Fit Teens Live Longer? Maybe Not, Says New Study

A new study from Uppsala University calls into question decades of research suggesting that physical fitness in youth leads to longer life.

The large-scale analysis of over 1.1 million Swedish men found that highly fit teenagers showed similarly reduced risks of dying from both disease and random accidents—suggesting that the often-reported health benefits of early-life fitness may be significantly overstated. This surprising finding, published in the European Journal of Preventive Cardiology, reveals widespread bias in previous research and could reshape public health approaches to fitness promotion.

The researchers tracked Swedish men from their military conscription at age 18 through their 60s, examining how their adolescent fitness levels correlated with premature death from various causes. While the results initially seemed to confirm previous research—showing that the fittest young men had a 53% lower risk of all-cause mortality compared to the least fit—the study took an innovative approach by also analyzing deaths from random accidents.

Same Fitness, Same Protection Against Random Accidents?

The research team, led by Marcel Ballin, used a method called negative control outcome analysis to test whether fitness truly provides the health protection many studies claim. They examined deaths from accidents such as car crashes, drownings, and homicides—outcomes that should theoretically have minimal connection to teenage fitness levels.

The results were striking: men with the highest fitness levels had a 53% lower risk of dying in random accidents compared to those with the lowest fitness—almost identical to their reduced risk of all-cause mortality.

“We found that people with high fitness levels in late adolescence had a lower risk of dying prematurely, for example from cardiovascular disease, compared to those with low fitness levels. But when we looked at their risk of dying in random accidents, we found an almost similarly strong association,” explains Marcel Ballin, associated researcher in epidemiology and lead author of the study.

This unexpected finding suggests that highly fit and less fit individuals likely differ in important ways beyond their cardiovascular capacity—differences that previous studies haven’t fully accounted for.

Sibling Comparisons Strengthen the Case

To further test their hypothesis, the researchers compared siblings with different fitness levels. This approach controls for shared family factors like genetics, behaviors, and environmental conditions.

Even when comparing brothers, the pattern remained: highly fit men showed reduced mortality risk for both disease and random accidents, though the overall association was somewhat weaker than in the general population analysis.

“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 notes.

Redefining Expectations for Fitness Interventions

The study tracked specific mortality outcomes among the 1.1 million participants, finding:

• 58% lower risk of cardiovascular mortality for the fittest vs. least fit group
• 31% lower risk of cancer mortality for the fittest vs. least fit group
• 53% lower risk of all-cause mortality for the fittest vs. least fit group
• 53% lower risk of accidental mortality for the fittest vs. least fit group

The comparable reduction in accidental death risk suggests what statisticians call “residual confounding”—unmeasured or unmeasurable differences between study groups that create misleading correlations.

What might these differences be? Individuals with higher fitness often come from higher socioeconomic backgrounds, have different behavioral patterns, and may possess genetic traits that both enhance fitness capacity and reduce disease risk. These factors appear to be extremely difficult to fully account for, even with sophisticated statistical adjustments.

Does Exercise Still Matter?

Should these findings discourage physical activity? Ballin emphasizes this is not the case: “Our results should not be interpreted as if physical activity and exercise are ineffective or that you should not try to promote it. But to create a more nuanced understanding of how big the effects of fitness actually are on different outcomes, we need to use several different methods.”

The research aligns with previous twin studies and genetic research suggesting overlapping genetic factors that influence both fitness capacity and disease risk independently. This growing body of evidence indicates that the protective effects of fitness may be more modest than commonly believed.

Why does this matter? Public health initiatives and policies need accurate estimates of potential benefits to properly allocate resources. As Ballin explains, “Large-scale interventions or policy changes intended to apply to the entire population must be based on reliable estimates—otherwise there is a risk of expecting effects that have in fact been overestimated.”

The researchers suggest that future studies should triangulate findings using multiple methods including cross-country comparisons, negative controls, sibling and twin comparisons, and various types of instrumental variable analyses to establish more reliable estimates of fitness benefits.

While regular physical activity remains important for numerous health reasons, this study provides a valuable reminder of the complexity of human health outcomes and the care needed when interpreting observational research. As science continues to refine our understanding of fitness and longevity, the most accurate conclusion may be that the relationship between adolescent cardiorespiratory fitness and long-term health is more nuanced than previously thought.