Quasi-causal associations of physical activity and neighborhood walkability with body mass index: A twin study

•Twin models can be used to compute quasi-experimental associations.•Moderate-to-vigorous activity is causally associated with body mass index.•Neighborhood walkability is causally association with neighborhood walking.•Neighborhood walking is causally associated with body mass index.

ObjectivePhysical activity, neighborhood walkability, and body mass index (BMI, kg/m2) associations were tested using quasi-experimental twin methods. We hypothesized that physical activity and walkability were independently associated with BMI within twin pairs, controlling for genetic and environmental background shared between them.MethodsData were from 6376 (64% female; 58% identical) same-sex pairs, University of Washington Twin Registry, 2008-2013. Neighborhood walking, moderate-to-vigorous physical activity (MVPA), and BMI were self-reported. Residential address was used to calculate walkability. Phenotypic (non-genetically informed) and biometric (genetically informed) regression was employed, controlling for age, sex, and race.ResultsWalking and MVPA were associated with BMI in phenotypic analyses; associations were attenuated but significant in biometric analyses (Ps < 0.05). Walkability was not associated with BMI, however, was associated with walking (but not MVPA) in both phenotypic and biometric analyses (Ps < 0.05), with no attenuation accounting for shared genetic and environmental background.ConclusionsThe association between activity and BMI is largely due to shared genetic and environmental factors, but a significant causal relationship remains accounting for shared background. Although walkability is not associated with BMI, it is associated with neighborhood walking (but not MVPA) accounting for shared background, suggesting a causal relationship between them.