Background

There has been a controversy on whether the association between weight and osteoporosis in postmenopausal women was the influence of body fat component or lean body mass. The purpose of this study was to examine the association between body composition and bone mineral density in postmenopausal women.

Methods
The subjects were 41 postmenopausal women older than 50 years old whose bone mineral density (BMD) and body composition were assessed using DEXA and bioelectrical impedence. Aspects of socioeconomic status, postmenopausal period, exercise, smoking and alcohol use were checked. Osteoporosis was defined as any BMD which was 2.5 SD or more below the "young normal" adult. Non-osteoporosis was defined as all BMD above 2.5 SD below the "young normal" adult. Body composition, as continuous variables or ordinal variables, was compared between the two groups.

Results
The prevalence of osteoporosis was 58.5% (24/51) in subjects. The women with osteoporosis were older and had lower education level compared to the women with non-osteoporosis. There were significant differences in height (P=0.009), weight (P=0.043), protein mass (P=0.004), mineral mass (P=0.004), and fat-free mass (P=0.005) between the two groups. In contrast, fat mass, % body fat, waist-hip ratio (WHR), and body mass index were not significantly different between the two groups. As the tertile of mineral mass and fat-free mass increased, the prevalence of osteoporosis showed decreased trends. However, there was no significant trend according to fat mass tertiles. When factors of age was adjusted, osteoporosis prevalence decreased 0.7 fold with 1 kg increase in fat-free mass.

Conclusion
This study suggests that fat-free mass rather than fat mass may be associated with BMD in postmenopausal women.