Dietary patterns and protein density associations with fat-free mass in a cross-sectional analysis of rural Iranian adults with overweight and obesity

Fat-free mass (FFM), predominantly skeletal muscle, is associated with metabolic health, functional capacity, and longevity. This cross-sectional study investigated the association between dietary patterns, protein density, and Fat-Free Mass Index (FFMI) in 2,299 adults (799 men, 1,500 women) with overweight or obesity (body mass index ≥ 25 kg/m²) from the Fasa Adult Cohort Study in rural Iran. Dietary intake was assessed using a validated food frequency questionnaire, identifying two major patterns as Healthy and Unhealthy. Body composition was measured via bioelectrical impedance analysis. Multivariable linear regression assessed associations, while partitioning models adjusted for Fat Mass Index (FMI) to isolate relationships independent of adiposity. Initially, the Healthy pattern was positively associated with FFMI in men (β = 0.17, P = 0.003) but lost significance after adjusting for FMI (β = 0.09, P = 0.077). Conversely, the Unhealthy pattern showed a positive association with FFMI in men only after FMI adjustment (β = 0.10, P = 0.050). Protein Density was consistently associated with higher FFMI in both men (β = 0.01, P = 0.008) and women (β = 0.01, P = 0.047), independent of adiposity. Physical activity significantly mediated this relationship. These findings suggest that while broad dietary patterns may be confounded by body size, protein density is a robust, independent factor associated with muscle mass Interventions should prioritize protein density and physical activity to optimize body composition.