175 BrJ Spors Med 1997 31 175 182 Reviews Factors influencing the restoration of fluid and electrolyte balance after exercise in the heat R J Maughan J B Leiper S M Shirreffs Summary University Medical School Foresterhill Aberdeen AB25 2ZD Scotland United Kingdom R J Maughan J B Leiper S M Shirreffs Correspondence to Professor Maughan Accepted for publication 29 May 1997 Maintenance of fluid balance is a major concern for all athletes competing in events held in hot climates This paper reviews recent work relating to optimisation of fluid replacement after sweat loss induced by exercising in the heat Data are taken from studies undertaken in our laboratory Issues investigated were drink composition volume consumed effects of consuming food with a drink effects of alcohol on rehydration effectiveness voluntary intake of fluid and considerations for women related to the menstrual cycle The results are presented as a series of summaries of experiments followed by a discussion of the implications The focus of this review is urine output after ingestion of a drink fluid excreted in urine counteracts rehydration Also included are data on the restoration of plasma volume losses Ingestion of large volumes of plain water will inhibit thirst and will also promote a diuretic response If effective rehydration is to be maintained for some hours after fluid ingestion drinks should contain moderately high levels of sodium perhaps as much as 50 60 mmol l and possibly also some potassium to replace losses in the sweat To surmount ongoing obligatory urine losses the volume consumed should be greater than the volume of sweat lost Palatability of drinks is important in stimulating intake and ensuring adequate volume replacement Where opportunities allow the electrolytes required may be ingested as solid food consumed with a drink There are no special concerns for women related to changes in hormone levels associated with the menstrual cycle Ingestion of carbohydrate electrolyte drinks in the post exercise period restores exercise capacity more effectively than plain water The effects on performance of an uncorrected fluid deficit should persuade all athletes to attempt to remain fully hydrated at all times and the aim should be to start each bout of exercise in a fluid replete state This will only be achieved if a volume of fluid in excess of the sweat loss is ingested together with sufficient electrolytes Introduction Sustained hard exercise in a hot environment presents a greater challenge to the body s homoeostatic mechanisms than any other circumstance The combination of a high rate of metabolic heat production and a restricted capacity for heat dissipation leads to hyperthermia which may progress to heat illness this will inevitably impair exercise performance and may in extreme cases be fatal The 1996 Olympic Games held in Atlanta in July and August were but one example of a major sporting event held in unfavourable climatic conditions many others are scheduled for the near future The combination of high temperature and high humidity has major implications for everyone attending this includes spectators officials and team management as well as competitors Performance in all outdoor endurance events which we can define as those lasting longer than a total time of about 20 30 minutes is generally reduced and there seems to be no way of avoiding some impairment of performance A recent study under laboratory conditions2 has shown that endurance time on a bicycle ergometer at an exercise intensity that could be sustained for 92 minutes at a temperature of 11 C was reduced to 83 minutes when the temperature was increased to 21 C and to 51 minutes when the temperature was increased to 30 C in Atlanta conditions the reduction would be even greater If the athlete is dehydrated before exercise begins the reductions in performance observed in the heat are greatly magnified For those unaccustomed to living in the heat the stress will be no different from that experienced by those who are accustomed to the heat but the way in which the athletes deal with the conditions may be the largest factor influencing the impact of the climatic stress on their performance The successful competitor will have prepared a coping strategy that includes acclimatisation rehydration and behavioural and psychological components Aside from behavioural mechanisms to minimise or prevent overheating such as moving indoors to an air conditioned cool environment the physiological mechanisms activated by the acclimation process include an earlier onset of sweating and an increased sweating rate Sweating can be very effective in removing heat from the body and substantial amounts of fluid are likely to be lost in this way by many individuals The acclimation process results in an increased sweat loss and therefore increases rather than decreases the need for fluid replacement The water lost must be replaced in order to Maughan Leiper Shirreffs 176 maintain body water balance and allow the best possible athletic performance to be achieved In addition it is well established that dehydration removes the thermoregulatory advantage and improved exercise tolerance that result from acclimatisation Although the physician and physiologist disapprove it seems to be beneficial at least in terms of performance for athletes in weight category sports including wrestlers boxers and weightlifters to perform while in a dehydrated state the disadvantage arising from competing with a body water deficit has been found by trial error and the experience of coach and athlete to be outweighed by the advantages of competing in a weight category below the athlete s natural weight 4 The advantage probably arises from biomechanical factors including limb leverage advantages However the optimisation of rehydration after sweat loss induced to make weight at the time of the weigh in may be of particular importance to performers in these sports There are also many other athletes including those who have to compete in more than one round of a competition on the same day or on successive days for whom recovery and rehydration between events is of crucial importance The difficulty of achieving effective rehydration after thermal dehydration has long been recognised Ingestion of water causes a prompt diuresis even when the individual is still hypohydrated effectively preventing a return to euhydration 5 More recent studies have confirmed that plain water is not the