U-shaped association between non-protein calorie-to-nitrogen ratio and mortality in adults with overweight and obesity: a population-based cohort study

The non-protein calorie-to-nitrogen ratio (NPC/N, kcal/g) is recognized as a valuable metric for assessing the balance between non-protein energy intake and nitrogen derived from protein. This study aimed to investigate the association between NPC/N and mortality among overweight and obese adults. Data were obtained from the National Health and Nutrition Examination Survey (1999–2018), with mortality follow-up through December 31, 2019. Among 22,892 participants, 3,332 all-cause deaths occurred over up to 20 years of follow-up. Kaplan–Meier curves showed that the intermediate NPC/N group (90–160 kcal/g) had lower mortality than both the low (< 90 kcal/g) and high (≥ 160 kcal/g) groups (log-rank test P = 0.012). Restricted cubic spline analysis demonstrated a U-shaped association between NPC/N and all-cause mortality, with the risk nadir at approximately 120 kcal/g ( P for non-linearity = 0.027, P for overall = 0.022). In fully adjusted models, NPC/N was inversely associated with all-cause mortality below the nadir (HR = 0.61, 95% CI: 0.39–0.94, P = 0.030) and positively associated above it (HR = 1.12, 95% CI: 1.01–1.25, P = 0.040); log-likelihood ratio test P = 0.006. These suggest that both lower and higher NPC/N are associated with increased mortality.