Exercise is essential for building and maintaining bone mass and strength, but our recent work has raised the possibility that current exercise recommendations for bone health may not be appropriate. We have strong evidence that a single bout of vigorous exercise has an acute catabolic effect in bone (i.e., increased resorption) that lasts several hours. This is mediated by a decrease in serum calcium (Ca) during exercise, which stimulates parathyroid hormone (PTH) secretion. PTH then activates bone resorption to mobilize Ca from bone, presumably to prevent the decrease in serum Ca from progressing to a harmful level. This cascade of events can be markedly attenuated by minimizing the decline in serum Ca during exercise via either intravenous or oral Ca administration. The timing of Ca supplementation relative to exercise is likely important, because it must be available for gut absorption during exercise. Interestingly, repeated pharmacologic stimulation of the PTH receptor with PTH analogs (teriparatide, abaloparatide) has anabolic effects on bone, suggesting that repeated exercise-induced increases in PTH could have a chronic anabolic skeletal effect, in addition to the acute catabolic effect, which may be apparent only after repeated exercise sessions. If this is the case, suppressing the PTH response with pre-exercise Ca supplementation may not be appropriate. In this context, the proof-of-concept phase will include a short exercise intervention consisting of treadmill exercise at 70% to 80% of maximal heart rate, 60 minutes per day, 4 days per week, for 4 weeks. Serum markers of bone formation and resorption will be measured before, during, and for 24 hours after the 1st, 8th, and 16th exercise sessions to address two questions: 1) Does the acute catabolic response of bone to a single bout of exercise continue to occur with repeated exercise sessions (i.e., exercise training)? 2) Does exercise training also generate an anabolic PTH-mediated bone response, similar to the anabolic response to PTH analog therapy? If the answers to questions 1 and 2 are YES (persistent catabolic signal) and NO (lack of anabolic signal), this will support the need for the randomized controlled trial (RCT), which will evaluate whether taking Ca before exercise to attenuate the acute catabolic response improves skeletal adaptations to exercise training. The RCT will be a 36-week supervised exercise intervention, with participants randomized to take supplemental Ca citrate plus vitamin D3 (Ca+D3) or vitamin D3 alone (D3; control) about 60 minutes before each exercise. Primary aims are to determine 1) whether taking Ca before exercise enhances bone mineral density (BMD) adaptations to exercise, and 2) whether this occurs by attenuating the increase in bone resorption during and after exercise sessions. The overarching goal is to improve the currently imprecise recommendations for exercise to improve and maintain bone health. This research is of high relevance to Vet...