The present study assessed the effects of mild dehydration on cognitive performance and mood of young males. A total of twenty-six men (age 20·0 (SD 0·3) years) participated in three randomised, single-blind, repeated-measures trials: exercise-induced dehydration plus a diuretic (DD; 40 mg furosemide); exercise-induced dehydration plus placebo containing no diuretic (DN); exercise while maintaining euhydration plus placebo (EU; control condition). Each trial included three 40 min treadmill walks at 5·6 km/h, 5 % grade in a 27·78C environment. A comprehensive computerised six-task cognitive test battery, the profile of mood states questionnaire and the symptom questionnaire (headache, concentration and task difficulty) were administered during each trial. Paired t tests compared the DD and DN trials resulting in .1 % body mass loss (mean 1·59 (SD 0·42) %) with the volunteer's EU trial (0·01 (SD 0·03) %). Dehydration degraded specific aspects of cognitive performance: errors increased on visual vigilance (P¼0·048) and visual working memory response latency slowed (P¼ 0·021). Fatigue and tension/anxiety increased due to dehydration at rest (P¼ 0·040 and 0·029) and fatigue during exercise (P¼0·026). Plasma osmolality increased due to dehydration (P,0·001) but resting gastrointestinal temperature was not altered (P¼0·238). In conclusion, mild dehydration without hyperthermia in men induced adverse changes in vigilance and working memory, and increased tension/anxiety and fatigue.Key words: Vigilance: Reaction time: Reasoning: Memory: FurosemideThe most comprehensive studies that evaluated the effects of dehydration on cognitive performance were conducted more than 20 years ago in laboratories located in a hot climate (1,2) . These studies employed a combination of highheat and aerobic exercise to rapidly produce dehydration. Subsequently, at least six other studies have evaluated the influence of body water loss via exercise in the heat (3 -5) and suggested that mild dehydration adversely affects cognitive function at approximately 2 % or more body mass loss, but not at 1 % body mass loss (1,2,5) . However, because body water loss and hyperthermia (i.e. resulting from exercise in a hot environment) were both present in these studies, the effects of dehydration alone could not be examined. Recently, three studies have examined dehydration without inducing hyperthermia (3,6,7) , via passive water restriction or prolonged exercise in a mild environment. These studies induced a narrow range of moderate dehydration (2·6-2·8 %) and therefore did not determine whether more modest levels of dehydration affect cognitive function.The present investigation was designed to assess the effects of mild dehydration, between 1 and 2 % body water loss, on cognitive performance and mood of healthy young males. Few studies have evaluated the effects of such mild dehydration in either men or women (7,8) . To isolate mild dehydration as the sole independent variable, body water loss was induced by mild exercise in a mild environ...

Context: Authors of most field studies have not observed decrements in physiologic function and performance with increases in dehydration, although authors of well-controlled laboratory studies have consistently reported this relationship. Investigators in these field studies did not control exercise intensity, a known modulator of body core temperature.Objective: To directly examine the effect of moderate water deficit on the physiologic responses to various exercise intensities in a warm outdoor setting.Design: Semirandomized, crossover design. Setting: Field setting. Patients or Other Participants: Seventeen distance runners (9 men, 8 women; age 5 27 6 7 years, height 5 171 6 9 cm, mass 5 64.2 6 9.0 kg, body fat 5 14.6% 6 5.5%).Intervention(s): Participants completed four 12-km runs (consisting of three 4-km loops) in the heat (average wet bulb globe temperature 5 26.56C): (1) a hydrated, race trial (HYR), (2) a dehydrated, race trial (DYR), (3) a hydrated, submaximal trial (HYS), and (4) a dehydrated, submaximal trial (DYS).Main Outcome Measure(s): For DYR and DYS trials, dehydration was measured by body mass loss. In the submaximal trials, participants ran at a moderate pace that was matched by having them speed up or slow down based on pace feedback provided by researchers. Intestinal temperature was recorded using ingestible thermistors, and participants wore heart rate monitors to measure heart rate.Results: Body mass loss in relation to a 3-day baseline was greater for the DYR (24.30% 6 1.25%) and DYS trials (24.59% 6 1.32%) than for the HYR (22.05% 6 1.09%) and HYS (22.0% 6 1.24%) trials postrun (P , .001). Participants ran faster for the HYR (53.15 6 6.05 minutes) than for the DYR (55.7 6 7.45 minutes; P , .01), but speed was similar for HYS (59.57 6 5.31 minutes) and DYS (59.44 6 5.44 minutes; P . .05). Intestinal temperature immediately postrun was greater for DYR than for HYR (P , .05), the only significant difference. Intestinal temperature was greater for DYS than for HYS postloop 2, postrun, and at 10 and 20 minutes postrun (all: P , .001). Intestinal temperature and heart rate were 0.226C and 6 beats/min higher, respectively, for every additional 1% body mass loss during the DYS trial compared with the HYS trial.Conclusions: A small decrement in hydration status impaired physiologic function and performance while trail running in the heat.Key Words: environmental physiology, dehydration, rehydration, core temperature, heart rate Key Points N The physiologic and performance decrements associated with dehydration that exist in laboratory settings also exist in field settings. N Methodologic challenges in the field setting make isolating these effects difficult.

The FULL and PART conditions resulted in greater physiologic strain than the CON condition. These findings indicated that critical internal temperature and hypotension were concurrent with exhaustion during uncompensable (FULL) or nearly uncompensable (PART) heat stress and that anthropomorphic characteristics influenced heat storage and exercise time to exhaustion.