Sex Differences in the Association Between Ultra-Processed Food Consumption and NAFLD: An Analysis of KNHANES 2013–2021 Data

<b>Background/Objectives:</b> Ultra-processed food (UPF) consumption is increasingly implicated in metabolic diseases; however, evidence for non-alcoholic fatty liver disease (NAFLD) and potential sex differences remains limited. Thus, this study aimed to examine the relationship between UPF consumption and NAFLD stratified by sex in Korean adults. <b>Methods</b>: This was a cross-sectional analysis of Korean adults from the Korea National Health and Nutrition Examination Survey 2013-2021 (<i>n</i> = 24,587). UPF intake was quantified as the percentage of NOVA Group 4 items consumed in total daily food weight based on a 24 h recall. The participants were grouped into quartiles of UPF intake. NAFLD was defined using the hepatic steatosis index. Survey-weighted models were used to summarize the characteristics and estimated odds ratios (ORs) for NAFLD across the UPF quartiles with adjustment for factors associated with both NAFLD and dietary intake. Linear trend tests across quartiles and continuous analyses of UPF intake were performed, with sex-stratified models to assess potential effect modification. <b>Results</b>: NAFLD prevalence increased as UPF intake quartile increased, from 19.1% in Q1 to 24.1% in Q4. With Q1 as reference, the fully adjusted OR for Q4 was 1.24 (95% CI, 1.10-1.41, <i>p</i> for trend = 0.001). In the sex-stratified analyses, the association was only significant in women (Q4 vs. Q1: OR, 1.52, 95% CI, 1.28-1.81; <i>p</i> for trend < 0.001). When UPF intake was modeled as a continuous variable, NAFLD risk showed a modest overall increase, with a nearly flat pattern in men and a clear linear increase in women. <b>Conclusions</b>: Higher UPF intake is associated with a greater risk of NAFLD in Korean adults, with a more pronounced association in women. Thus, UPF consumption is a feasible modifiable target for liver health.