Bone turnover and body weight relationships differ in normal-weight compared with heavier postmenopausal women

Low body weight is associated with increased risk for fractures, whereas higher body weight has been shown to be protective against osteoporosis. This study evaluated whether body weight plays a role regulating bone turnover and mass in normal-weight (body mass index (BMI) <25 kg/m²), overweight (BMI 25–29.9 kg/m²) and obese (BMI≥30 kg/m²) postmenopausal women who were either receiving hormone replacement therapy [HRT(+)] or not [HRT(−)] (total of six groups). Body weight, BMI, total body bone mineral content (TBBMC), and markers of bone formation (serum osteocalcin) and bone resorption (urinary pyridinoline (PYD) and deoxypyridinoline) were retrospectively analyzed in 210 postmenopausal women. The mean age was 67±6 years, with mean body weight of 70.8±14.2 kg, ranging from 45.0 to 115.5 kg. Body weight was positively correlated with TBBMC (r=0.50, p<0.0001). There was a lower TBBMC and higher bone formation rate in normal-weight than obese HRT(−) women, but in women taking HRT there were no differences between BMI categories. In addition, in normal-weight HRT(−) women only, PYD and body weight showed a negative correlation (r=−0.39, p=0.01). Among normal-weight, but not overweight or obese subjects, we observed higher TBBMC and lower bone turnover in the HRT(+) compared with the HRT(−) group. Regression models explained 36% of the variance in TBBMC, mainly through body weight. Additional models could only explain 11–15% of the variance in bone turnover. Taken together, these data suggest that among normal-weight but not obese postmenopausal women, higher bone turnover is associated with lower bone mass, and that only normal-weight women show a different bone turnover profile with HRT treatment. Body weight should be considered an important factor in bone metabolism with relevant clinical implications.