So how much fluid should runners drink? Guidance from the International Marathon Medical Directors Association (IMMDA), drawn up by Prof Noakes in 2003, advises that “drinking to thirst” is enough to maintain the body’s fluid balance – and that to drink more than that contradicts our natural biological signals. However, the influential American College of Sports Medicine (ACSM) argues that to avoid dehydration, athletes should not lose more than two per cent of body weight while exercising, a position also taken by the big sports drinks companies, Lucozade and Gatorade. The Gatorade Sports Science Institute (GSSI) advises would-be athletes to “drink before you’re thirsty or you’ll just be playing catch-up. Drink before, during and after exercise.” Athletes should “train their guts” to tolerate more fluids in the same way “they train their muscles to tolerate more exercise”.