Important considerations for recommending hydration studies.

Dear Editor, We read with interest the study by Sun and colleagues1 published in the recent April Sports Medicine edition. The study investigated fluid balance associated with endurance kayaking and evaluated the efficacy of hydration with water versus a commercially available sports drink. We congratulate the authors for profiling the fluid loss incurred during kayaking, in which information specific to the sport has not been well documented. However, we are concerned with the proposed hydration recommendations made by the authors. While several of the limitations in the study are already mentioned by the authors, we would like to highlight other concerns to ensure a more balanced reading of the “take home message” for the readers. In both trials, it was reported that the paddlers were dehydrated by 0.72 ± 0.38% (~ 0.44 kg) and 1.10 ± 0.52% (~ 0.68 kg) when consuming (ad libitum) the sports drink and water respectively. Based on this observation, the authors recommended that paddlers should replace about 1.04 L/h of sports drinks (an additional of ~ 0.46 L above the 0.58 L ingested by the subjects during the sports drink trial) to prevent a loss in body mass. With the recent debates on evidence-based hydration guidelines,2-4 and the revision in Position Stand/Statement on fluid guidelines during exercise5,6 such that “the goal of drinking during exercise is to prevent excessive (>2% body weight loss from water deficit) dehydration”,6 it is hard to understand the recommendation proposed by Sun and colleagues.1 The body masses lost in both the water and the sports drink trials reported in this study were within the 2% threshold. Indeed, the authors had correctly pointed out in the opening paragraph that only in events of longer distance, hence longer duration, are dehydration likely to have a negative impact on performance. And this degree of dehydration is in the order of at least 2% body mass loss. Complete fluid replacement should not be confined to the exercise period, but should be extended to the period after exercise. To suggest replacing 100% of fluid loss during exercise, especially when the magnitude of dehydration is <2% of body mass, is excessive. Moreover, for optimal fluid replenishment, consensus in the literature suggests replacing 150% (including the recovery period) and not 100% of the fluid lost during exercise.6 In this study, sweat losses was calculated from the differences in body masses before and after exercise, fluid intake and urine output. The precision of the weighing scale used to measure body mass needs to be sensitive Important Considerations for Recommending Hydration Studies