Voluntary dehydration and alliesthesia for water.

The purpose of this experiment was to explore the complex relationship between fluid consumption and consumption factors (thirst, voluntary dehydration, water alliesthesia, palatability, work-rest cycle) during a simulated 14.5-km desert walk (treadmill, 1.34 m X s-1, 5% grade, 40 degrees C dry bulb/26 degrees C wet bulb, and wind speed of approximately 1.2 m X s-1). Twenty-nine subjects were tested (30 min X h-1, 6 h) on each of two nonconsecutive days. The subjects were randomly assigned to one of three groups: tap water (n = 8), iodine-treated tap water (n = 11), or iodine-treated flavored tap water (n = 10). The temperature of the water was 40 degrees C during one trial and 15 degrees C on the other. Mean sweat losses (6 h) varied between 1.4 kg (warm iodine-treated; 232 +/- 44 g X h-1) and 3.0 kg (cool iodine-treated flavored; 509 +/- 50 g X h-1). Warm drinks were consumed at a lower rate than cool drinks (negative and positive alliesthesia). This decreased consumption resulted in the highest percent body weight losses (2.8 and 3.2%). Cooling and flavoring effects on consumption were additive and increased the rate of intake by 120%. The apparent paradox between reduced consumption concomitant with severe dehydration and hyperthermia is attributed to negative alliesthesia for warm water rather than an apparent inadequacy of the thirst mechanism. The reluctance to drink warm iodine-treated water resulted in significant hyperthermia, dehydration, hypovolemia, and, in two cases, heat illness.