Hinton J.V.Raykateeraroj N.Ker C.J.Waldman B.Suh J.M.Pilcher D.Bellomo R.Lee D.K.Weinberg L.Mahidol University2026-02-062026-02-062026-04-01Anaesthesia Critical Care and Pain Medicine Vol.45 No.2 (2026)https://repository.li.mahidol.ac.th/handle/123456789/114354Background: The impact of body mass index (BMI) on morbidity and mortality in very old individuals with acute, critical illness is poorly understood. Methods: We conducted a bi-national, retrospective, entropy-weighted cohort study of 12,510 nonagenarians and centenarians admitted to the intensive care unit (ICU) over a 15-year period. Based on their admission BMI, patients were classified as underweight (BMI < 20 kg/m²), normal weight (20 kg/m² ≤ BMI < 25.0 kg/m²), overweight (25.0 kg/m² ≤ BMI < 30 kg/m²), or obese (BMI ≥ 30 kg/m²). We estimated the association between admission BMI and clinical outcomes, with in-hospital mortality as the primary outcome. Results: The median BMI of the cohort was 24.4 kg/m²(IQR 21.8–27.5). Of the 12,510 eligible patients, 5,471 (43.7%) were categorized as normal weight, 1,526 (12.2%) underweight, 3,914 (31.3%) overweight, and 1,599 (12.8%) obese. After entropy weighting, increasing BMI was negatively associated with mortality (P < 0.001). In-hospital mortality occurred in 699 (12.8%) normal weight, 223 (14.6%) underweight (relative risk [RR] 1.18; 95% CI 1.02–1.36; P = 0.030), 504 (12.9%) overweight (RR 0.97; 95% CI 0.87–1.08; P = 0.580), and 172 (10.8%) obese (RR 0.76; 95% CI 0.64–0.90; P = 0.001) patients. BMI was not associated with binary requirements for inotropes, invasive ventilation, or renal replacement therapy. Conclusions: In nonagenarians and centenarians admitted to the ICU, higher BMI was linked to lower in-hospital mortality, suggesting a protective effect. These results emphasize the need for individualized risk assessment in older ICU patients. Australian New Zealand Clinical Trials Registry number: ACTRN12625000297426.MedicineArticleSCOPUS10.1016/j.accpm.2025.1016342-s2.0-1050197544862352556841033448