Another win for South Africa’s Ryan Sandes!

Losing Fluids on the Run


I usually struggle with cramps in ultra distance events. Research has shown that hydration and mineral salt levels play an important role in cramp prevention. I recently weighed myself before a controlled workout on a treadmill and lost 1kg after a moderate 30 minute workout. This really scares me because it means I lost a litre of fluid in half an hour. This equates to two litres an hour. I guess this won’t be the case for every hour of an endurance event, but I dare not drink too much fluid as this will dilute my mineral salt levels. What advice do you have for individuals like me who lose a lot of fluid on the run? – ROY McGREGOR

This is one of those questions that requires more information in order to provide an answer, but it also highlights some misconceptions that should be corrected. It is not correct that ’inadequate‘ hydration (as may be experienced by a marathon runner) plays a role in the development of muscle cramps. Although both fluid loss and lowered mineral salt levels are popularly believed to be the cause of cramping in races, there is a large body of research evidence that shows that these factors are not the cause.

Unfortunately, while we know what does not the cause cramps, exercise physiologists are yet to establish exactly what the cause actually is. What we do know is that some runners are more prone to cramping than others, no matter how hard they train; we also know that there is a much higher incidence of cramping amongst those runners who have not trained adequately for the distance that they are racing, or if runners are racing at a higher intensity than they should, based on the training done.

One litre of fluid lost in 30 minutes sounds quite excessive, but fluid loss measured after treadmill running does not represent fluid loss outdoors. Specifically, because of the lack of the cooling effect caused by the evaporation of sweat from the skin surface when running indoors, the sweat rate is much higher for a runner training indoors than when running at the same speed outdoors. I therefore suggest that the experiment is repeated with a run outside to establish more realistic fluid losses, so that a more appropriate recommendation for fluid and carbohydrate replacement can be given.