Low levels of endogenous estradiol protect bone mineral density in young postmenopausal women.

Objective: Low levels of endogenous estrogens may play a role in the protection of bone mineral density (BMD) in healthy postmenopausal women. The aim of this study was to evaluate the effect of endogenous estradiol and testosterone on bone mass in young and older healthy postmenopausal women.

Methods: The study involved 99 postmenopausal women aged 55-75 years. The BMDs of the lumbar spine, proximal femur and total skeleton were determined. Measurements were taken of serum calcium, bone alkaline phosphatase, Crosslaps, estradiol, estrone, sex hormone binding globulin, testosterone, bioavailable testosterone and urine calcium. Estradiol was measured using a sensitive assay with a lower detection limit at 5 pg/ml.

Results: A multivariate analysis showed that the BMD of the lumbar spine was significantly predicted by estradiol (p < 0.05), and testosterone (p < 0.0001). Likewise, testosterone was found to be an independent predictor of the BMD of the total femur (p < 0.001) and the total skeleton (p < 0.001). The population was divided into two groups: < or = 65 (Group 1) and > 65 years (Group 2) of age and also stratified according to estradiol levels: > 10 and < or = 10 pg/ml. Significant differences in BMD were found in women in Group 1 in whom estradiol levels higher than 10 pg/ml were associated with a higher BMD of the lumbar spine (+ 14%, p < 0.01), proximal femur (+ 6%, p < 0.05) and total skeleton (+ 7%, p < 0.05) compared with women with estradiol levels below 10 pg/ml. Bone alkaline phosphatase levels (p < 0.05) and serum Crosslaps (not significant) were lower in women in Group 1 with a level of estradiol more than 10 pg/ ml.

Conclusion: Endogenous estradiol levels higher than 10 pg/ml and testosterone protected bone mass in healthy postmenopausal women under 65 years of age. These results were not observed in the group of older women.