Submitted to: Journal of Applied Physiology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 4/20/2011
Publication Date: 6/14/2011
Publication URL: http://handle.nal.usda.gov/10113/58218
Citation: Ely, M.R., Kenefick, R.W., Cheuvront, S.N., Chinevere, T.D., Lacher, C.P., Lukaski, H.C., Montain, S.J. 2011. Surface contamination artificially elevates initial sweat mineral concentrations. Journal of Applied Physiology. 110:1534-1540. Interpretive Summary: There have been reports of decreasing mineral concentrations during prolonged sweating. It is unknown if the decreasing concentrations are a result of flushing of surface contamination or a result of a biological mechanism related to sweating. Therefore, the purpose of this study was to determine if surface contamination or a biological mechanism is responsible for decreasing mineral concentrations. To address the problem mineral concentrations of calcium, copper, iron, potassium, magnesium, sodium, and zinc were serially sampled from three body fluid pools (blood, transdermal fluid, sweat) during prolonged exercise heat-stress. Additionally, sweat was collected from two skin sites (upper back and whole arm) that were meticulously cleaned in an effort to remove surface contaminants. There were nominal shifts in transdermal iron (+15%) and zinc (-18%) concentrations, but were no alterations in blood or sweat mineral concentrations collected from the upper back. There were decrements in Ca, Cu, Mg, and Zn concentrations from the arm bag which declined 26 to 76% from initial to the subsequent samples, becoming similar to sweat pouch concentrations from the upper back. These results suggest that the decreasing mineral concentrations found during prolonged sweating are not due to a biological conservation mechanism but to suspected surface contamination that remained on the relatively difficult to clean arm (due to fingernails, arm hair) than on the smooth surface of the back. Accurate knowledge of sweat mineral concentrations are an area of concern outlined by the Institute of Medicine for proper formulation of Dietary Reference Intakes and for formulation of military rations.
Technical Abstract: During exercise in the heat, sweat is initially concentrated in minerals, but serial sweat samples appear more dilute. Possible causes include reduced dermal mineral concentrations or flushing of surface contamination. PURPOSE: To simultaneously sample mineral concentrations in transdermal fluid (TDF), sweat, and serum during extended exercise-heat stress to characterize the serial changes in sweat mineral concentrations. METHODS: Sixteen heat-acclimated individuals walked on a treadmill (1.56m/s, 3.0% grade) in a 35°C, 20% RH, 1 m/s wind environment 50 min each hour for 3-hours. Mineral concentrations of Ca, Cu, Fe, K, Mg, Na, and Zn were measured each hour from serum, sweat from upper back (sweat pouch) and arm (bag), and TDF from the upper back. Sites were meticulously cleaned before the experiment to minimize surface contamination. Mineral concentrations were determined by ICP-AES. RESULTS: There was a modest increase in TDF [Fe] (15%) and decrease in TDF [Zn] (-18%) while all other mineral concentration in TDF, serum, and pouch sweat exhibited small intra-subject variability. The alterations in TDF [Fe] and [Zn] occurred without concomitant changes in sweat [Fe] and [Zn] from the pouch. In contrast, the initial arm bag sweat mineral concentrations were greater than those in the sweat pouch and [Ca], [Cu], [Mg], and [Zn] declined 26 to 76% from initial to the subsequent samples, becoming similar to sweat pouch. CONCLUSION: The alterations in Fe and Zn TDF concentrations occurred without variations to mineral concentrations from the sweat pouch. After prolonged sweating, the elevated mineral concentrations measured in the arm bag decreased and became similar to those measured in the sweat pouch. It is concluded that the initially concentrated sweat measured in the arm bag was due to surface contamination that could not be thoroughly cleaned (e.g. fingernails, arm hair) and nominal TDF mineral shifts do not affect sweat mineral concentrations.