Exercise can cause a decrease in serum ionized calcium (iCa) concentration, which stimulates parathyroid hormone (PTH) secretion and activates bone resorption. We postulated that dermal Ca loss during cycling exercise is the major determinant of the serum iCa, PTH, and bone resorption (C-terminal telopeptide of type 1 collagen [CTX]) responses.
To investigate this, women (n = 13) and men (n = 12) age 18 to 45 yr performed the same exercise bout under cool (18°C) and warm (26°C) conditions. Exercise was 60 min of cycling at ~75% of peak aerobic power. Sweat samples were obtained during exercise using a skin patch method, and blood samples were obtained before and during exercise and during 60 min of recovery.
Sweat volume and estimated sweat Ca loss were 50% higher for the warm condition than the cool condition. Despite this, there were no differences between thermal conditions in the changes (mean, 95% confidence interval [95% CI]) in iCa (cool, −0.07 mg·dL−1; 95% CI, −0.16 to 0.03); warm, −0.07 mg·dL−1; 95% CI, −0.20 to 0.05), PTH (cool, 34.4 pg·mL−1; 95% CI, 23.6–45.2; warm: 35.8 pg·mL−1; 95% CI, 22.4–49.1), or CTX (cool, 0.11 ng·mL−1; 95% CI, 0.08–0.13; warm, 0.15 ng·mL−1; 95% CI, 0.11–0.18). Adjusting for exercise-related shifts in plasma volume revealed a marked decline in vascular iCa content in the first 15 min of exercise (cool, −0.85 mg·dL−1; 95% CI, −1.01 to −0.68; warm, −0.85 mg·dL−1; 95% CI, −1.05 to −0.66), before substantial sweat Ca loss had occurred.
This indicates that dermal Ca loss was not the primary trigger for the increases in PTH and CTX during exercise. Further research is necessary to understand the causes and consequences of the disruption in Ca homeostasis during exercise and specifically the extravascular shift in iCa.
1Division of Geriatric Medicine, Department of Medicine, University of Colorado Denver, Aurora, CO
2Eastern Colorado VA Geriatric Research, Education, and Clinical Center, Aurora, CO
3Department of Preventive Medicine and Biometrics, University of Colorado Denver, Aurora, CO
4Division of Endocrinology, Metabolism and Diabetes, Department of Medicine, University of Colorado Denver, Aurora, CO
5Department of Nutrition Science, Purdue University, West Lafayette, IN
Address for correspondence: Sarah J. Wherry, Ph.D., Campus Box B179 12631 E 17th Ave, Aurora, CO 80045; E-mail: firstname.lastname@example.org.
Submitted for publication December 2018.
Accepted for publication April 2019.
Online date: April 19, 2019