Joint effects of physical activity and diet quality on depressive symptoms and all-cause mortality in adults with diabetes: NHANES 2007–2018

Depressive symptoms are common among adults with diabetes and are associated with adverse clinical outcomes, including mortality. Evidence from general populations suggests that physical activity (PA) and diet quality (DQ) may have a joint relationship with mental health and survival; however, evidence focusing specifically on adults with diabetes remains comparatively sparse. We analyzed data from 3451 U.S. adults with diabetes in the NHANES 2007-2018 dataset. Depressive symptoms were assessed at the baseline examination using the Patient Health Questionnaire-9 (PHQ-9 score ≥ 10), and all-cause mortality was ascertained through linkage to the National Center for Health Statistics linked mortality files up to December 31, 2019. PA (MET-min/week) and DQ (Healthy Eating Index-2015 [HEI-2015]) were examined using survey-weighted logistic regression for prevalent depressive symptoms and survey-weighted Cox proportional hazards models for subsequent mortality, with restricted cubic splines to assess dose-response relationships. Multiplicative and additive interactions between PA and DQ were evaluated. Additionally, complementary machine learning analyses were conducted to evaluate predictive performance and feature contributions. Higher PA and better DQ were generally associated with lower odds of depressive symptoms and lower hazards of all-cause mortality. A combined favorable PA and DQ profile was also associated with more favorable outcomes. PA showed a nonlinear inverse association with depressive symptoms, with lower predicted probabilities around 2000-3000 MET-min/week, whereas HEI-2015 did not demonstrate a nonlinear association. These findings highlight the potential importance of integrated lifestyle behaviors in relation to mental health and survival among adults with diabetes.