The effect of calcium supplementation on bone calcium balance and calcium and bone metabolism during load carriage in women: a randomized controlled crossover trial.

Calcium supplementation before exercise attenuates the decrease in serum calcium and increase in PTH and bone resorption. This study investigated the effect of calcium supplementation on calcium and bone metabolism during load carriage in women. A total of 48 women completed two load carriage sessions (load carriage 1 n = 48; load carriage 2 n = 40) (12.8 km in 120 min carrying 20 kg) 60 min after consuming either 1000 mg calcium (Calcium) or nothing (Control) in a randomized order. Pre- and post-exercise urine samples were analyzed for calcium isotope ratio (δ44/42Ca). Fasted blood samples were taken before (pre-exercise), during (0, 20, 40, 60, 80, 100, 120 min), and after (+15, +30, +60, +90 min) exercise and analyzed for markers of calcium and bone metabolism. There was no effect of load carriage or supplementation on urine δ44/42Ca (p ≥ .110). Serum δ44/42Ca did not change with load carriage in Control (p = .617) but increased in Calcium (p = .003) and was higher at 120 min in Calcium vs Control (p = .018). Ionized calcium (iCa) decreased from pre-exercise to all exercise time-points (p < .001); iCa was higher in Calcium than Control throughout (p < .001). PTH increased from pre-exercise to 120 min in Control (p < .001) but decreased from pre-exercise to all time-points in Calcium (p < .001). PTH was higher in Control than Calcium from 0 to +90 min (p < .001). βCTX decreased from pre-exercise to 20 to +15 min in Control (p ≤ .004); βCTX decreased from pre-exercise to 0 to +90 min in Calcium (p < .001). βCTX was lower in Calcium than Control from 20 to +90 min (p ≤ .036). A 1000 mg calcium supplement before load carriage promotes bone calcium balance and prevents disruptions to bone and calcium homeostasis. Clinical trial registration: NCT04823156 (ClinicalTrials.gov).