Kohrt WM, et al. - In this investigation, researchers assessed whether dermal calcium loss during cycling exercise is the major determinant of the serum ionized calcium (iCa), parathyroid hormone (PTH), and bone resorption (C-terminal telopeptide of type 1 collagen; CTX) responses. For this investigation, under both cool (18°C) and warm (26°C) conditions, female (n=13) and male (n=12) participants aged 18-45 years completed the same exercise. Exercise included 60-minute cycling at approximately 75% of peak aerobic power. During exercise, sweat samples were collected via skin patch method, and blood samples were collected before and during exercise, as well as 60 minutes post-recovery. For the warm condition, sweat volume and estimated sweat loss Ca was 50% higher than the cool condition. Overall, dermal Ca loss during exercise was not the primary trigger for PTH and CTX increases.