Cardiorespiratory fitness

Cardiorespiratory fitness is the capacity of your heart, lungs, blood and muscles to take in oxygen and deliver it to working muscle during sustained exercise. It is the ceiling on how much aerobic work your body can do, and it is normally summarized by one number: maximal oxygen uptake, or VO2 max. It reads the whole oxygen transport chain at once rather than a single organ, from the air entering your lungs to the mitochondria in your legs. It is also one of the few health measures that responds strongly to training at almost any age, which is why Cardiorespiratory fitness (cardiorespiratory endurance) appears in longevity research far more often than its low profile in routine checkups suggests.

What it measures

The reference method is a graded exercise test: you walk, run or cycle at a workload that rises every few minutes while a mask samples the air you breathe out, until oxygen uptake stops rising or you stop. The peak value is VO₂max (maximal oxygen uptake). Most people never take that test, so fitness is usually estimated: from how long you last on a standard treadmill protocol, from a submaximal step or walk test, from the distance you cover in a fixed time, or from a watch that models it out of your heart rate at a given pace. Estimates follow the laboratory value only loosely, so use them to track your own trend over months rather than to compare yourself with someone else.

Typical values

Results are reported in milliliters of oxygen per kilogram of body weight per minute, or converted to METs, where one MET is defined as 3.5 mL/kg/min. There is no single normal range. Values are read against normative tables split by sex and by ten-year age band, so the same number can be average for a man in his sixties and poor for a woman in her twenties. Two things follow. Because the value is divided by body weight, losing fat mass raises it without the heart changing at all. And because fitness peaks in young adulthood and drifts down through the decades, holding a value steady across ten years is itself a good result.

Why it matters for longevity

In large cohorts of people who have had exercise testing, low cardiorespiratory fitness is associated with higher all-cause mortality, and in several of those datasets the association is as strong as the one seen for smoking, high blood pressure or diabetes. The gradient is steepest at the bottom of the range: the biggest difference in risk sits between the least fit group and the next one up, while gains among the already-fit are associated with smaller additional benefit. Higher fitness is also associated with lower rates of cardiovascular disease, type 2 diabetes, several cancers and dementia. The evidence is observational, and illness lowers fitness as well as the reverse. Trials show that training raises fitness and improves the risk factors that sit in between, but no trial has randomized people to a lifetime of high fitness and counted the deaths.

What changes it

Aerobic training, more than anything else. The usual structure is a base of longer easy-to-moderate sessions plus a small amount of hard interval work; in terms of Heart rate zones, that means most of your time low and a little of it near the top. Public health guidance of 150 to 300 minutes of moderate activity a week, or 75 to 150 of vigorous, is a reasonable floor. Changes are measurable in weeks to months, and lost at a similar speed when training stops. Other things pull the number down for reasons unrelated to training: anemia, untreated thyroid or lung disease, poor sleep, heavy alcohol use, added fat mass, and drugs such as beta blockers that cap heart rate. Genetics set both your starting point and how much you gain from a program. Chronological age you cannot change; the slope you travel on, largely, you can.

Related reading

Disclaimer. This article is for information only and does not replace medical advice. Talk to a qualified clinician before changing anything about your health.