Maintenance of Serum Ionized Calcium during Exercise Attenuates the Exercise-Related Increase in Bone Resorption

Abstract

Exercise is recommended to build or maintain bone mass, but there is evidence that vigorous or prolonged exercise is associated with bone loss under certain conditions. It is our contention that disruptions in calcium homeostasis during exercise lead to increases in parathyroid hormone (PTH) and bone resorption. PURPOSE: To determine if preventing the decline in serum ionized calcium (iCa) during exercise attenuates the increases in PTH and a marker of bone resorption (carboxy-terminal collagen crosslinks (CTX)). METHODS: Healthy, cycling-trained men (n=11, aged 18-45y) underwent two identical one-hour cycling bouts at ~75% VO2peak under conditions of calcium gluconate versus half-normal saline infusion. Blood was sampled every 5 minutes to adjust the calcium infusion rate, with a goal to maintain serum iCa at ~0.2 mg/dL above baseline. The same infusion protocol was replicated under the saline condition. Blood samples to assess PTH, CTX, bone formation (procollagen type 1 amino procollagen (P1NP)), and total calcium (tCa) were taken every 15 minutes during exercise and hourly for 4 hours post-exercise. RESULTS: Serum iCa was successfully maintained above baseline during Ca infusion and was significantly different from the saline condition during exercise (all time points p<0.01). Compared to saline, the Ca infusion markedly reduced the increase in serum PTH during exercise (p=0.004) and the suppression of PTH persisted throughout the 4-hour recovery period (all p<0.01). Similarly, the increase in CTX during exercise was suppressed with Ca infusion (p=0.003) and remained below the saline condition through recovery (all p<0.001). tCa was also a significantly higher during exercise (p<0.001) with Ca infusion, but values were similar during recovery (all p>0.10). There were no differences between conditions for P1NP at any time points (all p>0.10). CONCLUSIONS: The increase in bone resorption was attenuated when the exercise-related decline in serum iCa was prevented, suggesting a calcium-dependent relationship. There was no effect of Ca infusion on P1NP, but the duration of post-exercise sampling may have been too short to capture any changes. The results are limited to young, trained, men during cycling exercise. Future research should investigate sex- and age-differences and other exercise modalities.