Association between obesity and the prevalence of fractures: a population-based cross-sectional study in China.

Although obesity is generally associated with increased bone mass, recent data have challenged its potential protective effect. Our study found that increased BMI showed beneficial effects on BMD in a non-linear way. However, individuals with obesity, especially women, were more likely to have vertebral fractures. There was a U-shaped relationship between BMI and the prevalent fractures.

Purpose: To estimate the association of obesity with the prevalence of fractures in the Mainland Chinese population.

Methods: A total of 8251 individuals from the COPS cohort were enrolled in this cross-sectional study and grouped by BMI level. The Five-Repetition Sit-to-Stand Test (5R-STS) and the Sharpened Romberg test were used to evaluate the balance ability. Vertebral fractures (VFs) were confirmed by spine X-ray examination. Prevalent fractures were defined by a self-report questionnaire which happened in the recent 5 years. The restricted cubic spline (RCS) was used to explore the non-linear relationship. Multiple linear regression and multivariable logistic regression were conducted to adjust the covariates.

Results: Obesity was correlated with a reduced bone turnover rate and increased BMD. Nevertheless, there were significant non-linear correlations between BMI and BMD, with a rapid increase and plateau at extremely high BMI levels (p for non-linear < 0.001 for all). Individuals with obesity were associated with a longer time of the 5R-STS and more likely to have a positive Sharpened Romberg test, especially in women. Compared with the normal weight group, the likelihood of prevalence of VFs and the prevalent fractures were significantly increased in the obesity group, independent of the Sharpened Romberg test and lumbar spine BMD (VFs: OR = 1.88 [95% CI 1.38-2.56]; prevalent fractures: OR = 2.18 [95% CI 1.39-3.41]). Per standard deviations (SD) increase in BMI was associated with 21% and 22% increase in the prevalence of VFs and prevalent fractures, respectively. Moreover, the prevalence of prevalent fractures also elevated in the low-weight individuals (OR = 2.62 [95% CI 1.34-4.75]), which indicated a U-shaped relationship between BMI and the prevalence of prevalent fractures.

Conclusion: Obesity was associated with higher BMD in a non-linear manner. However, BMI was positively associated with the prevalence of VFs, and there was a U-shaped relationship between BMI and the prevalent fractures.