Bone Biomarker Response to Walking under Different Thermal Conditions in Older Adults

Endurance exercise can cause a decrease in serum ionized calcium (iCa) and increases in parathyroid hormone (PTH) and c-terminal telopeptide of type I collagen (CTX), which may be due to Ca loss in sweat. PURPOSE To determine if exercise in a warm environment exaggerates the decrease in iCa and increases in PTH and CTX compared to a cool environment in older adults. METHODS Twelve women and men aged 61-78 y performed two identical 60-minute treadmill bouts at ~75% of maximal heart rate under warm and cool conditions. Serum iCa, PTH, and CTX were measured every 15 minutes starting 15 minutes before and continuing for 60 minutes after exercise. Sweat Ca loss was estimated from sweat volume and sweat Ca concentration. RESULTS Sweat volume was low and variable; there were no differences in sweat volume or Ca concentration between conditions. iCa decreased after 15 minutes of exercise and the change was similar in both conditions. Increases in PTH (Warm: 16.4, 95% CI: 6.2, 26.5 pg/mL; Cool: 17.3, 95% CI: 8.1, 26.4 pg/mL) and CTX (Warm: 0.08, 95% CI: 0.05, 0.11 ng/mL; Cool: 0.08, 95% CI: 0.01, 0.16 ng/mL) from before to immediately after exercise were statistically significant and similar between conditions. Adjusting for plasma volume shifts did not change the results. CONCLUSION The increases in PTH and CTX, despite the low sweat volume, suggest that dermal Ca loss is not a major factor in the decrease in iCa and increases in PTH and CTX observed during exercise in older adults. ADDRESS FOR CORRESPONDANCE: Sarah J. Wherry, PhD, Division of Geriatric Medicine, University of Colorado Anschutz Medical Campus, Mail Stop B179, 12631 E. 17th Avenue, Room 8111, Aurora, CO 80045, 720-848-6475 (O), 720-848-7382 (F). Sarah.wherry@ucdenver.edu Funding Sources: This study was supported in part by NIH awards UL1 TR002535, T32 AG000279, K23 AR070275 and P30 DK048520 and the VA Eastern Colorado Geriatric Research, Education, and Clinical Center (GRECC). Contents are the sole responsibility of the authors and may not represent official views of the NIH or VA. The authors have no conflicts of interest to declare. Accepted for Publication: 25 February 2019 © 2019 American College of Sports Medicine

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