The danger of an inadequate water intake during prolonged exercise

Abstract

To prevent thermal injuries during distance running, the American College of Sports Medicine proposes that between 0.83 and 1.65 l of water should be ingested each hour during prolonged exercise. Yet such high rates of fluid intake have been reported to cause water intoxication. To establish the freely-chosen rates of fluid intake during prolonged competitive exercise, we measured fluid intake during, body weight before and after, and rectal temperature after competition in a total of 102 runners and 91 canoeists competing in events lasting from 170-340 min. Fluid intakes during competition ranged from 0.29-0.62 l.h-1; rates of water loss ranged from 0.69-1.27 l.h-1 in the runners; values were lower in the canoeists. Mean post-race rectal temperatures ranged from 38.0-39.0 degrees C. There was no relationship between the degree of dehydration and post-race rectal temperature. We conclude that hyperthermia is uncommon in prolonged competitive events held in mild environmental conditions, and that exercise intensity, not the level of dehydration, is probably the most important factor determining the postexercise rectal temperature. During prolonged exercise in mild environmental conditions, a fluid intake of 0.5 l.h-1 will prevent significant dehydration in the majority of athletes.

Induction and decay of short-term heat acclimation

“…In a review, on the fluid replacement of athletes Casa and colleagues (2000) reported that dehydration of 1-2% body weight increases physiological stress and decreases performance. However, in reality, it is often difficult and sometimes impossible to prevent at least some dehydration during repeated heat stress (Convertino et al 1996) and this has been considered of little consequence by some authors (Noakes et al 1988;Greenleaf 1991), on the basis that this readily occurs during repeated heat exposure. In summary, we postulate that the increased physiological strain of restricted fluid replenishment during DEH acclimation may have resulted in adaptation of the fluid-regulatory system.…”

“…The use of permissive dehydration during acclimation contradicts the existing fluid replenishment guidelines for heat acclimation, which recommend the maintenance of good hydration status during exposure to heat stress conditions (Armstrong and Maresh 1991;Convertino et al 1996;Cheung and McLellan 1998;Casa et al 2000). However, the reality for many people undergoing acclimation bouts is that some level of dehydration is normal, if not frequently inevitable (Noakes et al 1988;Greenleaf 1992).…”

Influence of Carbohydrate-Electrolyte Drinks on Muscle Metabolism and Endurance Running Performance in Man

“…Despite the relatively severe dehydration induced by the marathon in this study (5.5% weight loss), core temperature was maintained within expected range (36.6 -38.9 CC). This is consistent with previous studies (Pugh et al 1967;Maughan et al 1985;Myhre et al 1985;Noakes et al 1988). The rise in core temperature is directly related to the metabolic rate and thus running speed sustained during prolonged exercise (Maughan et al 1985;Noakes et al 1991a).…”

“…On the other hand, it has been proposed that the rate of fluid intake should be less than the rate of sweat loss due to the fact that metabolic water and water stored with glycogen is released during exercise, and thus should not be taken into account when estimating the rate of fluid intake. On that basis, it has been recommended that an average fluid intake of 0.5 l.h-l is an adequate rate of fluid replacement during prolonged exercise (Noakes et al 1988). This rate of fluid intake appears to be the ad libitum fluid intake during prolonged exercise such as marathon running.…”

Exercise-Associated Hyponatremia

“…Before 1981, athletes were encouraged to drink heavily during exertion to avoid dehydration (7,31). With the description of EAH in South Africa and New Zealand in 1985, new fluid consumption guidelines that restricted overzealous fluid intake for endurance events in these countries were promoted (32,33). Concomitant with these recommendations, the incidence of EAH fell in both of these regions (19,20).…”