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 This study aimed to determine whether exercise in a warm environment exaggerates the decrease in iCa and increases in PTH and CTX compared with a cool environment in older adults. Methods Twelve women and men 61-78 yr old performed two identical 60-min treadmill bouts at similar to 75% of maximal heart rate under warm and cool conditions. Serum iCa, PTH, and CTX were measured every 15 min starting 15 min before and continuing for 60 min 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 min of exercise, and the change was similar in both conditions. Increases in PTH (warm: 16.4, 95% confidence interval [CI] = 6.2, 26.5 pg center dot mL(-1); cool: 17.3, 95% CI = 8.1, 26.4 pg center dot mL(-1)) and CTX (warm: 0.08, 95% CI = 0.05, 0.11 ng center dot mL(-1); cool: 0.08, 95% CI = 0.01, 0.16 ng center dot mL(-1)) 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.