Sodium is the major ion in the extracellular fluid, thus it is intuitive that sweat sodium losses should be replaced if plasma volume is to be restored or maintained. In a systematic investigation of the relationship between whole-body sweat sodium losses and the effectiveness of beverages with different sodium concentrations in restoring fluid balance, Shirreffs and Maughan (1998) showed that, provided that an adequate volume is consumed, euhydration is achieved when the sodium intake is greater than the sweat sodium loss, although, as discussed below, not all studies have reported similar findings (Mitchell et al., 2000). However, to investigate this properly it is important that the study design allows for sufficient time for drink-induced diuresis to occur once drinking is finished. Generally a minimum of 2 h is required after drinking a bolus of fluid to allow sufficient time for any significant renal excretion of water to occur