Transition duration of ingested deuterium oxide to eccrine sweat during exercise in the heat

Abstract The time necessary for the initial appearance of ingested water as sweat during exercise in the heat remains unknown. Based on the current literature, we estimated fluid transition through the body, from ingestion to appearance as sweat, to have a minimum time duration of approximately three minutes. The purpose of this study was to test this prediction and identify the time necessary for the initial enrichment of deuterium oxide (D 2 O) in sweat following ingestion during exercise in the heat. Eight participants performed moderate intensity (40% of maximal oxygen uptake) treadmill exercise in an environmental chamber (40 °C, 40% rH) to induce active sweating. After fifteen minutes, while continuing to walk, participants consumed D 2 O (0.15 ml kg −1 ) in a final volume of 50 ml water. Scapular sweat samples were collected one minute prior to and ten minutes post-ingestion. Samples were analyzed for sweat D 2 O concentration using isotope ratio mass spectrometry and compared to baseline. Mean±SD ∆ sweat D 2 O concentration at minutes one and two post-ingestion were not significantly higher than baseline (0 min). Minutes three (9±3 ppm) through ten (23±11 ppm) post-ingestion had ∆ sweat D 2 O concentrations significantly (P