Acute effects of dehydration on sweat composition in men during prolonged exercise in the heat.
OBJECTIVE
To determine whether acute exercise-heat-induced dehydration affects sweat composition, eight males cycled for 2 h at 39.5 +/- 1.6% VO2peak on two separate occasions in a hot-humid environment (38.0 +/- 0.0 degrees C, 60.0 +/- 0.1% relative humidity).


METHODS
During exercise, subjects ingested either no fluid (dehydration) or a 20 mmol L(-1) sodium chloride solution (euhydration). The volume of solution, calculated from whole-body sweat loss and determined in a familiarization trial, was ingested at 0 min and every 15 min thereafter. Venous blood was collected at 0, 60 and 120 min of exercise and sweat was aspirated from a patch located on the dominant forearm at 120 min.


RESULTS
Following the 2-h cycling exercise, sweat [Na+] and [Cl-] was greater (P < 0.05) in the dehydration trial (Na+ 91.1 +/- 6.8 mmol L(-1); Cl- 73.3 +/- 3.5 mmol L(-1)) compared with the euhydration trial (Na+ 81.1 +/- 5.9 mmol L(-1); Cl- 68.5 +/- 3.3 mmol L(-1)). In addition, dehydration invoked a greater serum [Na+] (142.2 +/- 0.7 mmol L(-1); P < 0.05), [Cl-] (105.8 +/- 0.6 mmol L(-1); P < 0.05) and [K+] (5.27 +/- 0.2 mmol L(-1); P < 0.05) over the euhydration values for [Na+], [Cl-] and [K+], respectively (138.9 +/- 0.6, 102.9 +/- 0.5 and 4.88 +/- 0.1 mmol L(-1)). Plasma aldosterone was also significantly higher during exercise in the dehydration trial compared with the euhydration trial (53.8 +/- 3.8 vs. 40.0 +/- 4.3 ng dL(-1); P < 0.05).


CONCLUSIONS
Acute exercise-heat stress without fluid replacement resulted in a greater sweat [Na+] and [Cl-] which was potentially related to greater extracellular fluid [Na+], plasma aldosterone or sympathetic nervous activity.
Strathclyde Institute for Biomedical Sciences, University of Strathclyde, Glasgow, UK.
R M
RM
Morgan
M J
MJ
Patterson
M A
MA
Nimmo
D000328:Adult; D000450:Aldosterone / Q000097:blood; D001833:Body Temperature Regulation / Q000502:physiology; D002712:Chlorides / Q000032:analysis / Q000097:blood; D003681:Dehydration / Q000503:physiopathology*; D015444:Exercise / Q000502:physiology*; D006358:Hot Temperature; D006801:Humans; D006863:Hydrogen-Ion Concentration; D008297:Male; D011188:Potassium / Q000032:analysis / Q000097:blood; D012964:Sodium / Q000032:analysis / Q000097:blood; D013542:Sweat / Q000737:chemistry / Q000502:physiology*
D002712:Chlorides; D000450:Aldosterone; D012964:Sodium; D011188:Potassium