title: Fluid restriction during exercise in the heat reduces tolerance to progressive central hypovolaemia.
abstract: What is the central question of this study? Interactions between dehydration, as occurs during exercise in the heat without fluid replacement, and hyperthermia on the ability to tolerate central hypovolaemia are unknown. What is the main finding and its importance? We show that inadequate fluid intake during exercise in the heat can impair tolerance to central hypovolaemia even when it elicits only mild dehydration. These findings suggest that hydration during physical work in the heat has important military and occupational relevance for protection against the adverse effects of a subsequent haemorrhagic injury. This study tested the hypothesis that dehydration induced via exercise in the heat impairs tolerance to central hypovolaemia. Eleven male subjects (32 ± 7 years old, 81.5 ± 11.1 kg) walked (O2 uptake 1.7 ± 0.4 l min(-1) ) in a 40°C, 30% relative humidity environment on three occasions, as follows: (i) subjects walked for 90 min, drinking water to offset sweat loss (Hydrated, n = 11); (ii) water intake was restricted, and exercise was terminated when intestinal temperature increased to the same level as in the Hydrated trial (Isothermic Dehydrated, n = 11); and (iii) water intake was restricted, and exercise duration was 90 min (Time Match Dehydrated, n = 9). For each trial, tolerance to central hypovolaemia was determined following exercise via progressive lower body negative pressure and quantified as time to presyncope. Increases in intestinal temperature prior to lower body negative pressure were not different (P = 0.91) between Hydrated (1.1 ± 0.4°C) and Isothermic Dehydrated trials (1.1 ± 0.4°C), but both were lower than in the Time Match Dehydrated trial (1.7 ± 0.5°C, P < 0.01). Prior to lower body negative pressure, body weight was unchanged in the Hydrated trial (-0.1 ± 0.2%), but was reduced in Isothermic Dehydrated (-0.9 ± 0.4%) and further so in Time Match Dehydrated trial (-1.9 ± 0.6%, all P < 0.01). Time to presyncope was greater in Hydrated (14.7 ± 3.2 min) compared with Isothermic Dehydrated (11.9 ± 3.3 min, P < 0.01) and Time Match Dehydrated trials (10.2 ± 1.6 min, P = 0.03), which were not different (P = 0.19). These data indicate that inadequate fluid intake during exercise in the heat reduces tolerance to central hypovolaemia independent of increases in body temperature.
authors.affiliation: Institute for Exercise and Environmental Medicine, Texas Health Presbyterian Hospital of Dallas and the University of Texas Southwestern Medical Center, Dallas, TX, USA.|Department of Exercise and Nutrition Sciences, University at Buffalo, Buffalo, NY, USA.
authors.forename: Zachary J
authors.initials: ZJ
authors.lastname: Schlader
authors.affiliation: Institute for Exercise and Environmental Medicine, Texas Health Presbyterian Hospital of Dallas and the University of Texas Southwestern Medical Center, Dallas, TX, USA.
authors.forename: Daniel
authors.initials: D
authors.lastname: Gagnon
authors.affiliation: Institute for Exercise and Environmental Medicine, Texas Health Presbyterian Hospital of Dallas and the University of Texas Southwestern Medical Center, Dallas, TX, USA.|Department of Kinesiology, Texas Woman's University, Denton, TX, USA.
authors.forename: Eric
authors.initials: E
authors.lastname: Rivas
authors.affiliation: US Army Institute of Surgical Research, Fort Sam Houston, San Antonio, TX, USA.
authors.forename: Victor A
authors.initials: VA
authors.lastname: Convertino
authors.affiliation: Institute for Exercise and Environmental Medicine, Texas Health Presbyterian Hospital of Dallas and the University of Texas Southwestern Medical Center, Dallas, TX, USA.
authors.forename: Craig G
authors.initials: CG
authors.lastname: Crandall
mesh_terms: D000328:Adult; D001833:Body Temperature Regulation / Q000502:physiology; D003681:Dehydration / Q000150:complications / Q000175:diagnosis / Q000503:physiopathology*; D004326:Drinking / Q000502:physiology*; D015444:Exercise / Q000502:physiology*; D005440:Fluid Therapy / Q000379:methods; D006358:Hot Temperature / Q000009:adverse effects*; D006801:Humans; D020896:Hypovolemia / Q000175:diagnosis / Q000209:etiology / Q000503:physiopathology*; D008297:Male