Calcium Supplementation Attenuates Disruptions in Calcium Homeostasis during Exercise

An exercise induced decrease in serum ionized calcium (iCa) is thought to trigger an increase in parathyroid hormone (PTH), which can stimulate bone resorption. Purpose: The purpose of this study was to determine whether taking a chewable calcium (Ca) supplement 30 min before exercise mitigates disruptions in Ca homeostasis and bone resorption in competitive male cyclists. Methods: Fifty-one men (18 to 45 yr old) were randomized to take either 1000 mg Ca (CA) or placebo (PL) 30 min before a simulated 35-km cycling time trial. Serum iCa and PTH were measured before and immediately after exercise and a marker of bone resorption (C-terminal telopeptide of type I collagen) was measured before and 30 min after exercise. Results: Serum iCa decreased in both groups from before to after exercise (mean +/- SD, CA = 4.89 +/- 0.16 to 4.76 +/- 0.11 mg.dL(-1), PL = 4.92 +/- 0.15 to 4.66 +/- 0.22 mg.dL(-1), both P <= 0.01); the decrease was greater (P = 0.03) in the PL group. There was a nonsignificant (P = 0.07) attenuation of the increase in PTH by Ca supplementation (CA = 30.9 +/- 13.0 to 79.7 +/- 42.6 pg.mL(-1), PL = 37.1 +/- 14.8 to 111.5 +/- 49.4 pg.mL(-1), both P <= 0.01), but no effect of Ca on the change in C-terminal telopeptide of type I collagen, which increased in both groups (CA = 0.35 +/- 0.17 to 0.50 +/- 0.21 ng.mL(-1), PL = 0.36 +/- 0.13 to 0.54 +/- 0.22 ng.mL(-1), both P <= 0.01). Conclusion: It is possible that ingesting Ca only 30 min before exercise was not a sufficient time interval to optimize gut Ca availability during exercise. Further studies will be needed to determine whether adequate Ca supplementation before and/or during exercise can fully mitigate the exercise-induced decrease in serum iCa and increases in PTH and bone resorption.