Low body mass index is an important risk factor for low bone mass and increased bone loss in early postmenopausal women

Thinness (low percentage of body fat, low body mass index [BMI], or low bod y weight) was evaluated as a risk factor for low bone mineral density (BMD) or increased bone loss in a randomized trial of alendronate for prevention of osteoporosis in recently postmenopausal women with normal bone mass (n = 1609), The 2-year data from the placebo group were used (n = 417), Percen tage of body fat, BR;II, and body weight were correlated with baseline BMD (r = -0.13 to -0.43, p < 0.01) and 2-year bone loss (r = -0.14 to -0.19,p < 0.01). Women in the lowest tertiles of percentage of body fat or BMI had u p to 12% lower BMD at baseline and a more than 2-fold higher 2-year bone lo ss as compared with women in the highest tertiles (p less than or equal to 0.004). Women with a lower percentage of body fat or BMI had higher baselin e levels of urine N-telopeptide cross-links (r = -0.24 to -0.31, p < 0.0001 ) and serum osteocalcin (r = -0.12 to -0.15, p < 0.01). To determine if the magnitude of treatment effect of alendronate was dependent on these risk f actors, the group treated with 5 mg of alendronate was included (n = 403), There were no associations between fat mass parameters and response to alen dronate treatment, which indicated that risk of low bone mass and increased bone loss caused by thinness could be compensated by alendronate treatment . In conclusion, thinness is an important risk factor for low bone mass and increased bone loss in postmenopausal women. Because the response to alend ronate treatment is independent of fat mass parameters, prevention of postm enopausal osteoporosis can be equally achieved in thinner and heavier women .