Background <p>Body mass index (BMI) was used to classify overweight or obesity. The obesity paradox was observed in elderly hip fracture patients. However, obesity has been implicated as one of the major risk factors for hypertension and diabetes. This study aims to determine whether a high body mass index (BMI) remains a protective factor in hip fracture patients with comorbid hypertension or diabetes, and to identify the optimal BMI threshold that best supports motor function recovery.</p> Methods <p>This study included patients aged 65 years and older who have underwent hip fracture surgery. Harris Hip Score (HHS) was utilized to evaluate the functional recovery, the relationship between BMI and HHS was examined using both linear and generalized additive model (GAM). A threshold model was established with BMI of 24 kg/m² and the difference between the threshold model and the GAM was compared utilizing the likelihood ratio test (LRT).</p> Results <p>A total of 213 patients were enrolled in the study. A nonlinear relationship was identified between BMI and HHS in patients with either hypertension or diabetes and the HHS demonstrated a significant downward trend with increasing BMI. The LRT revealed no significant difference between the threshold effect model with a BMI value of 24 kg/m² and the GAM.</p> Conclusions <p>This study reveals that the protective effect of high BMI on postoperative motor function in hip fracture patients is significantly modulated by comorbidities. We recommend modulating the BMI to approximately 24 kg/m² for elderly patients with hip fractures and comorbid conditions such as hypertension and diabetes.</p>