title: Rehydration with fluid of varying tonicities: effects on fluid regulatory hormones and exercise performance in the heat.
abstract: This study examined the effects of rehydration (Rehy) with fluids of varying tonicities and routes of administration after exercise-induced hypohydration on exercise performance, fluid regulatory hormone responses, and cardiovascular and thermoregulatory strain during subsequent exercise in the heat. On four occasions, eight men performed an exercise-dehydration protocol of approximately 185 min (33 degrees C) to establish a 4% reduction in body weight. Following dehydration, 2% of the fluid lost was replaced during the first 45 min of a 100-min rest period by one of three random Rehy treatments (0.9% saline intravenous; 0.45% saline intravenous; 0.45% saline oral) or no Rehy (no fluid) treatment. Subjects then stood for 20 min at 36 degrees C and then walked at 50% maximal oxygen consumption for 90 min. Subsequent to dehydration, plasma Na(+), osmolality, aldosterone, and arginine vasopressin concentrations were elevated (P < 0.05) in each trial, accompanied by a -4% hemoconcentration. Following Rehy, there were no differences (P > 0.05) in fluid volume restored, post-rehydration (Post-Rehy) body weight, or urine volume. Percent change in plasma volume was 5% above pre-Rehy values, and plasma Na(+), osmolality, and fluid regulatory hormones were lower compared with no fluid. During exercise, skin and core temperatures, heart rate, and exercise time were not different (P > 0.05) among the Rehy treatments. Plasma osmolality, Na(+), percent change in plasma volume, and fluid regulatory hormones responded similarly among all Rehy treatments. Neither a fluid of greater tonicity nor the route of administration resulted in a more rapid or greater fluid retention, nor did it enhance heat tolerance or diminish physiological strain during subsequent exercise in the heat.
authors.affiliation: US Army Research Institute of Environmental Medicine, Thermal and Mountain Medicine Division, Natick, MA 01760, USA. Robert.Kenefick@us.army.mil
authors.forename: R W
authors.initials: RW
authors.lastname: Kenefick
authors.forename: C M
authors.initials: CM
authors.lastname: Maresh
authors.forename: L E
authors.initials: LE
authors.lastname: Armstrong
authors.forename: D
authors.initials: D
authors.lastname: Riebe
authors.forename: M E
authors.initials: ME
authors.lastname: Echegaray
authors.forename: J W
authors.initials: JW
authors.lastname: Castellani
mesh_terms: D000284:Administration, Oral; D000328:Adult; D000450:Aldosterone / Q000097:blood; D001127:Arginine Vasopressin / Q000097:blood; D001833:Body Temperature Regulation; D003681:Dehydration / Q000097:blood / Q000150:complications / Q000209:etiology / Q000503:physiopathology / Q000628:therapy* / Q000652:urine; D015444:Exercise; D005440:Fluid Therapy / Q000379:methods*; D006358:Hot Temperature / Q000009:adverse effects*; D006801:Humans; D007038:Hypotonic Solutions; D020896:Hypovolemia / Q000209:etiology* / Q000503:physiopathology / Q000628:therapy; D007262:Infusions, Intravenous; D007552:Isotonic Solutions; D008297:Male; D009994:Osmolar Concentration; D010953:Plasma Volume; D012964:Sodium / Q000097:blood / Q000652:urine; D012965:Sodium Chloride / Q000008:administration & dosage*; D013997:Time Factors; D014882:Water-Electrolyte Balance
chemical_list: D007038:Hypotonic Solutions; D007552:Isotonic Solutions; D001127:Arginine Vasopressin; D012965:Sodium Chloride; D000450:Aldosterone; D012964:Sodium