Estimated Dose-Dependent Associations Between Dietary Intake and Stress Responses in Adolescents and Young Adults: Evidence from a Regional Observational Study in Japan

Background/Objectives: The relationship between diet and mental wellbeing is bidirectional: nutritional psychiatry examines effects of diet on mental health, whereas emotional-eating research focuses on the reverse. Although dose–response links between nutrient intake and health outcomes are well studied, few have analyzed such relationships between food-group intake and stress. Using cross-sectional data from a regional observational study of Japanese adolescents and young adults (males: n = 48, age = 32.3 ± 5.23; females: n = 124, age = 29.8 ± 5.77), we estimated dose–response associations between intake frequency of various food groups and stress responses. Methods: Mental and physical health was assessed using the response values for the mental and physical stress-reaction items in the Brief Job Stress Questionnaire. Intake frequency for each food group was obtained from a food frequency questionnaire (FFQ). Although the FFQ provides ordinal frequency categories, we demonstrate that structural equation modeling allows these data to be treated as integer values representing monthly intake frequency, enabling the estimation of associations with stress responses. Results: The analysis showed that, in females, the presence or absence of intake—rather than the frequency of intake—of dark-green and yellow vegetables was associated with the magnitude of stress responses. In contrast, for bread, noodles, and starchy foods, a monotonic positive association was observed between intake frequency and fatigue. Conclusions: Although these associations do not disentangle the bidirectional causal relationship between diet and mental–physical health, they provide evidence on the linear and nonlinear associations between food-group–specific intake frequencies and stress responses in the AYA population.