Load Carriage Exercise Increases Calcium Absorption and Retention in Healthy Young Women

Objectives Aerobic exercise (EX) reduces circulating ionized Ca (iCa) and increases parathyroid hormone (PTH), but the cause and consequences on Ca handling are unknown. The objective of this study was to determine the effects of EX on Ca kinetics using dual stable Ca isotopes. Methods Twenty-one healthy women (26.2 ± 6.7 yr; BMI: 22.8 ± 2.1 kg/m2) completed a randomized, crossover study consisting of two 6 day iterations separated by ≥2 wk. One iteration consisted of 3 d of load carriage EX 2 h after breakfast (1 hr treadmill walking at 65% VO2max wearing a vest weighing 30% body wt) on days 0, 2, 4, and the other a no-EX control (CON). Diet was controlled during each iteration, and matched to the volunteer's usual intake. On day 0, volunteers received one IV bolus of 42Ca, and oral 44Ca with each meal (0600,1200, and 1700). Serial blood draws and 3 urine pools (U1: 0600–1000, U2: 1000–2000, U3: 2000–0800) were collected on day 0; spot urines were collected on days 1–5. Ca isotope ratios were analyzed by Thermal Ionization Mass Spectrometry and total Ca by Siemens Dimension; fractional Ca absorption (FCA) was measured and kinetic modeling determined, rates of Ca deposition (VO+) and resorption (VO–) from bone, and bone Ca balance (Vbal). Circulating PTH and iCa were also measured before, during and after exercise. Data were analyzed by paired T-test or linear mixed models using SPSS. Results On day 0, PTH increased 45 min into EX as compared to pre (+36 pg/ml, P < 0.001) with max increases 15 min post EX (+70.5 pg/ml, P < 0.001). No changes were observed during CON. Compared to pre-EX, iCa decreased at 45 min into EX (-0.14 mmol, P = 0.01) with max 15 min post EX (–0.273 mmol, P < 0.001). Urinary Ca (UCa) was lower during EX in the U1 pool (25 ± 11 mg) vs. CON (38 ± 16 mg, P = 0.001) with no differences in U2 and U3 (P > 0.05). FCA was greater during EX (26.6 ± 8.1%) compared to CON (23.9 ± 8.3%, P < 0.05). Vbal was less negative during EX (–61.3 ± 111 mg) vs. CON (–108 ± 23.5 mg, P < 0.05), but VO+ (574 ± 241 vs. 583 ± 260 mg) and VO– (–636 ± 243 vs. –692 ± 252 mg) were not different (P > 0.05). Conclusions During EX, an increase in FCA and decrease in UCa resulted in a positive shift in Ca balance. The rapid reduction in circulating iCa may be due to a change in the miscible Ca pool, resulting in increased PTH and changes in intestinal and renal Ca handling that support a more positive Ca balance. Funding Sources Defense Health Program.