A new study from Uppsala University, published in the European Journal of Preventive Cardiology, examines the association between high cardiorespiratory fitness in late adolescence and the risk of premature death. While previous observational studies have consistently found that fitter individuals have a lower risk of dying early from diseases such as cancer and cardiovascular disease, this new research suggests that these associations may be more complex than previously thought.
The study analyzed data from 1.1 million Swedish men conscripted for military service between 1972 and 1995, dividing them into five groups based on fitness at around age 18. Participants were followed into their 60s or until death, with causes of death identified through the National Cause of Death Register. Traditional analyses showed that those with the highest fitness levels had a 58% lower risk of dying from cardiovascular disease, a 31% lower risk from cancer, and a 53% lower risk from all causes compared to the least fit group.
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Researchers then used a negative control outcome analysis, examining deaths from random accidents—such as car accidents, drownings, and homicides—where no link to fitness was expected. Surprisingly, high fitness was also associated with a 53% lower risk of dying from these accidents. This finding, confirmed in sibling comparisons, suggests that other unmeasured factors may influence both fitness and mortality risk, making it difficult to create truly comparable groups in observational studies.
Lead author Marcel Ballin commented, “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. The consequences may be that you overestimate the magnitudes of the effects you find.”