Prognostic significance of body mass index in hip fracture hospitalizations: A nationwide database study.

BACKGROUND AND AIMS: Hip fractures are common among older adults and are associated with functional decline and increased mortality. Body mass index (BMI) is a known prognostic factor for adverse outcomes. However, evidence on nonlinear associations across the entire BMI spectrum remains limited. This study aimed to examine the association between BMI at admission and in-hospital mortality, complications, and changes in functional status as measured by the Barthel Index.

METHODS

This retrospective observational study used data from a nationwide inpatient database in Japan between 2014 and 2024. This study included patients aged 65-99 years who had undergone surgery for hip fractures. BMI at admission was categorized according to the World Health Organization Asian criteria: underweight (<18.5 kg/m2), normal (18.5-22.9 kg/m2), overweight (23.0-27.4 kg/m2), and obese (≥27.5 kg/m2). The primary outcome was in-hospital mortality, and the secondary outcomes were in-hospital complications and changes in the Barthel Index. Cox proportional hazards and linear regression models were applied and nonlinear associations were assessed using restricted cubic splines.

RESULTS

In total, 146,699 patients were included in the analysis (mean age 84.5 ± 7.5 years; 78.4% female). In-hospital mortality occurred in 2554 patients (1.7%). The associations of BMI at admission with in-hospital mortality and complications showed L-shaped patterns in the spline curves. The hazard ratio for in-hospital mortality at a BMI of 18.5 kg/m2 was 1.83 (95% CI: 1.69-1.99), compared with a BMI of 23.0 kg/m2 as the reference. A similar L-shaped pattern was observed for complications (BMI 18.5 kg/m2: HR = 1.06, 95% CI: 1.04-1.09). No substantial changes were observed for BMI ≥23.0 kg/m2. Changes in Barthel Index showed a reverse J-shaped pattern, decreasing progressively below 23.0 kg/m2 (18.5 kg/m2: β = -4.44, 95% CI: -4.92 to -3.96) and increasing above this threshold (27.5 kg/m2: β = 1.45, 95% CI: 0.78-2.12).

CONCLUSION

BMI at admission is an important prognostic factor of clinical outcomes in patients with hip fractures. Maintaining an adequate BMI through nutritional management before injury may improve survival and functional recovery. However, the exclusion of patients with missing BMI data and the absence of information on surgical delay and other potential confounders should be considered when interpreting these results.