Studies in mice have suggested that osteocalcin plays an important role in glucose and fat metabolism. Since anti-resorptive drugs reduce circulating levels of osteocalcin they might be associated with increased fat mass and an increased risk of diabetes. Positive changes in body weight have been found in trials of alendronate and denosumab, but no significant effect in a previous trial of zoledronate. Whether those weight differences were in fat or lean mass is unknown. There were no effects of anti-resorptive treatments on fasting glucose concentrations or incidence of diabetes in those three studies. We have used our recent trial comparing zoledronate and placebo over 6 years in 2000 older osteopenic women to re-examine these questions. Both treatment groups lost body weight during the study (placebo 1.65 kg, zoledronate 1.05 kg), and this was significantly greater in the placebo group (P = 0.01). Both groups lost lean mass, and this loss was marginally (0.17 kg) but significantly (P = 0.02) greater in those receiving zoledronate. The placebo group had a mean loss of fat mass of 0.63 kg but there was no change in fat mass in the zoledronate group (between-groups comparison, P = 0.007). In the placebo group, there were 20 new diagnoses of diabetes, and in the zoledronate group, 19 (P = 0.87). Zoledronate prevented age-related loss of fat mass in these late postmenopausal women. The present study is the first to document a significant effect of zoledronate treatment on body weight, confirming results previously found with alendronate and denosumab. It also demonstrates that this is principally an effect to maintain fat mass rather than influencing lean mass, raising an important physiological question as to how anti-resorptive drugs have this effect on intermediary metabolism. It is possible that this anti-catabolic action contributes to the beneficial effects of anti-resorptive drugs on bone and longevity.