Body muscle mass versus fat mass: gender-specific associations with NAFLD and liver fibrosis.

Background: This study aimed to investigate the gender-specific associations of skeletal muscle mass and fat mass with non-alcoholic fatty liver disease (NAFLD) and NAFLD-related liver fibrosis in two population-based studies.

Methods: Analyses were based on data from the MEGA (n = 238) and the MEIA study (n = 594) conducted between 2018 and 2023 in Augsburg, Germany. Bioelectrical impedance analysis was used to evaluate relative skeletal muscle mass (rSM) and SM index (SMI) as well as relative fat mass (rFM) and FM index (FMI); furthermore, the fat-to-muscle ratio was built. The fatty liver index (FLI) was calculated to identify NAFLD. To estimate the degree of liver fibrosis, liver stiffness was assessed in the MEGA study using ultrasound elastography. Multivariable gamma regression models with log-link were used to analyze the respective associations.

Results: After adjusting for confounders, FMI and rFM (p < 0.001) as well as SMI (p < 0.001) were significantly positive associated with FLI. The associations were non-linear, and effect modification by gender (p_{gender-interaction} < 0.001) existed in all models except for SMI, while age modified the rSM and SMI results. The effect estimates for FMI and rFMI were higher in men than in women but approached each other at the high FMI range. Increasing rSM was inversely associated with FMI. The fat-to-muscle ratio was positively associated with FLI in men and women. However, no associations were found between the exposure variables and liver fibrosis.

Conclusions: This population-based study demonstrated gender-specific positive associations between fat mass parameters and FLI, and an inverse association with rSM. Furthermore, for rSM effect modification by age was observed. Neither in men nor in women associations between body composition and liver fibrosis could be identified.