A study suggesting that exercise does not impact mortality offers little more than click-bait
In the realm of health and longevity, the notion that physical activity plays a crucial role in reducing mortality is widely accepted. Regular exercise, particularly increasing cardiorespiratory fitness (CRF), has been shown to correlate with reduced risk of chronic diseases and longer life expectancy. However, a preprint study shared on bioRxiv recently challenged this understanding by suggesting that physical activity might not have a causal relationship with lower mortality. While such studies can generate attention and media buzz, it is important to critically analyze their methodology, data collection, and analysis to evaluate their credibility.
About the Study
The study in question, conducted by Kankaanpää et al., aimed to determine the relationship between leisure-time physical activity (LTPA) and all-cause mortality risk. The research was based on data from the older Finnish Twin Cohort, including participants who were between 18 and 50 years old in 1975, with mortality data tracked from 1990 through 2020. LTPA was assessed using self-reported questionnaires administered in 1975, 1981, and 1990, categorizing participants as sedentary, moderately active, active, or highly active. The study concluded that while individuals in the sedentary group had higher mortality rates, after adjusting for factors like body mass index (BMI), smoking, and alcohol use, the link between physical activity and reduced mortality risk weakened significantly, and in some cases, disappeared entirely.
Study Strengths and Weaknesses
One strength of the study is its long follow-up period, spanning over 30 years. However, the weaknesses of this study are considerable and significantly impact the credibility of the conclusions drawn. The primary issue with the study lies in its reliance on self-reported data from 1975, 1981, and 1990, which are subject to numerous biases, including inaccurate recall and social desirability biases. Given that the data used to classify participants into different activity levels were collected decades before mortality data, it is highly questionable whether a participant’s reported activity level during those years accurately reflects their ongoing physical activity. For example, how likely is it that someone who was moderately active at 30 continued to be so when they reached 65? Furthermore, the data collection methods excluded key variables that could have influenced mortality risk, such as body composition, blood pressure, and adherence to preventive health measures like cancer screenings. The study also fails to consider the possibility that professional athletes, workers with physically demanding jobs, or people who engage in physical activity as part of their occupation were misrepresented in the leisure-time activity categories. These individuals could be classified as sedentary based on the study’s narrow definition, despite being highly active in their work.
Questionable Data Analysis and Interpretation
The analysis of the data also raises significant concerns. After adjusting for BMI, smoking, alcohol consumption, and education, the difference in mortality risk between the sedentary and moderately active or active groups diminished. While the highly active group initially showed a significant reduction in mortality risk, this effect was lost when the analysis excluded participants with preexisting conditions like angina, heart disease, or diabetes. This exclusion introduces a type of bias known as "informative censoring," where groups are defined based on a variable (in this case, physical activity) that is directly related to the outcome (mortality). This can result in misleading conclusions, as those who remained sedentary for health reasons (e.g., due to chronic disease) were removed from the analysis, skewing the results.
The Role of Preprints in Scientific Communication
This study was published as a preprint, which means it has not undergone the peer-review process, leaving its findings vulnerable to scrutiny. Preprints can serve as a valuable tool for quickly disseminating research, but they also carry the risk of premature conclusions that have not yet been rigorously tested. In this case, the lack of peer review and the significant methodological flaws undermine the study’s credibility. Although preprints can prompt important discussions, their findings should be taken with caution until they undergo the more thorough vetting process of peer-reviewed publication.
Evaluating the Larger Picture
While this study’s findings suggest that physical activity might not be causally linked to reduced mortality risk, the broader body of research strongly supports the opposite conclusion. Numerous studies have shown that increased physical activity is one of the most effective ways to reduce the risk of chronic diseases such as heart disease, diabetes, and certain types of cancer. The overwhelming consensus in the scientific community remains that exercise is beneficial for health and longevity. This study, with its reliance on self-reported data, exclusion of critical variables, and flawed statistical analysis, does little to overturn this well-established understanding.
Conclusion
Although the study by Kankaanpää et al. may seem to present a challenge to the well-established benefits of physical activity, its methodological flaws and limited scope undermine its conclusions. In the quest for reliable scientific knowledge, it is crucial to assess studies not only based on their novelty but also on the robustness of their data, analysis, and interpretations. Physical activity continues to be one of the most important factors for maintaining good health, and no single study, especially one that fails to meet rigorous scientific standards, should be allowed to change that fundamental truth.
Update from Peter Attia, on 2024-09-14Source