Sex- and Age-Specific Differences in the Joint Effects of Diet Quality and Physical Activity on Depressive Symptoms in Adults: A Cross-Sectional Study

<b>Background/Objectives:</b> Diet quality and physical activity are modifiable lifestyle factors linked to depressive symptoms, but their joint association-and potential variation by sex and age-remains unclear, particularly in Asian populations. This study examined the joint association of diet quality, measured by the Korean Healthy Eating Index (KHEI), and physical activity (PA) with depressive symptoms in Korean adults. <b>Methods:</b> We analyzed 17,737 adults aged ≥20 years from 2014, 2016, 2018, and 2020 KNHANES. Diet quality and PA were each dichotomized at the median to construct four lifestyle groups. Depressive symptoms were defined as Patient Health Questionnaire-9 (PHQ-9) ≥ 10. Survey-weighted multivariable logistic regression adjusted for sociodemographic factors, health-related behaviors, and major chronic diseases was performed, and stratified analyses were conducted by sex and age to explore potential variation across subgroups. <b>Results:</b> Overall, 4.6% of participants had depressive symptoms. Compared with the Low KHEI & Low PA group, the High KHEI & High PA group had substantially lower odds of depressive symptoms (aOR 0.55, 95% CI 0.42-0.73). In sex-stratified analyses, this joint association was significant only among women (aOR 0.48, 95% CI 0.35-0.66). In age-stratified analyses, significant associations were observed in adults aged 45-65 years (aOR 0.42, 95% CI 0.26-0.66) and ≥65 years (aOR 0.41, 95% CI 0.27-0.63). <b>Conclusions:</b> High diet quality combined with high physical activity was associated with markedly lower odds of depressive symptoms, particularly among women and middle-aged to older adults. These findings support integrated lifestyle strategies targeting both diet quality and physical activity, tailored to demographic and social contexts to enhance mental-health benefits across diverse populations.