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Water and your health Updated: 30 Apr 2010

Exercising

When you're losing water rapidly, as you do when carrying out strenuous physical activity, you could already be dangerously dehydrated by the time your thirst mechanism kicks in.

A water loss of 1 percent of body weight can impair physical and sporting performance during exercise, while a loss of 3 to 5 percent can diminish performance by around 20 percent. Apart from making you feel light-headed and nauseous, dehydration also increases muscle glycogen use - so you get tired faster.

Experts say you should:

Hydrate yourself two hours before exercising by drinking 400 to 600mL. This allows time for adequate hydration and excretion of excess water.
Drink sufficient fluid during exercise to prevent dehydration from exceeding two percent of your body weight. Small amounts (150 to 300mL) every 15 to 20 minutes should do the trick.
Weigh yourself before and after exercise, and make up the weight loss in water.
For exercise that lasts less than 90 minutes, plain water is fine. For longer periods of exercise, a sports drink with added glucose and electrolytes may be beneficial.