Journal Title (Medline/Pubmed accepted abbreviation): J Sports Sci
Page Numbers: 1233-1241
Summary of background and research design:
Background:Although evidence supports the concept that human thermoregulatory capacity is limited and may reduce exercise performance in extreme environmental conditions, there is some evidence that athletic performance can be maintained in extreme heat. Biologic control of thermoregulatory response may be sufficient to maintain homeostasis if exercisers are allowed to drink ad libitum and modify exercise intensity according to environmental conditions.
Hypothesis/Purpose: Evaluate the effects of ad libitum fluid intake and exercise self modification on exercise performance in extreme heat.
Subjects: Eighteen well-conditioned, heat-adapted men from the South African National Defense Force (average age, 26 y; maximal oxygen uptake [VO2max], 56 mL/kg/min) participated in this study.
Experimental design: Descriptive
Treatments and protocol:
Each participant marched 25 km carrying 26 kg. The march was individually paced (resting was allowed as necessary) and water was provided for ad libitum fluid intake. Core body temperatures were collected every minute (remote sensing system). Total body water was determined from deuterium abundance in salvia before and immediately after the march, as well as 2 hours after another deuterium dose following the march. Sweat and respiratory water losses were calculated. Blood samples for sodium levels and osmolality were collected before and 45 minutes after the march.
Summary of research findings:
- Overall, the group lost 2.73 kg (3.8%) during the march (P < .05 from baseline).
- Average water consumed was 1,264 mL/hr (range, 999 to 1,782 mL/hr).
- There was no association between body mass loss or fluid consumed and core temperature.
- There was no association between body mass loss and exercise time.
- Osmolality and sodium levels did not significantly change during the march.
- There was a significant association between sodium levels after exercise and body mass loss as well as total body water change (P < .05 for both).
- Mean peak body core temperature was 39.0° C; however, temperature over time shows a pacing strategy (up to 4 rest periods in the shade) that was not correlated with core temperature.
- Total body water decreased during the march (1.47 kg [2.0% body mass]; P < .05 from baseline).
- Body mass change was associated with total body water change (P < .05).
- There was no association between total body water change and exercise performance
Interpretation of findings/Key practice applications:
Body core temperatures during the march were maintained below the safe temperature (40.4° C) set by the American College of Sports Medicine for athletes. Fluid intake of the marchers exceeded that observed in competitive athletes (400 to 800 mL/hr); however, the marchers would be considered dehydrated according to their body mass changes. The magnitude of body mass loss observed in this study was approximately double that considered appropriate for athletes, although serum sodium changes were minimal among the marchers. A limitation of this study is that sweat losses were not assessed. In addition, the performance of the soldiers was not addressed as the march continued, so it is not clear how the dehydration may have affected marching speed. The results suggest that sodium intake is not necessary to maintain serum concentrations during exercise and that regulation of body water determines serum sodium concentration changes. Drinking ad libitum is sufficient for thermoregulation during prolonged exercise provided the participants are able to self-regulate their pace for the environmental condition.