Obesity as a protective factor for postmenopausal osteoporosis.

BACKGROUND: Obesity is considered a protective factor for osteoporosis improving bone mass and maintaining higher levels of estrogen during menopause.
OBJECTIVE: To determine the association of obesity with bone mineral density (BMD), and its relationship with sex hormone levels.
DESIGN: A case-control study in Caucasian obese and non obese postmenopausal women.
SUBJECTS: 113 obese and 50 non-obese postmenopausal women. MEASUREMENTS: BMD (dual-photon X-ray absorptiometry) at cervical femur. Ward's triangle, proximal radius and lumbar spine. Plasma levels of glucose, insulin, total estrogen, follicle stimulating hormone (FSH), sex hormone binding globulin (SHBG), dehydroepiandrosterone sulfate (DHA-S) and testosterone.
RESULTS: Mean BMD at femoral sites were significantly higher in obese women (femoral neck: 0.849 +/- 0.124 g/cm2 vs 0.753 +/- 0.095 g/cm2, P < 0.001; Ward's triangle: 0.634 +/- 0.134 g/cm2 vs. 0.553 +/- 0.100 g/cm2, P < 0.001). Mean BMD at lumbar spine was 0.906 +/- 0.138 g/cm2 in obese women and 0.849 +/- 0.137 g/cm2 in non obese, P < 0.017. A decreased risk of osteopenia in femoral neck (Age adjusted OR = 0.36, 95%CI 0.17-0.75) and in lumbar spine (Age adjusted OR = 0.43, 95%CI 0.20-0.91) in obese women was observed. Although total estrogen were similar in both groups, in obese women, SHBG was lower (68.6 +/- 26.84 nmol/l vs. 85.1 +/- 31.18 nmol/l, P < 0.001), and postglucose load insulin levels were higher, than in non obese (77.2 +/- 50.4 Ul/ml vs. 49.4 +/- 24.1 Ul/ml, P < 0.0005).
CONCLUSION: The findings confirm a higher BMD in obese women and suggest that obesity exerts protection due to a decreased SHBG thus increasing free sex steroids. Besides, hyperinsulinemia may produce a decline in the production of IGFBG-1, leading to an increase of IGF-1, that may stimulate the proliferation of osteoblasts.