Water, water everywhere and too much to drink
By Dr. Gabriel Symonds | Ask the Expert
Photo © Elena Derevstova
Have you noticed as the weather gets warmer, and even when it’s not so warm, there are many people walking around carrying bottles? Why should this be? It seems there is a widespread concern about ‘dehydration’ – something potentially dangerous that must be avoided by frequent swigs from a bottle of water or ‘sports drink’. Current received wisdom seems to be the more you exercise the more you should drink, or ‘as much as possible’ during sustained heavy exercise as in marathon running. Some people believe even if you’re not exercising you should drink X glasses of water a day.
But is this really a good idea? Is it necessary or could it actually be harmful?
The current popularity of water drinking during exercise seems to have started in medical circles in 1969 with a wrongly titled article in the South African Medical Journal: “The Danger of an Inadequate Water Intake during Marathon Running.” It was thought then that the risk of heatstroke was related to the degree of dehydration, and this could be measured by the weight lost during the race, which in turn could be prevented by frequent drinking. This reasoning was based on rather simplistic assumptions which subsequent studies have not supported; there is no reliable evidence that high rates of water drinking can influence the risk of heatstroke.
On the contrary, it is now realised that it is indeed possible to drink too much during prolonged exercise, with potentially harmful effects. The blood can become too ‘dilute’ (hyponatremic), and this can in extreme cases cause symptoms like confusion, fits, and collapse.
So, how much should one drink then? The advice to drink copious amounts of fluid is unwise. When running a marathon, for the average runner, 400 – 800ml per hour seems to be sufficient. Reports of interviews with top level athletes indicate that during marathons they tend to drink little – around 200ml per hour – which belies the belief that one can only perform at a high level when you drink a lot of fluid.
It is also sometimes said that you can’t tell how much you need to drink from your sense of thirst. Well, it seems one’s sense of thirst is, after all, a good indication of how much one should drink. Thus it is better – as in so many situations in life – to be guided by nature rather than to follow some arbitrary rule.
Incidentally, does it do any good to imbibe sports drinks? It certainly does good for the coffers of the sports drinks industry! But in fact, if you feel thirsty during or after exercise, all you really need is plain water.
Dr. Gabriel Symonds has been working as a general practitioner in Tokyo since 1984. A graduate of the University of London, he has wide experience in general and family medicine, and is one of the few foreign physicians licensed to practice in Japan. Tel: (03)5458-6099 www.tokyobritishclinic.com