Hydration status and the effects of hypohydration have been the topic of much public and scientific debate in recent years. While many physiological responses to hypohydration have been studied extensively, the subjective responses to hypohydration have largely been ignored. The present investigation was designed to investigate the physiological responses and subjective feelings resulting from 13, 24 and 37 h of fluid restriction (FR) and to compare these with a euhydration (EU) trial of the same duration in fifteen healthy volunteers. The volunteers were nine men and six women of mean age 30 (SD 12) years and body mass 71·5 (SD 13·4) kg. Urine and blood samples were collected and subjective feelings recorded on a 100 mm verbally anchored questionnaire at intervals throughout the investigation. In the EU trial the subjects maintained their normal diet. Body mass decreased by 2·7 (SD 0·6) % at 37 h in the FR trial and did not change significantly in the EU trial. Food intake in the FR trial (n 10) provided an estimated water intake of 487 (SD 335) ml and urinary losses (n 15) amounted to 1·37 (SD 0·39) litres. This is in comparison with an estimated water intake of 3168 (SD 1167) ml and a urinary loss of 2·76 (SD 1·11) litres in the EU trial. Plasma osmolality and angiotensin II concentrations increased from 0 -37 h with FR. Plasma volume decreased linearly throughout the FR trial amounting to a 6·2 (SD 5·1) % reduction by 37 h. Thirst increased from 0 -13 h of FR then did not increase further (P.0·05). The subjects reported feelings of headache during the FR trial and also that their ability to concentrate and their alertness were reduced.

This study measured fluid balance during a 90-min preseason training session in the first team squad (24 players) of an English Premier League football team. Sweat loss was assessed from changes in body mass after correction for ingested fluids and urine passed. Sweat composition was measured by collection from patches attached to the skin at 4 sites. The weather was warm (24-29 degrees C), with moderate humidity (46-64%). The mean +/- SD body mass loss over the training session was 1.10+/- 0.43 kg, equivalent to a level of dehydration of 1.37 +/- 0.54% of the pre-training body mass. Mean fluid intake was 971 +/- 303 ml. Estimated total mean sweat loss was 2033 +/- 413 ml. Mean sweat electrolyte concentrations (mmol/L) were: sodium, 49 +/- 12; potassium, 6.0 +/- 1.3; chloride, 43 +/- 10. Total sweat sodium loss of 99+/- 24 mmol corresponds to a salt (sodium chloride) loss of 5.8 +/- 1.4 g. Mean urine osmolality measured on pre-training samples provided by the players was 666 +/- 311 mosmol/kg (n = 21). These data indicate that sweat losses of water and solute in football players in training can be substantial but vary greatly between players even with the same exercise and environmental conditions. Voluntary fluid intake also shows wide inter-individual variability and is generally insufficient to match fluid losses.

To investigate how differing moderate sodium chloride concentrations affect rehydration after exercise and subsequent exercise capacity, eight males lost 1.98 +/- 0.1% body mass exercising in the heat, then consumed one of four drinks in a volume equivalent to 150% of mass loss. Drinks were identical except for sodium chloride content (1 +/- 1, 31 +/- 1, 40 +/- 1, 50 +/- 1 mmol/l). After 4 h recovery subjects cycled for 5 min at 70% VO(2peak) then at 95% VO(2peak) until volitional fatigue. Urine output was inversely related to sodium intake: more was produced with the 1 than the 40 and 50 mmol/l drinks (P < 0.01). Time to exhaustion in the exercise capacity test was not different between treatments (P = 0.883). The addition of 40 or 50 mmol/l of sodium chloride to a rehydration beverage reduced subsequent urine output, thereby providing more effective rehydration than a sodium-free drink. This did not, however, result in improved performance 4 h after the end of the rehydration period.