Association Between Body Mass Index and the Risk of Hip Fracture by Sex and Age: A Prospective Cohort Study.

The association between body mass index (BMI) and hip fracture may differ by ethnic group. We examined the association between BMI and hip fracture according to sex and age and to identify BMI ranges associated with the lowest risk in Korean men and women. We followed up 288,068 Korean adults (aged 50 to 80 years), who underwent health examinations in 2002-03 to 2013; we examined national hospital discharge records. Hazard ratios (HRs) were calculated using Cox proportional hazard models after adjusting for confounders. During a mean follow-up period of 10.5 years, 1502 men and 2432 women suffered a hip fracture. Nonlinear associations were observed between BMI and hip fracture: a U-curve for women and a reverse J-curve for men. Men with BMIs of 27.5 to 29.9 kg/m2 and women with BMIs of 25 to 27.4 kg/m2 showed the lowest incidence of hip fracture. The multivariate-adjusted HRs for hip fracture per 5 kg/m2 decrease in BMI were 2.09 (95% confidence interval [CI] 1.83-2.38) and 1.34 (95% CI 1.19-1.51) in men and women with BMI <25 kg/m2 . The HRs were generally highest in the age group of 50 to 59 years (HR = 3.42 in men and 2.27 in women) and thereafter decreased with age. Among participants with BMI ≥25 kg/m2 , the HRs for hip fracture per 5 kg/m2 increase in BMI were 1.26 (95% CI 1.08-1.47) in women and 0.91 (95% CI 0.62-1.33) in men. In conclusion, the overweight range of BMI was associated with the lowest risk of hip fracture. Lower BMI was a risk factor for hip fracture, whereas obesity was associated with an increased risk of hip fracture, particularly in women. Overweight may be protective against hip fracture in Asian adults but not obesity.