The relationship between body composition and bone mineral density in women
Purpose: The purpose of this study was to investigate the relationships between body composition (fat and lean mass), body weight, body mass index (BMI) and bone mineral density (BMD) and bone mineral content (BMC) in women across the age span. Methods: 1046 healthy Caucasian women (49.9 ± 15.9 years old) were recruited and categorized to four age groups, ranging from 18 to 35 years old, 35 years to before the age of menopause, menopause to 65 years and over 65 years old. Each different age group was further categorized according to subjects’ body mass index (BMI < 25, BMI =25-30, and BMI > 30). Measurements included anthropometrics, body composition and BMC/BMD by dual-energy x-ray absorptiometry (DXA). Results: Subjects in the younger groups were taller, weight less and had greater total body BMC/BMD and BMD at spine, femoral neck, and total femur (p<0.05) compared with older subjects. Obese women had higher total lean and fat mass, and the highest BMD/BMC in all skeletal sites (p<0.05) among different age groups. Weight, BMI, total lean, and total fat mass were positively correlated with total body BMC/BMD, BMD of all skeletal sites among different age groups. The results from multiple linear regression models revealed that weight was a significant predictor of total body BMC/BMD, BMD of all skeletal sites in women of different age groups, gonadal status, and different BMI, except overweight women. Both lean and fat mass were important determinants of total body BMC/BMD, BMD at femur and forearm in premenopausal women though total lean mass had greater effect than total fat mass. Total fat mass was the only significant predictor of total body BMC/BMD, BMD at other sites in postmenopausal women. Furthermore, total fat mass became more important with the increased body weight. Further analysis of covariance in subjects stratified by body weight and percent body fat revealed that high percent body fat seemed to have negative effect on bone mass in total body BMC/BMD and BMD at spine, femoral neck and forearm when mechanical loading effect of body weight was controlled. Conclusions: These results show that overweight/obese women had higher BMD in all skeletal sites than normal-weight women. Lean mass was an important predictor of BMD in premenopausal women and fat mass became more important in postmenopausal women. Higher fat mass however, may not have beneficial effect on bone mass when mechanical loading of weight is accounted for.