Early Body Mass Index Trajectory as a Marker of Metabolic and Nutritional Changes in Critically Ill Patients

Background: Body mass index (BMI) is a common nutritional marker, but admission-only measurements present limitations. Early dynamic BMI changes may better reflect metabolic stress and fluid balance. However, the clinical significance of early BMI trajectory during critical illness remains poorly understood. This study evaluated the impact of early BMI trajectory on mortality and ventilator weaning in critically ill patients. Methods: This retrospective cohort study included 1355 adult patients (ICU stay ≥ 7 days) admitted to the medical ICU between 2019 and 2025. BMI trajectory was defined as the percentage change from admission to day 7 and was categorized into three groups: decrease (>5% reduction), stable (±5%), and increase (>5% gain). Multivariable Cox proportional hazard and logistic regression analyses were performed to evaluate the association between BMI trajectory and clinical outcomes. Results: Of the 1355 patients, 15.9%, 57.7%, and 26.4% were in the decrease, stable, and increase groups, respectively. The increase group demonstrated significantly higher hospital mortality (52.5%) than the decrease (41.9%) and stable (40.0%) groups (p = 0.001). Multivariable analysis revealed that an increasing BMI trajectory was independently associated with higher hospital mortality (HR 1.25, 95% CI 1.05-1.48). A decreasing BMI trajectory strongly predicted successful ventilator weaning (OR 2.76, 95% CI 1.81-4.21). Conclusions: Early BMI trajectory significantly predicted ICU outcomes. Increasing and decreasing BMI were associated with higher mortality and improved ventilator weaning, respectively. These findings suggest that BMI trajectory may be a simple surrogate marker of metabolic stress, nutritional status, and fluid balance during early critical illness.