How fit are you, really? The number that predicts a long life

In the early 1950s, heart attacks had become common in middle-aged men, and no one could say why. A Scottish doctor named Jerry Morris went looking for an answer in an unlikely place, London’s red double-decker buses.

Each bus had a crew of two. The driver sat at the wheel for his whole shift. The conductor walked the length of the bus and climbed its stairs to collect fares, again and again through the day. Both men worked the same streets for the same hours, and they often came from similar backgrounds. That left one clear difference between them, which was how much they moved.

In 1953, Morris published his findings in The Lancet, one of the world’s leading medical journals. The conductors were less likely to develop heart disease than the drivers. When they did develop it, it came on later in life and in a milder form. Morris then saw the same pattern in postmen. Those who delivered on foot had less heart disease than colleagues who worked at desks. Together, these studies started a new field of medicine, the study of exercise and health. Morris followed his own findings. He stayed active all his life and lived to 99.

Seventy years on, the question has moved from what people do to what their bodies can do. How fit are you? The answer is one of the most useful numbers a person can know.

What fitness really means

In medicine, fitness has a precise meaning. It is how well the heart, lungs, blood and muscles bring in oxygen and use it during effort. Doctors call this cardiorespiratory fitness.

Scientists have been able to measure it for a century. In 1923, the British scientist A. V. Hill, who had just won the Nobel Prize, showed that every person has a limit on how much oxygen their body can use during hard exercise. That limit is now called VO2 max, short for maximum volume of oxygen. The higher a person’s VO2 max, the more their body can do before they run out of breath.

VO2 max depends on almost every part of the body at once. The lungs pass oxygen into the blood. The heart pumps that blood to the muscles. Inside each muscle cell, mitochondria, the tiny energy factories, turn the oxygen into movement. One number sums up the whole chain.

What fitness predicts

In 2018, cardiologists at Cleveland Clinic put fitness to the test. They studied 122,007 adults, with an average age of 53, who had done a treadmill test at the clinic between 1991 and 2014. Each result was ranked against people of the same age and sex, from low to elite. Then everyone was followed for about eight years.

The results showed a clear pattern. With each step up in fitness, the risk of death during the study went down. The elite group, the top 2 to 3 percent for their age, had about an 80 percent lower risk than the least fit group. They even did better than the high group just below them, so the researchers found no upper limit to the benefit.

To put this in context, the researchers compared fitness with well-known risk factors. Low fitness carried a risk as large as smoking, diabetes or heart disease. In other words, being fit was among the most protective things a person could have.

Research across millions of people agrees. At every level, more fitness means lower risk.

More than a longer life

Fitness adds years, and it adds life to those years. Stairs, a heavy suitcase and a game with children all draw on the same heart and muscles. The fitter a person is, the lighter each of these feels.

The protection also reaches beyond the heart. Higher fitness is linked to lower rates of type 2 diabetes, several cancers, depression and dementia. It is even linked to better recovery after surgery.

Fitness changes slowly with age, often too slowly to notice. Measuring it turns that slow change into a clear number.

The vital sign that is rarely measured

Yet fitness is one of the least measured numbers in medicine. Blood pressure is checked at almost every visit. Fitness predicts long-term health at least as well, and few people ever learn theirs.

In 2016, the American Heart Association called for this to change. It recommended that fitness become a clinical vital sign, checked for every adult once a year, just like blood pressure.

The most precise way to check it is a supervised exercise test. During the test, a person walks or cycles while the effort rises step by step. Their heart and breathing are tracked until they reach their peak. The result shows their fitness today and how it compares with others of the same age and sex.

Smartwatches can estimate fitness too, and they are useful for tracking trends. A supervised test gives the clearest starting point.

The body responds

The good news is that fitness changes. Unlike age or family history, it responds to training, often within weeks.

Harder effort is one of the most efficient ways to raise it. In one well-known trial, people did four bursts of four minutes at high effort, three times a week. After eight weeks, their VO2 max had risen by about 7 percent. Their hearts had grown stronger and pumped more blood with each beat.

Still, people respond to training in their own way. The same programme can bring large gains for one person and smaller gains for another, and much of that difference runs in families. This is a strong reason to measure fitness before and after a new routine. The numbers show what is working for each person.

Every gain counts, and those starting lowest have the most room to grow.

Four habits that build fitness

So how much is enough? Health guidelines, from the World Health Organization to the UK’s chief medical officers, recommend at least 150 minutes of moderate activity a week, or 75 minutes of vigorous activity, or a mix of the two. Moderate activity means a person can still talk but not sing. Vigorous activity means it is hard to say more than a few words. The guidelines also recommend strength exercises on at least two days a week.

Four habits turn these guidelines into daily life. The first is brisk walking on most days, at a pace that makes breathing a little harder. The second builds on that walking until the week adds up to 150 minutes of moderate activity, in any form a person enjoys, from swimming to dancing. The third adds one or two sessions of harder effort, such as a few minutes of fast cycling or a steep walk uphill, with easy recovery in between. As the trial above showed, these short bursts raise fitness quickly. The fourth is strength training twice a week. Stronger muscles can use more of the oxygen the heart delivers, so strength supports every other habit.

Start gently and build over several weeks. Anyone with a heart or lung condition, chest pain or unusual breathlessness during effort should see a doctor before harder training.

Back to the bus

Morris’s conductors never thought of themselves as athletes. They were doing their jobs, one staircase at a time, and it protected their hearts.

Today that protection has a number. It can be measured, and it can be raised at almost any age. For a long and capable life, it may be the most useful number to know.

At The Wellness, fitness is assessed as part of the longevity and executive health assessment, alongside heart health, blood markers and an unhurried review with a doctor who looks at the whole person.

Sources

Morris JN and colleagues. Coronary heart-disease and physical activity of work. The Lancet, 1953.

Hill AV, Lupton H. Muscular exercise, lactic acid, and the supply and utilisation of oxygen. Quarterly Journal of Medicine, 1923.

Mandsager K and colleagues. Association of cardiorespiratory fitness with long-term mortality among adults undergoing exercise treadmill testing. JAMA Network Open, 2018.

Kodama S and colleagues. Cardiorespiratory fitness as a quantitative predictor of all-cause mortality and cardiovascular events in healthy men and women. JAMA, 2009.

Lang JJ and colleagues. Cardiorespiratory fitness is a strong and consistent predictor of morbidity and mortality among adults. British Journal of Sports Medicine, 2024.

Ross R and colleagues. Importance of assessing cardiorespiratory fitness in clinical practice. A scientific statement from the American Heart Association. Circulation, 2016.

Helgerud J and colleagues. Aerobic high-intensity intervals improve VO2max more than moderate training. Medicine and Science in Sports and Exercise, 2007.

Bouchard C and colleagues. Familial aggregation of VO2max response to exercise training. Results from the HERITAGE Family Study. Journal of Applied Physiology, 1999.

UK Chief Medical Officers’ physical activity guidelines, 2019.

World Health Organization guidelines on physical activity and sedentary behaviour, 2020.

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