Hydration for Runners

Ruth McKean, ASICS Running Clinics Pro Team contributor and professional Dietician, gives us the lowdown on hydration for runners.

It is well established that sufficient dehydration before and during exercise can have an impact on performance as it increases both the physiological strain and perceived effort to train or compete.

The level of dehydration to negatively affect performance is individual, but for most people less than 2% loss of body weight can decrease aerobic (any running further than sprinting distance) performance, particularly in warm environments. Serious health problems can also be exaggerated by dehydration, such as heat illness.

However, it’s equally important to recognise that drinking too much can also be dangerous. Drinking too much fluid, especially fluid low in salt, can lead to a drop in the sodium in your blood. This is called ‘exercise associated hyponatremia’, which can, and has been fatal for some individuals in prolonged events.

In marathon runners this drop in sodium in the blood is more likely to occur in smaller and less lean individuals who run slowly, have excessive sweat loss and drink water heavily during and after the race. There are now guidelines which should be followed, particularly for those excising for 3 hours or longer. Women and older adults may be at greater risk from hyponatremia.

Individuals can sensibly monitor their hydration status by assessing urine colour, frequency and volume as well as body weight measures, which is encouraged.

There are now many commercial sports drinks available that aim primarily to replace or maintain hydration. These are typically called hypotonic drinks, compared to isotonic drinks which are aimed at recovery as a priority as they provide fuel rather than fluid replacement, meaning they can also contribute to fluid and sodium losses.