Association of Reallocation of Movement Behaviours with Ectopic Fat Accumulation: A Cross-Sectional Analysis of the UK Biobank Cohort

PURPOSE: Ectopic fat in non-adipose tissues is linked to higher chronic-disease risk. Using the UK Biobank, this study examined associations of device-measured physical activity (PA) and sedentary time with ectopic fat. METHODS: Liver and pancreas proton density fat fraction (PDFF), and anterior-thigh skeletal muscle fat infiltration (MFI) were assessed by magnetic resonance imaging. Reallocations among accelerometer derived time spent sedentary and in light intensity PA (LPA), and moderate-to-vigorous intensity PA (MVPA) were examined using isotemporal substitution. Multi-variable linear regression was employed with stepwise adjustment for sex, age, height, socioeconomic factors, and counts of cancer and non-cancer illnesses. Sex*activity interaction terms were then added to the models, and sex-stratified analyses were conducted where interactions were significant. RESULTS: The final sample comprised 9,213 participants with liver PDFF, 6,521 participants with pancreatic PDFF, and 9,197 participants with MFI (mean age ~64 years). Replacing 30 minutes of sedentary time with LPA was associated with lower ectopic fat, with relative percentage differences in fully adjusted models of -2.2% [-2.7%, -1.8%] for liver PDFF, 1.9% [-2.4%, -1.3%] for pancreas PDFF, and -0.5% [-0.7%, -0.4%] for MFI. Substituting sedentary time with MVPA showed greater differences (-9.9% [11.0%, 8.7%], 7.9% [-9.2%, -6.6%], and -3.6 % [-3.9%, -3.2%], respectively). Replacing LPA with MVPA was also associated with lower ectopic fat (-7.6% [-8.9%, -6.4%], -6.0% [7.5%, -4.6%], and -3.0% [-3.4%, -2.6%] for liver PDFF, pancreas PDFF, and MFI. Associations were stronger in females for liver and pancreas PDFF and MFI when sedentary time was replaced with LPA or MVPA. CONCLUSIONS: Reallocation of sedentary time or lower-intensity PA to higher intensity PA is linked to lower ectopic fat. These relationships are stronger in women than men.