The obesity paradox in bone health: high prevalence of osteoporosis and its non-linear association with BMI among urban elderly in China

Osteoporosis is a major public health concern in aging populations. Although data from rural Chinese populations are available, large-scale epidemiological studies on osteoporosis prevalence and its risk factors, particularly the complex relationship with body mass index (BMI), remain limited among urban community-dwelling elderly in China. A community-based cross-sectional study was conducted from April to August 2025 among elderly residents (aged ≥ 60 years) in Jiangling Sub-district, Suzhou. Participants were derived from a government-sponsored, non-mandatory health examination program through a two-stage voluntary process. Bone mineral density (BMD) was measured at the non-dominant distal radius using portable dual-energy X-ray absorptiometry (pDXA). Osteoporosis was defined as a T-score ≤ -2.5. Multivariate logistic regression was used to identify factors associated with osteoporosis. Among 6,496 participants (mean age 74.2 years, 55.2% female), the overall prevalence of osteoporosis was 46.7%. Prevalence was significantly higher in women (65.8%) than in men (23.2%) and increased markedly with age (from 32.6% in 60–69 years to 58.7% in ≥ 80 years). A non-linear relationship with BMI was observed: the highest prevalence was in the obese group (63.3%), followed by normal weight (50.1%), underweight (45.7%), and overweight (37.5%) groups. After adjusting for age and gender, female sex (OR = 6.28, 95% CI: 5.50–7.18) and obesity (OR = 2.18, 95% CI: 1.84–2.59) were independent risk factors, while overweight was protective (OR = 0.69, 95% CI: 0.60–0.80). This novel U-shaped relationship challenges conventional wisdom regarding the protective effect of higher body weight on bone health and suggests the existence of an optimal weight range for bone health in elderly populations. This study reveals a high prevalence of osteoporosis in an urban Chinese elderly population. The finding that obesity is an independent risk factor while overweight is protective challenges conventional wisdom and suggests that maintaining an optimal weight range (overweight by Chinese standards) may be crucial for bone health. Public health strategies should integrate targeted osteoporosis screening and promote evidence-based optimal weight management for bone health in urban elderly communities.