Long working hours and increased risks of lean non-alcoholic fatty liver disease among Korean men and women

Despite the increasing prevalence of lean nonalcoholic fatty liver disease (NAFLD), its risk factors are not well established. We examined the association between long working hours and incident NAFLD in lean Korean workers with emphasis on sex-based effect modification. This cohort study involved 46,113 non-overweight (BMI < 23 kg/m(2)) and NAFLD-free Korean workers (mean age, 35.5 years). Working hours were categorized into 35–40 (reference), 41–52, and ≥ 53 h. The presence of fatty liver and its severity were determined using ultrasonography and NAFLD fibrosis score (NFS), respectively. Hazard ratios (HRs) and 95% confidence intervals (CIs) were estimated using parametric proportional hazards models. Incident cases of 5901 lean NAFLD developed over a median follow-up of 3.8 years. The incidence of lean NAFLD increased with increasing working hours with a stronger association in men than in women (P for interaction < 0.001). For men, multivariable-adjusted HRs (95% CIs) for lean NAFLD in time-dependent models comparing working hours of 41–52 and ≥ 53 h compared to the reference category were 1.17 (1.07–1.28) and 1.25 (1.12–1.39), respectively. The excess relative risk of developing lean NAFLD with intermediate/high NFS was observed in working hours of 41–52 and ≥ 53 h with a corresponding HR of 1.66 (1.13–2.43) and 1.54 (0.94–2.51), respectively. Conversely, no significant associations were found between working hours and incidence of lean NAFLD in women. In conclusion, long working hours were significantly associated with an increased incidence of lean NAFLD and its severe form in men but not in women.