PurposeLow physical activity has been shown to be associated with a higher risk of nonalcoholic fatty liver disease (NAFLD). However, the strength and shape of this association are currently uncertain due to a reliance on self-reported physical activity measures. This report aims to investigate the relationship of median daily step count with NAFLD using accelerometer-derived step count from a large prospective cohort study.MethodsThe wrist-worn accelerometer substudy of the UK Biobank ( N = ~100,000) was used to characterize median daily step count over a 7-d period. NAFLD cases were ascertained via record linkage with hospital inpatient data and death registers or by using a measure of liver fat from imaging. Cox proportional hazards models were employed to assess the association between step count and NAFLD, adjusting for age, sociodemographic, and lifestyle factors. Mediation analyses were conducted.ResultsAmong 91,031 participants (709,440 person-years of follow-up), there were 762 incident NAFLD cases. Higher step count was log-linearly and inversely associated with risk of NAFLD. A 1000-step increase (representing 10 min of walking) was associated with a 12% (95% confidence interval, 10%-14%) lower hazard of NAFLD. When using imaging to identify NAFLD, a 1000-step increase was associated with a 6% (95% confidence interval, 6%-7%) lower risk. There was evidence for mediation by adiposity, accounting for 39% of the observed association.ConclusionsDaily step count, a modifiable risk factor, is log-linearly and inversely associated with NAFLD. This association was only partially explained by adiposity. These findings from a large cohort study may have important implications for strategies to lower NAFLD risk.