Effects of skeletal loading on bone mass and compensation mechanism in bone: a new insight into the mechanostat theory

We have suggested that: (i) osteocalcin carboxylation is related to bone material properties (bone quality), and (ii) impairment of bone material properties could be compensated by bone mass increase. The aim of the present prospective study was to investigate whether the effects of skeletal loading on bone mass were associated with the compensation mechanism between bone mass and bone material properties. The subjects were 56 healthy female volunteers aged around 50 years. They were classified into pre- and postmenopausal groups, and each group was then subdivided into a no-exercise (control) and a vertical jumping exercise group. Bone mineral density (BMD) was measured at baseline and after the 6-month study period. Urinary gamma-carboxyglutamate (Gla), a possible parameter of osteocalcin carboxylation, was also measured at baseline. Among the premenopausal women, hip BMD in the exercise group increased significantly in comparison to the control value. Among the postmenopausal women, however, there was no significant difference in the BMD change between the control and the exercise group. In addition, the baseline urinary Gla level showed an inverse correlation with the change in whole body BMD in the premenopausal exercise group. These results suggest that: (i) estrogen plays a certain role in the high-impact exercise-induced bone gain, and (ii) the effects of skeletal loading on bone mass are involved in the compensation mechanism, i.e., bone gain due to high-impact exercise becomes greater in accordance with the degree of deterioration in bone material properties. Our concept of the compensation mechanism could provide a new insight into the understanding of the skeleton's adaptability to load-bearing.