Body mass index and mortality: The "Obesity Paradox" in critically ill patients with intra-abdominal infection or sepsis - An international cohort study

Objective: To assess the relationship between body mass index (BMI, i.e., kg/m²) and mortality in ICU patients with intra-abdominal infection or sepsis. Methods: This is a secondary analysis from AbSeS, an international, observational cohort study including patients with intra-abdominal infection (n = 2588). Patients were classified as underweight (BMI < 18.5), normal weight (BMI 18.5–24.9), overweight (BMI 25–29.9), or obese (BMI ≥ 30). Independent relationships with mortality were assessed by logistic and Cox regression. Results reported as odds ratios (OR) or hazard ratios (HR) 95 % and confidence interval (CI). Sensitivity analyses were performed for age (<65 or ≥ 65 years), sex, and sepsis or septic shock. Results: Overall mortality was 29.1 %. Logistic regression showed underweight to be associated with increased mortality (OR 1.76, 95 % CI 1.12–2.78), while obesity was independently associated with a decreased mortality risk compared to normal weight patients (OR 0.75, 95 % CI 0.58–0.97). Mortality in overweight patients was not different from patients with a normal weight. In patients < 65 years, the association with underweight remained, while obesity was no longer associated with decreased mortality. All significant relationships between BMI and mortality disappeared when only considering patients ≥ 65 years. Underweight was associated with increased mortality in patients with sepsis, but not in septic shock. In Cox regression, underweight remained an independent risk factor for death (HR 1.72, 95 % CI 1.25–2.35), whereas obesity was no longer associated with a decreased mortality risk. Conclusion: In the context of intra-abdominal infection in ICU patients, underweight is independently associated with increased mortality. The possible association of obesity with decreased mortality risk is less robust. Implications for Clinical Practice: Nutritional screening should be part of preoperative evaluation in elective abdominal surgery. If undernutrition is identified, targeted nutritional support should be initiated early to improve postoperative resilience and potentially reduce mortality risk in case of critical illness.