Family Affluence and Lifestyle Behaviors as Determinants of Fat Mass Index in University Students: A Sex-Specific Structural Equation Modeling Approach

<b>Background/Objectives</b>: Family affluence is commonly considered an important contextual determinant of adiposity; however, its relative contribution compared with current lifestyle behaviors during early adulthood remains unclear. This study examined direct associations between family affluence, specific lifestyle indicators (physical activity, sedentary time, and dietary indices), and fat mass index (FMI) in university students. <b>Methods</b>: This cross-sectional study included 418 students (199 males, 219 females). Family affluence (FAS), physical activity (PA), sedentary time (SITT), and dietary behaviors (positive and negative dietary indices) were assessed using standardized questionnaires. To evaluate how family affluence and behavioral variables relate to fat mass index (FMI), a structural equation modeling approach was implemented. Sex-specific models were estimated for males and females independently. <b>Results</b>: In both sexes, physical activity was inversely associated with fat mass index (FMI) and represented the strongest protective factor (males: β = -0.36, 95% CI: -0.46 to -0.26; females: β = -0.35, 95% CI: -0.45 to -0.25; both <i>p</i> < 0.001). The negative dietary index showed a significant positive association with FMI in males (β = 0.34, 95% CI: 0.21 to 0.48; <i>p</i> < 0.001) and females (β = 0.26, 95% CI: 0.10 to 0.42; <i>p</i> = 0.001). Sedentary time was positively associated with FMI only in females (β = 0.15, 95% CI: 0.02 to 0.28; <i>p</i> = 0.022). No statistically significant direct effect of family affluence on FMI was observed in either males or females. The SEM models explained 30% of FMI variance in males and 37% in females. <b>Conclusions:</b> In this cross-sectional university sample, proximal lifestyle indicators showed stronger direct associations with FMI than family affluence. These findings suggest that interventions targeting physical activity and unhealthy dietary behaviors may be particularly relevant for adiposity prevention in early adulthood, although longitudinal research is required to clarify causal pathways.