Institutional fast-food availability and sustainable diet quality among shift workers: the mediating role of food environment capacity and the moderating effect of physical activity

Shift workers are disproportionately exposed to institutional food environments shaped by long working hours and irregular schedules. In rapidly industrializing cities, institutional fast-food availability (IFFA) has become a routine component of daily food access for this population. However, existing evidence remains limited regarding how such exposure influences sustainable diet quality (SDQ), particularly through individuals’ capacity to navigate their food environments and the potential conditioning role of physical activity. This cross-sectional study surveyed 1,027 shift workers in Dongguan, China. Institutional fast-food availability, food environment capacity (FEC), physical activity (PA), and sustainable diet quality were measured using validated instruments. Partial least squares structural equation modeling was employed to estimate direct associations, test the mediating role of food environment capacity, and examine the moderating effect of physical activity. Conditional indirect effects were analyzed to assess moderated mediation. Institutional fast-food availability showed a significant positive association with sustainable diet quality (β = 0.281, p < 0.001). Food environment capacity partially mediated this relationship (indirect effect β = 0.056, p < 0.001), indicating that greater exposure to institutional food provision was associated with enhanced capacity to access and utilize healthier food options. Physical activity significantly moderated the pathway from institutional fast-food availability to food environment capacity (β = 0.108, p < 0.001) as well as the pathway from food environment capacity to sustainable diet quality (β = 0.088, p = 0.004). Conditional effect analyses further demonstrated that both the direct and indirect associations were substantially amplified among individuals with higher levels of physical activity, underscoring the conditional nature of the observed positive relationship. Among shift workers in Dongguan, institutional fast-food availability demonstrated a positive association with sustainable diet quality; however, this relationship was conditional rather than intrinsic. The beneficial association emerged primarily when individuals possessed stronger food-environment capacity and engaged in higher levels of physical activity. These findings indicate that institutional fast-food exposure does not directly enhance dietary sustainability per se but may function as a structural “safety net” under time-constrained working conditions when supported by individual competencies and health-oriented behaviors. Policies should therefore prioritize strengthening food environment capacity and promoting active lifestyles rather than focusing solely on restricting fast-food access. The findings should not be interpreted as evidence that fast food inherently promotes sustainable diets; rather, they highlight the importance of contextual constraints and individual adaptive capacity in shaping dietary outcomes within institutional food systems.