The influence of physical activity on the response of bone mineral density to 5 years tibolone

Menopausal hormone replacement therapy (HRT) maintains bone mineral density (BMD) and reduces risk of fracture in postmenopausal women. It has been suggested that sex steroids and loading may have synergistic effects on bone. We therefore investigated whether habitual physical activity influences the response of BMD to tibolone in postmenopausal women.

The subjects were 42 postmenopausal women aged mean (SE) 65.8+/-6.2 year who had taken tibolone for prevention/ treatment of osteoporosis over 5 years. Bone mineral density was measured annually by dual X-ray absorptiometry and physical activity was assessed using accelerometers after 5 years therapy.

Twenty-six women were classified as having low physical activity (LPA; <15 min day(-1)) and sixteen as high physical activity (HPA; >15 min day(-1)). Spine BMD did not differ significantly between groups at baseline and increased significantly by 2 years of treatment with further increase to 5 years. The magnitude of increase did not differ between groups. Hip BMD at baseline was 7.3% higher in HPA women (P=0.07). Hip BMD increased over 2 years tibolone treatment in LPA women (+5.6%, P<0.01) whilst no significant change occurred in the HPA group (-0.5%). This difference in response between groups was statistically significant (P=0.002) and persisted after adjustment for age and body mass (P=0.002). Hip BMD was maintained in both groups over the subsequent 3 years of treatment.

Spine BMD increased significantly in response to tibolone irrespective of physical activity participation. The more physically active women had higher hip BMD at baseline but the response to tibolone was greater in the less physically active women. The difference in response between groups may be due to physically active women having lower resorption at the hip and hence reduced response to anti-resorptive effects of HRT.