VO2 max and cardiorespiratory fitness
The best-evidenced single predictor of mortality that you can deliberately change, and the one that most training programmes ignore.
goodWell supported, with the usual caveats about effect size.Revised 17 Jul 2026
Most of the gain comes from easy, boring volume rather than from intervals.Photograph: Cam Adams / Unsplash
Definition
The maximum rate at which the body can take up and use oxygen during exercise, in millilitres per kilogram per minute. It integrates heart, lungs, blood, vasculature and muscle into one number, which is why it predicts so much and why it is hard to fake.
The association
Across large cohorts, low cardiorespiratory fitness is associated with higher all-cause mortality, and the gradient is steep at the bottom end — the difference between the least fit group and the next one up is larger than any step further along the scale.
That shape matters for what to do about it. Going from sedentary to modestly active is where nearly all the health return is. Going from fit to very fit is a performance decision, not a health one.
How it changes with age
VO2 max declines with age by default, and the decline accelerates after roughly the sixth decade. Training does not stop the slope, but it moves the whole line up — and because disability arrives when capacity falls below the demands of ordinary life, starting higher buys years of independence rather than years of racing.
What raises it
- Volume of easy aerobic work. Conversational pace, most of the time. This is the unglamorous majority of the effect.
- Some hard intervals. Four to six minutes near maximum, repeated, is the classic protocol and it does add on top of the base.
- Consistency over months. Meaningful change takes 8–12 weeks and keeps coming for a year or more in untrained people.
The WHO’s 2020 guidelines are the sensible floor: 150–300 minutes of moderate-intensity or 75–150 minutes of vigorous-intensity activity per week, plus muscle-strengthening on two or more days.
Measuring it
A laboratory test with a mask is the real measurement. Everything else — watch estimates, the Cooper test, submaximal step tests — is an estimate with meaningful error, though the error is often consistent enough that the trend is usable.
For most people the practical proxy is simpler: how you feel on a familiar climb, or how long a given pace stays conversational. If that is improving across a year, the number is going the right way.
Open questions
Whether high-intensity interval training is genuinely superior to equivalent volumes of moderate work, or merely faster, remains argued. What is not argued is that both beat nothing by a distance that makes the comparison academic.