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Abstract Background Osteoporotic fractures are a growing problem in an aging society. The association between body mass index (BMI) and osteoporotic fractures varies by fracture site and ethnicity. Limited knowledge exists regarding this association in native Chinese, particularly utilizing local databases as reference sources. Objective To investigate the association between BMI and osteoporotic fractures at different sites in Chinese women. Methods Three thousand ninety-eight female patients with radiographic fractures and 3098 age- and sex-matched healthy controls without fractures were included in the study. Both of them underwent assessment using dual-energy X-ray absorptiometry (DXA), with BMD measurements calculated using our own BMD reference database. Participants were classified into underweight (BMI < 18.5 kg/m2), normal weight (18.5 ≤ BMI < 24.0 kg/m2), overweight (24 ≤ BMI < 28 kg/m2) and obese (BMI ≥ 28 kg/m2) according to the Chinese BMI classification standard. Results There were 2296 (74.1%) vertebral fractures, 374 (12.1%) femoral neck fractures, and 428 (13.8%) other types of fractures in the case group. Bone mineral density (BMD) was almost lower in the fracture groups compared to the control groups (p = 0.048 to < 0.001). Compared with normal weight, underweight had a protective effect on total [odds ratio (OR) = 0.61; 95% confidence interval (CI), 0.49 –0.75; P< 0.001], and lumbar fractures (OR = 0.52; 95% CI, 0.41 – 0.67; P < 0.001), while obesity was associated with an increased risk for total (OR = 2.26; 95% CI, 1.85 – 2.76; P < 0.001), lumbar (OR = 2.17; 95% CI, 1.72 – 2.73; P < 0.001), and femoral neck fractures (OR = 4.08; 95% CI, 2.18 – 7.63; P < 0.001). Non-linear associations were observed between BMI and fractures: A J-curve for total, lumbar, and femoral neck fractures, and no statistical change for other types of fractures. Underweight was found to be a risk factor for other types of fracturess after adjusting for BMD (OR = 2.29; 95% CI, 1.09 – 4.80; P < 0.001). Osteoporosis and osteopenia were identified as risk factors for almost all sites of fracture when compared to normal bone mass. Conclusions Underweight has a protective effect on total and lumbar spine fractures in Chinese women, while obesity poses a risk factor for total, lumbar, and femoral neck fractures. The effect of BMI on fractures may be mainly mediated by BMD.

Abstract Background Fragility fracture is associated with bone mineral density (BMD), and most databases used in related researches are instrument-matched. Little is known about the relationship between BMD and fragility fracture risk of native Chinese, especially using local databases as reference databases. Objective To investigate relationship between BMD and risk of fragility fracture in native China. Methods 3,324 cases, including 2,423 women (67.7 ± 8.9 years) and 901 men (68.4 ± 11.6 years) having radiological fragility fractures and 3,324 age- and gender-matched controls participated in the study. We measured BMD at posteroanterior spine and hip using dual-energy X-ray absorptiometry (DXA), calculated BMD measurement parameters based on our own BMD reference database. Results BMDs and mean T-scores were lower in case group (with clinical fragility) than in control group (without clinical fragility). In patients with fragility fractures, prevalence of lumbar osteoporosis, low bone mass, and normal BMD were 78.9 %, 19.3 %, and 1.8 %, respectively, in women, and 49.5, 44.8 %, and 5.7 %, respectively, in men. In hip, these prevalence rates were 67.2 %, 28.4 %, and 4.4 % in females, and 43.2 %, 45.9 %, and 10.9 % in males, respectively, showing differences between females and males. Multivariate Cox regression analysis showed that after adjusting age, height, weight, and body mass index, fracture hazard ratio (HR) increased by 2.7–2.8 times (95 % CI 2.5–3.1) and 3.6–4.1 times (95 %CI 3.0–5.1) for women and men respectively with decreasing BMD parameters. In both sexes, risk of fragility fracture increased approximately 1.6–1.7 times (95 % CI 1.5–1.8) for every 1 T-score reduction in BMD. Conclusions Risk of clinical fragility fracture increases with decreasing BMD measurement parameters and anthropometric indicators in native China, and fracture HR varies from gender and site.