce the risk of type 2 diabetes.

Low certaintyFurther research is likely to change this.
No known concerns at usual amountsBenefit

Who this is about

adults without type 2 diabetes at baseline

Practical note

Graded low: observational evidence from 22 cohort studies (over 1.6 million participants in the primary analysis) with a linear dose-response (8% lower relative risk per 1-MET higher fitness after adjustment for adiposity or body size); not upgraded because the effect per MET is small and residual confounding is likely.

Sources

In a small number of prospective cohort studies, adults with low or medium cardiorespiratory fitness had a higher rate of new-onset depression than those with high fitness, and in children and adolescents higher fitness is linked to fewer depressive symptoms.

Very low certaintyThis is uncertain; treat it as a lead, not a finding.
No known concerns at usual amountsBenefit

Who this is about

adults and young people in the general population

Practical note

Graded very low: the adult evidence pools only two prospective cohorts (described by the authors as preliminary), and the youth evidence is cross-sectional with high heterogeneity (I2 75%), so the direction of cause cannot be inferred.

Sources

In controlled trials of healthy young to middle-aged adults, both continuous endurance training and high-intensity interval training produced large increases in maximal oxygen uptake (VO2max) compared with no exercise, with somewhat larger gains after interval training.

Moderate certaintyFurther research may change the size of the effect.
No known concerns at usual amountsBenefit

Who this is about

healthy adults aged 18-45

Practical note

Graded moderate: controlled trials downgraded once for risk of bias, because not all trials were randomised and the analysis used magnitude-based inference. Trials lasted at least two weeks; endurance training raised VO2max by about 4.9 mL per kg per minute and interval training by about 5.5 mL per kg per minute compared with no-exercise controls.

Safety note

The trials enrolled healthy adults aged 18-45, so they do not describe training safety in older adults or in people living with heart conditions.

Sources