Recovery from osteoporosis through skeletal growth: early bone mass acquisition has little effect on adult bone density.

It is often assumed that bone mineral accretion should be optimized throughout childhood to maximize peak bone mass. In contrast, we hypothesized that bone mineral acquisition early in life would have little or no effect on adult bone mass because many areas of the juvenile skeleton are replaced in toto through skeletal growth. To test this hypothesis, we induced osteoporosis by administering dexamethasone to 5-week-old rabbits for 5 weeks and then allowed them to recover for 16 weeks. Tibial bone mineral density (ash weight/volume) was decreased in the dexamethasone-treated animals at the end of treatment but recovered completely. Bone structure in the femur was assessed by histomorphometry. Trabecular and cortical bone in the distal metaphysis was made osteoporotic by dexamethasone, but was then replaced through endochondral bone formation and recovered. Periosteal bone formation rate in the diaphysis was decreased during dexamethasone treatment but afterwards rebounded above controls and normalized cortical width. Our data suggest that bone mineral acquisition early in life has little effect on adult bone density because the juvenile bone is largely replaced through growth. If this concept generalizes, then interventions to maximize peak bone mass should be directed at adolescents rather than young children.