Requests for sponsorship from individuals planning to run the London marathon have made me aware that this sporting spectacular is not too far down the road. While I am enthusiastic about the health benefits of physical exercise, endurance activities such as marathon running are, however, not without risk. For instance, while competitors in such events are generally advised to consume plenty of water, this can be overdone: in extreme circumstances, a surfeit of water may lead to lower-than-normal levels of sodium in the body that may lead to complications such as convulsions, coma and even death. The scientific literature contains more than 250 reports of this syndrome, though underreporting means that the actual numbers may well be far higher. The evidence suggests that individuals consuming an excess of water during exercise may be at risk of running to ground.

Although the replenishment of lost fluid during extended exercise is both widespread and well-accepted, this practice only came into vogue into the 1970s after being advocated as a way of staving off heat stroke. More latterly, studies have shown that dehydration can significantly impair athletic performance. Perhaps as a result of this research, drinking during exercise has filtered through to other sports too. Witness, for example, the relatively recent phenomenon of professional football and rugby players taking slurps from sports bottles during breaks in play.

While there are good reasons to keep fluid levels topped up during exercise, the potential for low sodium in the system (the medical term for which is hyponatraemia) and the complications associated with it show that it is possible to have one too many for the road. Fortunately, the risk of hyponatraemia seems to be confined to individuals engaged in sport for several hours at a time. During prolonged exercise the body may lose significant amounts of sodium via sweat. Also, extended activities such as marathon running provide sufficient time for potentially harmful quantities of water to be imbibed. The risks associated with endurance exercise seem to have been compounded by commonly-given advice that competitors should drink the maximum amount of fluid that they can tolerate. It was not until this maxim became widespread that reports of water intoxication in endurance events began flowing in.

Rather than drinking as much as can be stomached, it may be safer for individuals to consume more moderate amounts of fluid during marathons and other endurance events. This approach not only helps protect against hyponatraemia, but also reduces the risk that competitors will be slowed down by a surplus of water sloshing around the system. On the day of the event, quaffing about half a litre of water a couple of hours before hitting the road will help ensure the body is well topped-up with fluid, but also gives ample time for the body to shift any excess.

During the race itself, drinking 500 - 1000 mls (1/2 - 1 litre) of water per hour should maintain a good level of hydration with a minimum of risk. Volumes at the upper end of this range will be most appropriate for larger individuals, especially in the heat when fluid losses due to sweating are likely to be considerable. Smaller individuals, especially in more temperate climes, can generally make do with less. It seems that drinking adequate, though not excessive, quantities of water during endurance exercise is best in the long run.

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