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The fitter you are, the better your odds for a long and healthy life. But does it make any difference whether your fitness is the lucky result of good genes rather than hard work at the gym?

In general, of course, people who exercise most also tend to have the highest values of fitness when measured with objective tests like walking or running to the point of exhaustion on a treadmill.

But exercise habits and fitness levels don’t always match up. By some estimates, as much as half of the variation in objectively measured fitness can be explained by genetic and environmental factors outside your control, rather than by your exercise habits. That means some people can ace a treadmill test despite being mostly sedentary, while others fare poorly on the treadmill despite arduous workout routines – and it’s not clear who is healthier.

In a new report published in the European Journal of Preventive Cardiology, Louise de Lannoy of the School of Kinesiology and Health Studies at Queen’s University in Kingston, Ont., and her colleagues draw on a database of almost 30,000 people who had multiple fitness assessments at the Cooper Clinic in Dallas, Tex., starting in the 1970s. They identified 1,014 matched pairs of men and women who had similar age, body-mass index, and – crucially – aerobic fitness as measured in a treadmill test.

One person from each pair reported being totally sedentary both at their initial baseline testing session and at a follow-up session an average of 15 years later. The other was sedentary at baseline but then changed their habits and reported exercising at least once a week at follow-up, significantly increasing their fitness. The result was a comparison of pairs of people with identical fitness levels, one of whom had to work for it while the other didn’t.

If this were a kids’ story with a nice tidy moral, the people who’d sweated for their fitness would reap the benefits by living longer. In reality, the 209 deaths that occurred during the study were split roughly evenly between the active and inactive groups, with no statistically significant difference between them. Being fitter was associated with dramatically lower risk of dying during the study, but it didn’t seem to matter whether your fitness was acquired through hard work or genetics.

While that may be disappointing to the puritans among us, it’s actually good news for people with low fitness, de Lannoy says, because it confirms that your current state can be altered: “By becoming physically active, you can improve fitness and lower your risk.”

For those whose fitness is already high, in contrast, it may be tempting to rest on your laurels. Why sweat at the gym if you stay reasonably fit even when you don’t exercise?

But don’t be too hasty. Another recent study, published in October in the journal JAMA Network Open, explored the relative benefits of high levels of fitness and found no evidence of diminishing returns. In a sample of 122,000 patients who completed treadmill fitness testing at the Cleveland Clinic in Ohio, those with “elite” levels of fitness – defined as roughly the top 2 per cent compared to their age and sex – were 29 per cent less likely to die during the study period than those with merely “high” levels of fitness. Whatever cards you’ve been dealt, you can make them better.

The findings about elite fitness stand in contrast to recent debates about whether too much exercise can be bad for your health. These claims rely on imperfect self-reported estimates of exercise. When you deal with objective measures of fitness, the Cleveland Clinic results, along with other similar studies, suggest there’s no such thing as being too fit.

So is lab-measured fitness the new gold standard for health prediction? In theory, yes. “However,” de Lannoy points out, “the only known way to improve fitness is through physical activity. That’s an important message, one that I hope is not lost here.”

Alex Hutchinson is the author of Endure: Mind, Body, and the Curiously Elastic Limits of Human Performance. Follow him on Twitter @sweatscience.

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