Is Overweight Protective Against Fracture Occurrence? Age and Site-Dependent Different Association Between Body Mass Index and the Incidence of Hip and Vertebral Fractures.

Study Design: Retrospective observational study.

Objective: To investigate the effects of body mass index (BMI), age, and sex on the occurrence of hip and vertebral fractures in the South Korean population.

Methods: We enrolled participants who underwent routine health screenings conducted by the Korean National Health Insurance Service in 2009 and followed up from 2010 to 2018. Cox proportional analyses assessed the hazard ratios for hip and vertebral fractures compared to individuals with a normal BMI.

Results: A total of 2,515,078 individuals were included in this analysis, the mean age was 53.8 years, and 50.1% were men. Low BMI emerged as a risk factor for both hip and vertebral fractures, with a stronger association observed for hip fractures ( < 0.01). Hip fracture incidence decreased with increasing BMI in all patients. However, compared to the normal BMI group, the incidence of vertebral fractures in patients under 65 years was 10.4%, 12.8%, and 24.1% higher in the underweight, obese I, and obese II groups, respectively ( < 0.001) and that of women, 4.4%, 4.1%, 9%, and 13.4% higher in the underweight, overweight, obese I, and obese II groups, respectively ( < 0.001).

Conclusions: Our findings showed that regardless of sex and age, higher BMI consistently correlates with a lower risk of hip fractures. However, elevated BMI may increase the risk of vertebral fractures in women and those under 65 years. This study underscores the complexity of the relationship between BMI and fracture risks, highlighting the necessity for specific guidelines to mitigate fracture risks across diverse populations.