Nutritional Imbalance and Oral Functional Limitation Jointly Associated with Depressive Symptoms in Adults Living Alone: A Nationally Representative Cross-Sectional Study

Background/Objectives: Adults living alone are vulnerable to nutritional inadequacy and depressive symptoms; however, their combined influence remains poorly understood. This study examined the independent and joint associations of nutritional imbalance and oral functional limitation with depressive symptoms among adults living alone and explored potential indirect pathways involving metabolic syndrome (exploratory). Methods: This cross-sectional study used nationally representative data from the Korea National Health and Nutrition Examination Survey (KNHANES) 2022 and 2024, including 1572 adults aged ≥19 years living alone. Nutritional imbalance was assessed by the number of essential nutrients consumed below the Estimated Average Requirement (EAR; range 0–8). Oral functional limitation was defined as chewing or speaking difficulty. Depressive symptoms were evaluated using the Patient Health Questionnaire-9 (PHQ-9). Survey-weighted regression and bootstrapped indirect-effect analyses were performed. Results: Nutritional imbalance was associated with higher PHQ-9 scores (β = 0.216; 95% CI: 0.098–0.335). Oral functional limitation (β = 1.278; 95% CI: 0.739–1.816) and metabolic syndrome score (β = 0.156; 95% CI: 0.006–0.306) were independently associated with depressive severity. Participants with both high nutritional imbalance and oral functional limitation had substantially higher odds of clinical depression (PHQ-9 ≥ 10; OR = 4.268; 95% CI: 2.037–8.943) than the reference group. Exploratory analyses suggested that indirect effects via oral functional limitation and metabolic syndrome were directionally consistent but did not reach statistical significance. Conclusions: Nutritional imbalance and oral functional limitation were jointly associated with depressive symptoms among adults living alone. These findings highlight the importance of integrated strategies targeting dietary quality, oral health, and metabolic health in vulnerable single-person households.