Association of Weight-Adjusted Waist Index with All-Cause and Cardiovascular Mortality in Cardiovascular, Kidney, and Metabolic Syndrome Population: A Cohort Study Based on NHANES 1999–2018

Background Weight-adjusted waist index (WWI), an emerging obesity metric integrating body weight and waist circumference, is posited to reflect central obesity more accurately than traditional indices. This study investigated the association between WWI and all-cause and cardiovascular mortality in adults with cancer within the NHANES 1999-2018 cohort, exploring potential nonlinear relationships. Methods A total of 23,981 participants with cardiovascular, kidney, or metabolic syndrome from NHANES 1999–2018 were included, with a mean baseline age of 49.56 (SD 18.0) years. WWI was computed as waist circumference (cm) divided by the square root of weight (kg). Mortality data were obtained through linked National Death Index records until December 31, 2019. Cox proportional hazards regression models and Kaplan-Meier survival curve were used to explore associations between WWI and all-cause mortality (primary outcome) and cardiovascular mortality (secondary outcome). Results During follow-up, 3,622 deaths (15.1%) occurred. WWI showed a significant nonlinear association with all-cause mortality, with a threshold effect at 11.24. Below this inflection point, WWI was not significantly related to mortality (HR=0.98, 95% CI 0.88–1.09, P=0.6948) while above this point, higher WWI was associated with increased risk (HR=1.20, 95% CI 1.11–1.29, P<0.0001). A similar nonlinear relationship was observed for cardiovascular mortality, with threshold at 12.36 (HR below=1.09, 95% CI 0.98–1.21, P=0.0964; HR above=1.45, 95% CI 1.08–1.95, P=0.0137). In fully adjusted models, WWI was significantly associated with all-cause mortality (HR=1.11, 95% CI 1.06–1.17, P<0.0001) and cardiovascular mortality (HR=1.14, 95% CI 1.04–1.25, P=0.0037). Conclusion Elevated WWI is associated with increased all-cause and cardiovascular mortality risk among adults with cardiovascular, kidney, and metabolic syndromes, demonstrating a nonlinear threshold effect. WWI could play a key role in stratifying mortality risk in this high-risk population.