Smoking and Risk of Hepatic Steatosis Within the MASLD Spectrum in Korean Young Adults

Metabolic dysfunction-associated steatotic liver disease (MASLD) is an increasing public health concern, particularly among young adults. Although smoking has been implicated in hepatic steatosis, large-scale longitudinal evidence remains limited.

We analyzed data from the Korean National Health Insurance Service, including 3 496 144 adults aged 20-39 years who underwent health examinations between 2004 and 2007 and were followed through 2022. Hepatic steatosis was assessed using the Fatty Liver Index (FLI), a validated surrogate marker, and categorized as < 30, 30-59, or ≥ 60. Smoking exposure was evaluated at baseline according to smoking amount, duration, and pack-years. Cox proportional hazards models were used to estimate adjusted hazard ratios (HRs) and 95% confidence intervals (CIs).

Smoking was associated with an increased risk of incident FLI-defined hepatic steatosis across multiple exposure metrics. Among men, smoking ≥ 20 cigarettes/day was associated with the highest risk of FLI ≥ 60 (HR 1.41, 95% CI 1.40-1.43), whereas 10-19 years of smoking was associated with an HR of 1.15 (95% CI 1.14-1.15). Among women, 10-19 years of smoking was associated with a 49% higher risk of FLI ≥ 60 (HR 1.49, 95% CI 1.43-1.56). Associations were generally stronger among participants with BMI < 25 kg/m² or alcohol intake < 25 g/day. Risk estimates varied according to FLI threshold.

Smoking was independently associated with an increased risk of FLI-defined hepatic steatosis in young adults. These findings support smoking prevention and cessation strategies as potential measures to reduce early-onset steatotic liver disease.