Effect of body mass index on vertebral and hip fractures in older people and differences according to sex: A retrospective Japanese cohort study

Objectives The purpose of this study was to investigate the incidence of vertebral and hip fractures in the older people and to clarify the relationship between these fractures and body mass index (BMI) along with the impact of sex differences. Design This was a retrospective cohort study. Setting We used administrative claims data between April 2010 and March 2018. Participants Older people aged ≥75 years who underwent health examinations in 2010 and were living in the Fukuoka Prefecture, Japan were included in the study. A total of 24 691 participants were included; the mean age was 79.4±4.3 years, 10 853 males and 13 838 females, and an the mean duration of observation was 6.9±1.6 years. Primary and secondary outcome measures We estimated the incidence of vertebral and hip fractures by BMI category (underweight: <18.5 kg/m 2, normal weight: 18.5-24.9 kg/m 2, overweight and obese: ≥25.0 kg/m 2) using a Kaplan-Meier curve in males and females and determined fracture risk by sex using Cox proportional hazards regression analyses. Results The incidence of vertebral and hip fractures was 16.8% and 6.5%, respectively. The cumulative incidence of vertebral and hip fracture at the last observation (8 years) in each BMI groups (underweight/normal weight/overweight and obese) estimated using the Kaplan-Meier curve was 14.7%/10.4%/9.0% in males and 24.9%/23.0%/21.9% in females, and 6.3%/2.9%/2.4% in males and 14.1%/9.0%/8.1% in females, respectively, and both fractures were significantly higher in underweight groups regardless of sex. Multivariable Cox proportional hazards models showed that underweight was a significant risk factor only in males for vertebral fractures and in both males and females for hip fractures. Conclusion Underweight was associated with fractures in the ageing population, but there was a sex difference in the effect for vertebral fractures.