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In a nutshell
- A major study of 1.1 million Swedish men found that teenage fitness is linked to lower risk of death — but the connection may not be causal.
- Fit teens were just as protected against accidental deaths as they were against disease, suggesting other hidden factors at play.
- Researchers urge caution in assuming teenage fitness guarantees longer life and recommend deeper studies using genetic and social controls.
UPPSALA, Sweden — For decades, parents have pushed their children to play sports and stay fit, believing it would help them live longer. But that connection we’ve all taken for granted might be built on shaky ground.
Swedish scientists tracking over a million men found that the link between adolescent fitness and living longer could be largely due to hidden factors that previous research missed.
“Our findings suggest that observational estimates supporting the role of adolescent cardiorespiratory fitness in preventing premature mortality may be characterized by widespread bias,” the researchers write in their paper, published in the European Journal of Preventive Cardiology.
The Surprising Discovery
What types of “bias” might the authors be referring to? Consider how being in shape might prevent death in random accidents unrelated to the ailments we typically associate as outcomes of one’s physical health. Study authors examined whether teenage fitness levels affected not only deaths from diseases but also deaths from accidents like car accidents and drownings.
What they uncovered was eye-opening. Men who were fitter as teenagers were 53% less likely to die in accidents compared to less fit peers—almost identical to the 58% reduced risk of heart disease deaths.
The data revealed something unexpected: being fit as a teenager shouldn’t logically protect someone from accidental death decades later, yet the study found almost identical protective effects for both disease and accident outcomes.
When the team compared brothers with different fitness levels but similar genetics and upbringing, the pattern remained, though somewhat weaker.
Behind the Numbers
The study followed 1,124,049 Swedish men who underwent military service between 1972 and 1995. Each 18-year-old completed a standardized bicycle test measuring cardiorespiratory fitness (basically, how efficiently your heart and lungs deliver oxygen during exercise).
Researchers then tracked these men for nearly four decades. During this time, approximately 65,000 men died—about 16,800 from cancer, 21,000 from heart problems, and 14,400 from accidents.
Initially, the results seemed to confirm conventional wisdom: the fittest men had a 53% lower chance of dying compared to the least fit. Cancer risk was 31% lower and heart disease risk 58% lower among the fittest group.
But the revelation that these same men showed a 53% lower risk of accidental deaths raised serious questions about the entire relationship.
The Hidden Factors
“People with higher fitness tend to differ from their peers in many ways besides just being fit. They often have different socioeconomic resources, behaviors, and genetic predispositions,” the researchers write in their paper. They add that “there is a well-known social gradient in both fitness and major somatic diseases,” highlighting how social background influences both fitness levels and longevity.
Of course, this doesn’t mean exercise is all for naught early on. Substantial research demonstrates that physical activity offers numerous benefits, from improved mood to better sleep.
Rather, the specific claim about teenage fitness directly causing a longer life appears more complicated than previously understood.
Rethinking Health Claims
Lead author Marcel Ballin says the study points to the need for more careful examination of cause-and-effect claims in health research.
“It surprised us that the association with accidental mortality reflected the other associations, even after we controlled for all the factors that siblings share. This underlines how strong the assumptions are that you make in observational studies, since it appears to be very difficult to create comparable groups,” Ballin explains in a statement. “The consequences may be that you overestimate the magnitudes of the effects you find.”
The study recommends future research use multiple approaches to better understand these relationships, including “cross-country comparisons (with different bias structures), negative controls, sibling and twin-comparisons, and various types of instrumental variable analyses, including (but not limited to) Mendelian randomization.”
Until then, parents and health officials might want to focus on the immediate, proven benefits of physical activity rather than making broad claims about longevity.
“That the effects of good cardiorespiratory fitness may be overstated might sound controversial to some, but the fact is that if you look at the results from studies others than traditional observational studies, a more nuanced picture does emerge,” Ballin adds. “A number of twin studies for example have found similar results. Some genetic studies also suggest that there are genes that affect both the propensity to be physically active or have a good fitness level, and the risk of developing diseases such as cardiovascular disease.
“If we just ask the question in the same way, we will always get the same answer,” he concludes. “It’s only when we get the same answer to a question that we have asked in slightly different ways that we can be sure that the findings are accurate.”
Paper Summary
Methodology
The researchers analyzed data from 1,124,049 Swedish men who underwent mandatory military conscription between 1972-1995 at around age 18. Each man completed a standardized bicycle ergometer test to measure cardiorespiratory fitness, with results recorded as Watt-max. The researchers then linked this data with national registers to track deaths through December 31, 2023. They used two key analytical approaches: first, comparing fitness levels to accidental deaths as a “negative control outcome” (something not expected to be causally affected by fitness), and second, comparing siblings with different fitness levels to control for shared family factors. All analyses adjusted for factors including age, conscription year, BMI, and parental education and income.
Results
Higher cardiorespiratory fitness was associated with significantly lower risk of all-cause mortality (53% reduction for top fitness quintile vs. bottom), cancer mortality (31% reduction), and cardiovascular disease mortality (58% reduction). However, a nearly identical reduction (53%) was observed for accidental deaths, which theoretically shouldn’t be affected by fitness. When comparing siblings, all associations were weakened but the pattern persisted. This suggests strong confounding factors (socioeconomic, genetic, behavioral) influence both fitness and mortality risk in ways that typical statistical adjustments cannot fully address.
Limitations
The study only included male participants and followed them until their early 60s on average, limiting generalizability to women and older age groups. While the study used objective fitness measures and high-quality national registers with minimal attrition, it cannot definitively prove the absence of causal effects. The authors suggest additional research using different methodological approaches is needed.
Funding and Disclosures
The authors received no specific funding for this work, though one author (V.H.A.) is funded via grants from the National Institute on Aging and the National Institute of Neurological Disorders and Stroke. One author (M.B.) is employed at the Swedish Medical Products Agency but noted the views expressed do not necessarily represent those of the agency. M.B. also reported receiving a lecture honorarium from the Swedish Heart Lung Foundation.
Publication Information
This study titled “Cardiorespiratory fitness in adolescence and premature mortality: widespread bias identified using negative control outcomes and sibling comparisons” was published on May 15, 2025, in the European Journal of Preventive Cardiology. It was authored by Marcel Ballin, Anna Nordström, Peter Nordström, and Viktor Hugo Ahlqvist from various Swedish and Scandinavian institutions, primarily Uppsala University.
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