Estimating the effect of physical activity on cognitive function within the UK Biobank cohort

BACKGROUND: Physical activity (PA) has been associated with benefits for cognitive function (CF), but previous estimates of the strength of this relationship may have been biased due to limitations in statistical modelling practices that are common among observational studies. We aimed to address this by using a rigorously constructed conceptual causal model to guide an empirical analysis estimating the effect of PA on CF in the UK Biobank cohort of middle-aged and older adults. METHODS: This study analysed a subsample of 334 227 adults from the UK Biobank prospective cohort study. PA was measured subjectively by self-report and by device using accelerometry, and CF was measured using objective cognitive tests. Composite CF measures were derived to represent general and domain-specific performance. Effect coefficients were estimated using regression models, adjusting for a wide range of confounders specified by the assumed causal model, including genetic risk factors, and relevant health, sociodemographic and behavioural variables from across the lifespan. RESULTS: Results indicated very small effect sizes (standardized mean difference estimates all <0.01) of inconsistent direction, for both cross-sectional and longitudinal analyses. CONCLUSIONS: The expected protective effect of PA on CF was not observed. This may reflect selection bias within UK Biobank, or the relatively young age of the sample at follow-up.