A recent study published in the European Journal of Preventive Cardiology has identified a widespread systematic bias in previous studies that may have caused them to overestimate the impact of physical fitness on mortality risk.
The study included data from over one million Swedish men who were conscripted for military service between 1972 and 1995. The participants were divided into five groups based on their fitness levels when they entered military service. Each man was 18 at the time of conscription, and the study followed all of the participants until their 60s or until they died. Those who died had their cause of death identified using the National Cause of Death Register. Using this data, the relationship between fitness levels at late adolescence and premature death was analyzed. The study controlled for various factors in their models, including BMI, age, age at conscription, parents’ income, and parents’ education level.
The results showed that the group with the highest levels of fitness had a 58% lower risk of death from cardiovascular disease, a 31% lower risk of dying from cancer, and a 53% lower risk of dying from any cause compared to the group with the lowest fitness levels. However, those at the highest fitness levels also showed a 53% lower risk of dying from random accidents, such as car accidents and homicides, compared to the group with the lowest fitness levels. Dying from random accidents was used in this study as a negative control outcome; there should be no relationship between fitness and death from random accidents, so finding an association may indicate that the groups being compared are not actually comparable. This issue was further confirmed when they compared results for siblings with different fitness levels, which also showed an association when there should have been none.
The study authors stated that their results do not suggest that exercise is unimportant; instead, the results suggest that the relationship between fitness and mortality is more nuanced than previously thought.
Sources: European Journal of Preventive Cardiology, Science Daily