Fluid intake recommendations during exercise have changed substantially over the past half century. Up until the 1970s, marathon runners were advised to avoid drinking during competitive racing.1 In 1996, the American College of Sports Medicine (ACSM) advised athletes to replace all sweat lost during exercise by drinking the maximum amounts that can be tolerated.2 Then, in 2007, the ACSM recommended drinking 0.4-0.8 L/h of fluid during exercise with lower fluid volumes for slower and lighter individuals competing in cool environments, and higher volumes for faster and larger individuals competing in warm environments. These guidelines also state that athletes drink enough to ensure that they do not lose more than 2% of body mass because a greater loss is thought to impair exercise performance.3 These guidelines have emerged from studies modelling parameters that influence the rate of sweating. One study4 predicted that a fluid intake rate of 0.4-0.8 L/h kept body mass loss within 3% and prevented body mass gain in 50-90 kg subjects running marathons at 8.5-15.0 km/h in cool and warm ambient conditions—that is, 18o C and 28o C, respectively. The …