Association of Body Mass Index (BMI) Changes with Short-term Mortality Risks in ICU Patients with Sepsis across different admission BMI states: Analysis of the MIMIC-IV database

Zhenhua Huang¹Wei Liu²Wenfei Zeng³Qinghua Yuan^4*

• ¹Shenzhen Second People's Hospital, Shenzhen, China
• ²Huangpu People's Hospital of Zhongshan, Zhongshan, China
• ³Hunan Provincial People's Hospital, Changsha, China
• ⁴Sun Yat-sen University Sixth Affiliated Hospital, Guangzhou, China

Background: Static body-mass index (BMI) is a known predictor of mortality in sepsis; however, the prognostic value of dynamic BMI trajectories across various admission BMI states remains unclear. This study aims to quantify the dose–response relationship between ICU-acquired BMI changes and 30-day mortality, and to determine BMI-specific thresholds for risk stratification. Methods: This retrospective multinational cohort study analyzed 5,577 adult sepsis patients from the MIMIC-IV database (2008-2022). To delineate the nature of the relationship between BMI change rate and 30-day mortality across distinct baseline BMI strata, we employed multivariable Cox proportional-hazards regression coupled with restricted cubic splines. A two-segment linear regression model with a recursive algorithm was then applied to pinpoint inflection points for each BMI-defined subgroup. Results: Among 5,577 ICU sepsis patients (mean age 66.5 ± 15.8 y; 57.4 % male), 2,068 deaths (37.1 %) occurred within 30 days. BMI change during ICU stay ranged from −39 % to +49 %. After multivariable adjustment, each 1 % increase in BMI change rate was associated with a 2 % higher 30-day mortality (hazard ratio [HR] 1.02; 95 % confidence interval [CI] 1.02–1.02; P < 0.001). However, in the underweight subgroup (<18.5 kg/m²) no significant association was observed (HR 0.99; 95 % CI 0.98-1.00; P= 0.093). Restricted cubic spline analyses revealed BMI-specific inflection points: −2 % in underweight, +4 % in normal-weight (18.5–24.9 kg/m²), and −1 % in overweight/obese (≥25 kg/m²) patients (p non-linear< 0.001 for all). Dynamic BMI metrics significantly outperformed admission BMI for predicting 30-day death (P< 0.001). Conclusion: In critically ill ICU patients with sepsis, the relationship between BMI change (%) and 30-day mortality is non-linear and varies across baseline BMI. Among patients with an admission BMI ≥18.5 kg/m², increase in BMI during the ICU stay is associated with higher mortality risk, indicating that weight gain is deleterious. Conversely, in patients with an admission BMI <18.5 kg/m², a decline in BMI markedly amplifies the risk of death. Tailored and dynamic weight-management strategies, accounting for baseline BMI trajectories, may therefore mitigate sepsis-related mortality.