An L-shaped association between prognostic nutritional index and all-cause mortality among adults with depressive symptoms: a cohort study using NHANES 2005–2018

BACKGROUND: depressive symptoms has risen sharply globally and increases the risk of death. However, the prognostic role of prognostic nutritional index (PNI) in this population remains limited. This study aimed to explore the association between PNI and all-cause mortality (ACM) in patients with depressive symptoms. METHODS: Using data from the National Health and Nutrition Examination Survey (NHANES) 2005-2018, the cohort study included 2,964 adults with depressive symptoms and a mean follow-up of 87.52 months. PNI was calculated using serum albumin and total lymphocyte count. Cox regression models were applied to estimate hazard ratios (HRs), while smooth curve fitting and threshold effect analysis assessed nonlinear relationships between PNI and ACM. Kaplan-Meier survival curves and subgroup analyses were conducted. RESULTS: During follow-up, 345 deaths occurred. Cox regression revealed a significant association between PNI and ACM. The highest PNI quartile had a 42% lower risk of mortality compared to the reference group (HR = 0.58, 95% CI: 0.41-0.80). Smooth curve fitting showed a L-shaped relationship, with an inflection point at 44.00. Significant interaction was observed for diabetes status (P for interaction = 0.001). CONCLUSIONS: This study demonstrated a L-shaped association between PNI and ACM in individuals with depressive symptoms. A higher PNI was associated with a lower mortality risk up to a threshold, beyond which further increases showed no additional protective effect in mortality risk. These findings suggest that PNI may serve as a potential marker for risk stratification and early intervention in patients with depressive symptoms.