Like many marathon runners, Kate Mori always drank plenty of fluids before, during and after a race, rather than waiting until she got thirsty. “I’d always been taught you had to \'stay ahead’ of thirst and that being thirsty was a sign you were already dehydrated,” says the 42-year-old sports scientist. In 2007, Mori took part in the London Marathon – her fourth, and the hottest on record, with temperatures peaking at 23.5C. Conscious of the repeated advice to maintain fluid intake coming over the PA system, she took frequent drinks at the water stations along the route. By the 18th mile, Mori felt “quite poorly” but was determined to finish; she was raising funds for a children’s cancer charity and wanted to ensure they got their sponsorship money. Near the end, she needed help from other runners to stay upright; hours later she was in casualty at the Royal London Hospital in Whitechapel, suffering from severe diarrhoea, vomiting and increasing confusion, with her legs endlessly mimicking a running motion. “I thought I was still in the marathon,” she says. Mori was not dehydrated from drinking too little fluid, as might be at first assumed. She had drunk too much. As a result she had developed a dangerous but little-recognised condition called exercise-associated hyponatraemia (EAH). Sometimes called water intoxication, EAH is marked by a low blood sodium concentration and can cause the brain to swell, causing confusion, loss of consciousness and seizures. Mori made a full recovery after intravenous treatment with sodium chloride to redress the low concentration of sodium in her blood. Another contestant in that year’s marathon was not so fortunate. David Rogers, a 22-year-old fitness instructor, died of EAH after finishing the race in three hours 30 minutes. Mori isn’t sure how much she drank during the race. “But the consultant reckoned it was probably about three litres of water,” she says. “Ironically, at the finish I was taken into a first aid tent and offered more water. “I feel ashamed that with my job [she teaches an MA in sports development at Gloucestershire University] I did not have the awareness about this condition,” she adds. “It is far more dangerous than dehydration.” With the 2012 London Marathon taking place next month, most would-be runners will be similarly advised on the need to avoid dehydration, especially if temperatures rise. Dehydration occurs when the body’s normal water content is reduced and its balance of salts and sugar upset. But according to many medical experts, thousands of runners could be risking their health if not their lives by drinking too much rather than too little. Timothy Noakes, professor of exercise and sports science at the University of Cape Town, South Africa, who has spent the last 30 years researching the topic, says that the dangers of dehydration during endurance exercise have been exaggerated, with the result that cases of EAH are on the rise. He says runners need to be warned that overconsumption of fluids (whether water or sports drinks) before, during, or after exercise can have a potentially fatal outcome. “There is not a single report in the medical literature of dehydration being a proven, direct cause of death in a marathon runner,” adds Prof Noakes. “But EAH, caused by drinking excessively, has resulted in at least 12 deaths among sportsmen and women.” As well as Rogers in 2007, fatalities include a female US marathoner in 1993 and an American footballer, Paul Allen, in 2010. “In addition,” says Noakes, since 1981, “there have been over 1,600 documented cases of EAH worldwide.” Although the symptoms of EAH were first noted by Prof Noakes and colleagues in the 1980s, the cause – voluntary over-drinking – was finally established in 1991. The condition results from abnormal water retention throughout the body, which reduces the blood sodium concentration, causing excess fluid to move from the blood into tissue cells, including those in the brain, and makes them swell. Consultant cardiologist Professor Sanjay Sharma of St George’s Healthcare NHS Trust, London, and the London Marathon medical director, warns that the higher temperatures rise, the more people are encouraged to drink – especially, he says, by sports drinks advertisements – and the more cases of EAH can be expected. “Many runners feel they need to drink at water stations whenever they can, regardless of how thirsty they feel,” he says. Slow runners are particularly at risk because they are on the road for longer – and it usually takes four hours of over-drinking to accumulate the excess fluid which causes EAH. 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”. But Dr Courtney Kipps, specialist in sport and exercise medicine and the London Marathon’s assistant medical director, argues that in a marathon one should expect to lose up to two per cent of body weight and that “losing less than two per cent body weight after a marathon means that you’ll probably be over-hydrated and at risk of EAH.” Prof Noakes sees a conflict between science and commerce. “Promoting an abnormal and unphysiological behaviour – drinking to stay ahead of thirst – produced a novel disease, EAH, with tragic consequences,” he says. However Dr Ian Rollo of Gatorade argues that Prof Noakes “does not factor in the dangers associated with the more common condition of dehydration”, while a spokeswoman for the ACSM told The Daily Telegraph that “if anyone is at fault, it’s the marketing teams for the commercial sports drink companies.” Given the controversy, perhaps it’s not surprising that many runners know disturbingly little about the dangers of over-drinking. Dr Jonathan Williams is a marathon runner and sports physician at Bristol University who has himself suffered symptoms of EAH. His recent research published this month in the Clinical Journal of Sports Medicine, found that 12 per cent of 217 runners in the 2010 London Marathon planned “to drink volumes which would put them at risk of EAH, and that only 35 per cent had a basic understanding of the cause and effects of EAH.” Kate Mori, now pregnant with her second child, will be watching this year’s London Marathon from the sidelines. “I haven’t felt like running a marathon since I had EAH,” she says. “But I would like to do another eventually. I am very fortunate to have made a recovery.”