Aerobic fitness, much more than being thin or losing weight, matters more when calculating overall health and expected longevity.
Getty Images

Being in shape is far more crucial for a long, healthy life than being slim.

That’s the conclusion of the largest, most comprehensive study yet of the relationship between aerobic fitness, body mass index and longevity. A review and analysis of reams of earlier research, it found that being out of shape doubled or tripled the risk of dying prematurely, whatever someone’s age or body mass index.

On the other hand, if someone had obesity but was aerobically fit, he or she was about half as likely to die young as someone whose weight was normal but their aerobic fitness low.

“This tells us that it’s much more important, all things considered, to focus on the fitness aspect” of health and longevity, “rather than the fatness aspect,” said Siddhartha Angadi, an exercise physiologist at the University of Virginia and the study’s senior author.

The study, published in November in the British Journal of Sports Medicine, adds to the growing research that people can be healthy and long-lived at any weight, if they are also active and fit. That message may be especially timely now, as New Year’s resolutions are in full swing, since the findings suggest even a little exercise could be enough to improve our fitness and drop our mortality risk, whether we gained pounds in the last year or not.

Can you be heavy and healthy?

The question of whether you can be heavy but healthy has long interested scientists, as well as anyone else with a growing waistline. So far, the evidence has been mixed. Overall, people with obesity are at heightened risks for other serious conditions, such as diabetes, cancer and heart disease, and generally more likely to die at a younger age than people whose weight is normal.

Your BMI doesn’t factor into your overall health as much as aerobic fitness, experts say.
Getty Images

But recent science has hinted that being fit and active changes those outcomes, no matter someone’s BMI. In a 2021 review of prior research, for instance, researchers, including Angadi, compared the longevity gains from beginning a program of either exercise or dieting among people with obesity. Exercise dropped the risk of premature death by about 30%, even if people lost zero weight, which was about double the gains from shedding weight by dieting.

But many of these past studies involved somewhat small groups of people, the bulk of them men and Americans, and the research’s definitions of “fitness” often relied on subjective data, such as people’s memories of how much they’d exercised recently.

So, for the new study, Angadi and his colleagues decided to broaden their scope.

The link between BMI, fitness and longevity

“We wanted to include more women and get representation from other nations,” Angadi said.

They began by scrolling through research databases, looking for any past studies probing BMI, fitness and longevity that included objective measures of people’s aerobic fitness, usually from a cardiovascular stress test.

They wound up with 20 studies involving nearly 400,000 middle-aged or older people from multiple nations, about 30% of them women.

Pooling these studies, they divided the participants into the unfit, defined as anyone whose stress test placed their endurance into the bottom 20% of people of their age and gender, and the fit, whose testing put them into the top 80% of people of their age and gender.

They also pulled data about who’d died during follow-up periods of up to about two decades.

Finally, they compared BMI, fitness and death.

Brisk walks are enough to boost fitness

Obesity, as expected, was strongly associated with mortality. Men and women with obesity, if they also were unfit, were about three times more likely to have died prematurely than fit people whose BMI was normal.

But poor fitness had its own hazards. In fact, people of normal weight who landed in the bottom 20% of fitness were about twice as likely to have died young as people with obesity who qualified as fit.

“From a statistical standpoint, fitness largely eliminated the risk” of early death from obesity-related conditions, Angadi said.

“This study is important because it confirms that cardiorespiratory fitness is strongly protective against mortality at any BMI and strengthens the evidence that this relationship is true of women as well as men,” said Barry Braun, who is director of the Human Performance Clinical Research Laboratory at Colorado State University and who studies exercise and body weight but wasn’t involved in the new research.

The study also suggests requires little effort to move from being unfit to fit. Someone in the bottom 20% of fitness for his or her age just needs to exercise enough to rise into the 21st percentile of fitness, Angadi said.

For most of us, that could entail plenty of “brisk walks,” Angadi said. Moderate exercise — meaning any exertion that’s strenuous enough that you can talk but not sing, such as a swift, arm-swinging walk — reliably improves fitness. (If you’d like a precise measure of your current fitness, ask your doctor or a physiology lab about a stress test and check your numbers against charts of typical fitness levels by age and gender online.)

Then, maybe spend more time walking and otherwise working out than worrying about weight, said John Thyfault, a professor at the University of Kansas Medical Center who studies obesity, exercise and health. (He wasn’t involved with the new study.) The science now overwhelmingly shows that “aerobic fitness is more important for mortality risk than body weight status,” he said.

“Yes, people may want to lose weight for a variety of reasons,” he continued. “But it should be known that you can get more healthy at your current body weight just by moving more.”