If you could measure a single number that best predicts how long and in what shape you will live, it would not have to be cholesterol or blood pressure. It would be VO2max – the maximum amount of oxygen your body can take in and use during exercise. It is the size of your biological engine: it depends on your lungs, heart, blood vessels, blood and the mitochondria in your muscles.
How to read the numbers: Risk = what to watch out for · Fact = a finding from research or biology
versus the highest fitness
of Norwegian 4×4 intervals
the health benefit clearly shows
Why VO2max
In a Cleveland Clinic study of more than 122,000 people who underwent a treadmill exercise test, those with the lowest fitness had roughly a five times higher risk of death during follow-up than those with the highest. For comparison, smoking in the same data was associated with a risk about 41% higher. The difference between below-average and above-average fitness was as large as the effect of smoking or diabetes. And no upper limit of benefit was found.
This is not an isolated result. A 2024 overview of meta-analyses summarising data from nearly 200 cohort studies showed that high cardiorespiratory fitness is linked to about half the risk of death from any cause, and every additional MET (a unit of metabolic output) means an 11–17% lower risk. As early as 2016, the American Heart Association therefore proposed that doctors measure fitness as a routine vital sign – just like blood pressure or heart rate.
The catch is that VO2max declines with age, and not evenly. Long-term follow-up of healthy adults in Baltimore showed a decline of 3–6% per decade in the twenties and thirties, but more than 20% per decade in the seventies. That is why you need to build your reserve for old age today.
Poor fitness is statistically as large a risk factor as smoking – and at the extremes an even larger one. Unlike your genes, however, you can change it within a few months.
Three pillars: zone 2, intervals and strength
The body produces energy in three “power plants”. The first handles only a few seconds of explosive effort, the second runs on muscle sugar and lasts roughly minutes, and the third – the oxidative one – burns both fat and sugar with oxygen and powers you throughout your life. The body improves at exactly what you train, so for a long life you need a hybrid: endurance, intensity and strength.
Zone 2 is a moderate, almost “boring” intensity – roughly around 70% of maximum heart rate. You can recognise it because you can still speak in full sentences, but your partner can hear that you are working. It trains slow muscle fibres, their mitochondria and the ability to burn fat. It is easiest to control on a stationary bike, during brisk uphill walking, rowing or on an elliptical trainer.
The most common mistake we see in the biohacking community is training in the so-called grey zone: too hard to build an endurance base and too easy to push VO2max. Easy days should be truly easy and hard days truly hard.
Norwegian 4×4 intervals: the gold standard for VO2max
Ordinary walking will not move your VO2max. For that, you need to go close to your ceiling. The protocol from the Norwegian University of Science and Technology in Trondheim is simple: four minutes at 90–95% of maximum heart rate, three minutes of active recovery at about 70%, four times in total. In a classic study of 40 men, after eight weeks (three sessions a week) VO2max in the 4×4 group rose by 7.2%, while the same amount of work at a moderate pace or at the lactate threshold did not improve it significantly.
Intervals, however, are a strong stress stimulus and raise cortisol. If you have had poor sleep, a demanding week at work or an illness, one more hard session can be the last straw. For most people one, at most two intense sessions a week are enough, and the rest of the movement should be calm.
Warm up easily for 10 minutes. Then go almost all-out for 4 minutes – on a bike, uphill or on a rowing machine – so that in the last minute you cannot say a coherent sentence. Follow with 3 minutes at a slow pace. Repeat four times in total and finish with 5 minutes of easy cool-down. If you have heart disease, high blood pressure or have not exercised for a long time, talk to your doctor first.
Strength training: the other half of the equation
VO2max is the engine, muscles are the chassis. After forty, it is above all the fast muscle fibres that start to disappear – the ones that catch you when you trip. A meta-analysis of 16 cohort studies showed that muscle-strengthening activity is associated with a 10–17% lower risk of death, cardiovascular disease, cancer and diabetes – independently of endurance activity. The greatest benefit appeared already at 30–60 minutes a week. Combining strength and endurance came out best.
The World Health Organization recommends strengthening all major muscle groups at least twice a week. Basic movements are enough: squat, push, pull, lunge and carrying loads.
How to estimate and measure VO2max
- Cardiopulmonary exercise testing – the most accurate measurement: an exercise test on a bike or treadmill with a mask that analyses the air you breathe out.
- Sports watches – estimate VO2max from heart rate and pace during running or walking. Treat the absolute number as a rough guide; the trend over time with the same type of measurement is what is valuable.
- Cooper test – how many metres you can run or walk in 12 minutes. A simple field estimate, suitable for comparing yourself three months later.
- Exercise test at the doctor’s – even without a mask, METs can be calculated from the workload achieved, which is exactly the measure used by the large mortality studies.
The minimum effective dose for busy people
Most people do not have ten hours a week for training. The good news is that much of the benefit comes with a relatively small dose. The REHIT protocol (reduced-exertion high-intensity training) takes ten minutes: easy pedalling on a bike and just two short all-out sprints, of 20 seconds each in the final phase. In a study of sedentary young adults, after six weeks (three sessions a week) VO2max rose by 15% in men and 12% in women. The condition is that those two sprints are truly maximal.
For everyday movement you do not need the magic 10,000 steps. A 2025 meta-analysis showed that 7,000 steps a day, compared with 2,000, is associated with a 47% lower risk of death, and the biggest jump in benefit occurs at around 5,000–7,000 steps. You can spread your steps throughout the day and add small “movement snacks”, such as squats before lunch.
- Once a week – Norwegian 4×4 intervals, about 40 minutes.
- Once or twice a week – full-body strength training, five basic exercises, 30 minutes.
- As time allows – one or two longer zone 2 sessions (45–60 minutes), or REHIT on days when you only have ten minutes.
- Every day – walking around 7,000 steps and short breaks from sitting.
A week like this takes roughly two to three hours and also covers the WHO recommendations: 150–300 minutes of moderate or 75–150 minutes of vigorous activity a week and muscle strengthening at least twice a week.
- Strong evidence Higher cardiorespiratory fitness is consistently associated with lower mortality and fewer chronic diseases; high-intensity intervals increase VO2max more effectively than the same amount of moderate exercise.
- Moderate evidence The benefit of 30–60 minutes of strength training a week and around 7,000 steps a day for lifespan – large but observational data.
- Weak evidence Specific benefits of zone 2 compared with other moderate endurance exercise, the accuracy of VO2max estimates from watches and the long-term results of shortened protocols such as REHIT.
- No evidence That “the harder, the better” applies, or that daily exhausting workouts extend life faster than sensibly dosed exercise.
Conclusion
VO2max is the currency of a long and independent life. You build it with patient moderate endurance work, one or two hard interval sessions a week and regular strength training that protects your muscles and bones. Measure your starting point, check it every few months and train today so that in thirty years you can still do everything you want to do.
