The Effect Of Calcium Supplementation On Calcium And Bone Metabolism During Load Carriage In Women: A Randomised Controlled Crossover Trial

PURPOSE: Women are at increased risk of stress fractures compared with men in military training. Exercise can decrease circulating serum calcium and increase parathyroid hormone (PTH) and bone resorption; these disturbances can be attenuated with calcium supplementation immediately before exercise, but this effect during load carriage in women is unknown. This study investigated the effect of calcium supplementation on calcium and bone metabolism during load carriage exercise in women. METHODS: Women completed an eligibility pre-screen followed by two experimental sessions. Experimental sessions involved completing 12.8 km in 120 mins (6.4 km·h-1) on a treadmill carrying 20 kg, 60 min after consuming a 1000 mg calcium supplement (SUPPLEMENT) or nothing (CONTROL) in a randomised order. Fasted venous blood samples were taken before (baseline), during (0, 20, 40, 60, 80, 100, 120 min), and after (+15, +30, +60, +90 min) exercise. Samples were analysed for biochemical markers of bone resorption and formation, and calcium metabolism. Differences between CONTROL and SUPPLEMENT were analysed using linear mixed effects models. Significance was accepted at p < 0.05. RESULTS: Forty-eight women (n = 38 military, n = 10 civilian, age 29 ± 4 yrs, body mass 65.3 ± 8.5 kg, height 1.67 ± 0.06 m, V̇O2max 45 ± 5 mL·kg·min-1, total 25(OH)D 78.7 ± 24.0 nmol·L-1) volunteered. Serum ionized calcium (iCa) decreased from baseline to all exercise time points (p < 0.001) and recovered by +15 min. iCa was higher in SUPPLEMENT compared with CONTROL across the trial (p < 0.001). PTH increased from baseline to 120 min in CONTROL (p < 0.001), but was lower than baseline at all time points in SUPPLEMENT (p < 0.001). PTH was higher in CONTROL compared with SUPPLEMENT at all time points from 0 min to +90 min (p < 0.001). βCTX decreased from baseline to all time points between 20 min and + 15 min in CONTROL (p ≤ 0.004); βCTX decreased from baseline to all time points from 0 to +90 mins in SUPPLEMENT (p < 0.001). βCTX was lower in SUPPLEMENT compared with CONTROL at all time points from 20 min to +90 min (p ≤ 0.036). P1NP did not change with exercise, but was higher in CONTROL compared with SUPPLEMENT (p < 0.001). CONCLUSIONS: Consuming a 1000 mg calcium supplement prior to load carriage exercise prevents exercise-induced distruptions to bone and calcium homeostasis. UK Ministry of Defence (Army)