With the summer coming on Dr Bob is turning to drinking, and not the alcoholic kind this time. Road runners, gym bunnies and mountain bikers all have a drinking problem. They won’t go beyond a few steps without starting to imbibe copious quantities of fluids usually from large pipes slung over their shoulders. They will tell you that this to avoid ‘dehydration’. Fell runners as a rule don’t do this..
So who is right? A fascinating article in the British journal of Sports Medicine summarised the evidence and the hazards of over drinking during exercise.The story starts back in 1985 when three cases of runners who had collapsed and had epileptic fits after being treated with intravenous fluids for ‘dehydration’ . In all the cases there was a low level of the main salt in the blood, sodium, and it appeared that it had been diluted by too much drinking exacerbated when the patient was ‘treated’ with intravenous fluid.
This condition hyponatraemia became increasingly common especially at the Comrades marathon in South Africa with 17 cases in 1987. A study in 1988 at the Comrades demonstrated that the 8 cases that year who collapsed had between 1.2 and 5.9 litres of excess body water.
Although the evidence was mounting against high fluid intake in 1990 the US Army introduced a policy that forced recruits in temperatures of above 30C to drink 1.8 litres per hour. Between 1989 and 1996 125 recruits required hospital admission and 6 died. The US army woke up to the problem in 1996 when it changed its rules to 0.9-1.3 litres per hour and less than 10 litres per day. The military hyponatraemia problem melted away.
However just as the US Army was dealing with drinking induced hyponatraemia the American College of Sports Medicine brought in guidelines in 1996 to encourage fluid intake in recreational runners backed up by a vigorous publicity campaign by the sports drinks industry! Guess who funds the American College of Sports Medicine. There were further high profile deaths following this in 1998 and 2002.
But you will be asking what about dehydration. Don’t we lose water and stop sweating and then develop hyperthermia? The answer is that sweating is maintained until extreme water depletion and water loss is only associated with slight temperature gain even if there is no water intake. Humans appear to be adapted to tolerate quite large quantities of fluid loss. Those at risk of hyperthermia are fit individuals doing very intensive exercise.
It took a further huge study by scientists from Harvard Medical School at the 2002 Boston Marathon published by the prestigious New England Journal of Medicine to begin to get the message across that drinking large quantities of fluid, particularly by slower women runners risked hyponatraemia. USA track and field the governing body for long distance running in the states have provided guidance for coaches which come down to drinking when thirsty which usually equates to 400-800ml per hour
Any residual doubters might like to ponder the example of South African bushmen from whom we are genetically a whisker away, who carry no water with them on 30km hunts in the dessert as it hinders their speed. So maybe fell runners are in good company!
