Sex-Specific Obesity Paradox in Critically Ill Patients with Severe Acute Kidney Injury: A Retrospective Analysis

OBJECTIVES: Although obesity is typically correlated with adverse outcomes in various diseases, certain acute critical illnesses exhibit a phenomenon known as the obesity paradox. This study evaluated sex-specific differences in the prognostic implications of the body mass index (BMI) of patients with severe acute kidney injury (AKI) requiring continuous renal replacement therapy (CRRT). DESIGN: A multicenter retrospective cohort study. SETTING: Data from eight tertiary hospitals in Korea. PATIENTS: A total of 3805 critically ill patients receiving CRRT. Patients were categorized into four BMI groups: underweight, normal weight, overweight, and obese. INTERVENTIONS: None. MEASUREMENTS AND MAIN RESULTS: There were 2308 male and 1497 female patients. The 90-day mortality risk significantly differed among BMI groups in the overall patient population and the male subpopulation but not the female subpopulation. Following adjustment for confounding variables, the 90-day mortality risk was higher in the underweight group than in the obese group (hazard ratio [HR], 1.20; 95% CI, 1.05-1.36; p = 0.006). Among male patients, the 90-day mortality risk was higher in both the underweight and normal weight groups than in the obese group (underweight: HR, 1.30; 95% CI, 1.10-1.53; p = 0.002 and normal weight: HR, 1.18; 95% CI, 1.04-1.35; p = 0.010); however, no such association was observed among female patients. Subgroup analysis demonstrated the obesity paradox in male patients of old age, with septic AKI, or with low albumin levels. CONCLUSIONS: There were sex differences in the association between BMI and mortality in critically ill patients with severe AKI. Although the precise distribution of fat mass and muscle mass was not identified, obese male patients had a more favorable prognosis, which was not evident among female patients. These findings highlight the importance of considering sex-specific factors in understanding the complex relationship between obesity and mortality in critically ill patients with AKI.