Adult obesity and mid-life physical functioning in two British birth cohorts: investigating the mediating role of physical inactivity

BACKGROUND: Associations between obesity and physical inactivity are bi-directional. Both are associated with physical functioning (PF, ability to perform physical tasks of daily living) but whether obesity influences PF via inactivity is unknown. We investigated whether mid-adult obesity trajectories were associated with subsequent PF and mediated by inactivity. METHODS: Body mass index (BMI; kg/m(2)) and inactivity were recorded at: 36, 43, 53 and 60-64 years in the 1946 Medical Research Council (MRC) National Survey of Health and Development (1946-NSHD; n = 2427), and at 33, 42 and 50 years in the 1958 National Child Development Study (1958-NCDS; n = 8674). Poor PF was defined as the lowest (gender and cohort-specific) 10% on the Short-form 36 Physical Component Summary subscale at 60-64 years (1946-NSHD) and 50 years (1958-NCDS). Estimated randomized-interventional-analogue natural direct (rNDE) and indirect (rNIE) effects of obesity trajectories on PF via inactivity are expressed as risk ratios [overall total effect (rTE) is rNDE multiplied by rNIE]. RESULTS: In both cohorts, most individuals ( approximately 68%) were never obese in adulthood, 16-30% became obese and