The causal relationships between leisure-time physical activity and body mass index in adulthood: A triangulation study

Abstract Background Previous studies have presented conflicting findings regarding the potential causal relationships between leisure-time physical activity (LTPA) and body mass index (BMI). Here, we use individual-level data and apply a triangulation framework that incorporates three complementary methods to investigate the bidirectional causal associations between LTPA and BMI. Methods We used data from a longitudinal Finnish twin cohort with four measurement points spanning 36 years. The data included 22,696 twin individuals aged 18-50 years at baseline (52.4% women); 8,527 had genetic data available. We applied three analytical approaches suggested to strengthen causal inference in observational studies: Random intercept cross-lagged path model (RI-CLPM) for longitudinal data, one-sample Mendelian Randomization (MR) and Direction of Causation (DoC and MR-DoC) twin models for cross-sectional data at each measurement point. Results All three approaches provided evidence for a causal effect of higher BMI on lower LTPA, particularly at the later follow-up stages. Only twin models suggested a negative causal effect of LTPA on BMI. Men and women showed mainly similar effects. Conclusions Evidence triangulation across the three methodologies provided support for a causal effect of higher BMI on lower LTPA, whereas the evidence for a reverse effect was less convincing. Our results indicate that the role of high BMI in limiting LTPA becomes more important with advancing age, while also highlighting the importance of accounting for timing when studying the causal effects of LTPA on BMI and vice versa.