Relative fat mass and risk of metabolic dysfunction associated steatotic liver disease and severe hepatic steatosis in U.S. adults: analysis of NHANES 2017-2020 data.

Background: Relative fat mass (RFM) is a novel, easily calculated, and cost-effective index of fat content and distribution in the body, associated with the odds of developing various obesity-related diseases. However, its association with metabolic dysfunction associated steatotic liver disease (MASLD) and severe hepatic steatosis (SHS) is underexplored. This study aims to examine the relationship between RFM and the odds of having MASLD or SHS in the general adult population.

Methods: This was a population-based cross-sectional study using data from the National Health and Nutrition Examination Survey (2017.01-2020.03). The aim of the statistical analysis was to examine the association between RFM and the prevalence of MASLD and SHS. Logistic regression was applied to explore this relationship. Nonlinear associations between RFM levels and MASLD or SHS prevalence were assessed using smoothed curve fitting and threshold effect models. Subgroup analyses were conducted to evaluate the consistency of this association across different population groups.

Results: A total of 6699 participants were included in this study, of whom 2825 had MASLD and 1834 had SHS. After adjusting for confounders, significant positive associations were observed between RFM and the prevalence of MASLD and SHS (odds ratio [OR]: 1.22, 95% confidence interval [CI: ] 1.18-1.26 and OR: 1.26, 95% CI: 1.21-1.30). Smoothed curve fitting and threshold effect analysis showed a nonlinear relationship between RFM and the prevalence of MASLD and SHS, with thresholds of 41.96 for MASLD prevalence and 40.42 for SHS prevalence. When the subgroups were analyzed according to sex, age, race, education level, smoking status, household income, body mass index, hypertension, and diabetes, no significant interactions were found between RFM and most subgroups.

Conclusions: Our results demonstrated a positive nonlinear relationship between RFM and the prevalence of MASLD and SHS, with a threshold effect. Lower RFM levels are associated with lower odds of MASLD and SHS. These findings suggest that RFM may serve as a simple, cost-effective tool for identifying individuals at increased odds of NAFLD and SHS in the general population.