The study of Twerenbold et al¹ is important for a number of reasons, not all of which may have been emphasised sufficiently by the authors.

Firstly, it confirms that a rate of fluid intake of 1000 ml/h is too high for a group of female runners running at ∼ 10 km/h and who would therefore complete a 42 km marathon in about 4.25 hours. As the athletes drank 4 litres and gained 2 kg during the trial, their average rate of weight loss (as opposed to sweat rate) was about 500 ml/h. As not all of the weight lost during exercise is sweat and as much as 1–3 kg of this weight loss may result from fuel and water losses that do not contribute to dehydration,^2,3 the absolute maximum rate at which these athletes should have ingested fluid during exercise was probably even less than 500 ml/h. This is substantially less than the drinking guidelines of the American College of Sports Medicine^4,5 and the Gatorade Sports Science Institute,⁶ which have promoted rates of fluid ingestion of up to 1200–1800 ml/h. As there is no evidence that gaining weight during exercise improves performance^7,8 whereas there is good evidence that athletes who either lose no weight or who gain weight during exercise are increasingly likely to (a) have an impaired performance,⁷ (b) develop troubling gastrointestinal symptoms,^7,8,9,10 or (c) finish the race with serum sodium concentrations below about 128 mmol/l causing hyponatraemic encephalopathy,^11–13 it is not immediately clear why the authors chose such high rates of fluid intake in these athletes. Except, perhaps, if they wished to “prove” the value of sodium ingestion during exercise. I note, for example, that the study was funded by a commercial company …