Low dose parathyroid hormone maintains normal bone formation in adult male rats during rapid weight loss.

A persistent negative energy balance results in bone loss. It is not clear whether the bone loss associated with chronic negative energy balance can be prevented. The objective of this study was to assess the efficacy of intermittent low dose parathyroid hormone (PTH) treatment in maintaining normal bone formation during severe energy restriction. Six-month-old male Fisher 344 rats were divided into 4 treatment groups: (1) baseline, (2) ad libitum (ad lib)-fed control, (3) energy-restricted (to consume 40% ad lib caloric intake), or (4) energy-restricted+low dose (1 μg/kg/d) PTH. Severe energy restriction for 14 d decreased body weight and serum leptin levels. Compared to ad lib-fed controls, energy-restricted rats had lower cancellous bone formation, higher osteoclast perimeter/bone perimeter and higher bone marrow adiposity in the proximal tibial metaphysis. Also, the energy-restricted rats had a lower periosteal bone formation rate at the tibia-fibula synostosis. Administration of PTH to energy-restricted rats had no effect on weight loss or osteoclast perimeter/bone perimeter. In contrast, energy-restricted rats treated with PTH had higher rates of cancellous and cortical bone formation compared to energy-restricted rats, and did not differ from the ad lib-fed control animals. Furthermore, PTH treatment maintained normal bone marrow adiposity. In conclusion, rapid weight loss in adult male rats was accompanied by decreased bone formation and increased bone marrow adiposity and these changes were prevented by low dose PTH treatment. Taken together, the results suggest that the energy cost of bone formation in adult rats is low and PTH therapy is effective in preventing the reduced bone formation associated with rapid weight loss.