When to drink and what to drink? These concerns preoccupy the thoughts of many a distance runner and it can be baffling to decipher the mixed messages of sports drink manufacturers (who insist we should consume more fluids) and sports scientists (who often encourage us to drink less). 
Confused? Don’t worry, we spell out the rules of fluid intake as determined by the leading experts:

MILD HYDRATION CAN BOOST PERFORMANCE

A report by New Zealand sport scientists suggested that a 3% drop in body weight through fluid loss doesn’t slow down athletes, while a 2012 paper in the Clinical 
Journal of Sports Medicine found that elite runners in the heat of the Dubai Marathon recorded fluid losses of almost 10%, yet still their finishing times were unaffected. 

How is this the case? John Brewer, professor of applied sport science at St Mary’s University, Twickenham, says that “tactical dehydration” is a growing phenomenon among endurance runners, triathletes and cyclists, at club and elite level. 

“Each litre of fluid lost equates to approximately 1kg of body weight,” Brewer explains. “Carrying around less weight has a positive impact on endurance performance, so people train their bodies to cope with dehydration.”

HOW MUCH FLUID DO YOU NEED?

Fluid needs are highly individual. In general, for every pound you lose, you need to drink approximately needs are highly individual two 10 fl oz glasses (half a litre) of fluids to replace the shortfall. 

Checking the quantity and quality of your urine can be used as an adjunct – dark and scanty urine suggests high concentrations of metabolic waste and that you need to increase your fluid intake. 

Remember that your urine will be darker if you take supplements (particularly vitamin C), so volume is a better indicator.

DON’T PRE-HYDRATE TO EXCESS

It’s the risk of over-hydration – and potential hyponatremia – that is greater than the risk of dehydration for most. Dr Mitchell Rosner, a kidney specialist from the University of Virginia school of medicine, recently chaired a group of 16 independent experts from four countries to look into the issue. 

“We’ve documented at least 14 deaths (from Exercise-Associated Hyponatremia) since 1981,” he says. “The common feature in all cases is excessive water consumption during athletic events. This is driven by misbeliefs that over-hydration can improve performance and even prevent dehydration.”

DRINK WHEN YOU FEEL THIRSTY

When it comes to when to drink during endurance events, the guidelines have changed. 

In an updated consensus statement recently published in the Clinical Journal of Sport Medicine, Dr Rosner and his panel stress a reliance on listening to your body. 

“We recommend using your thirst as a guide,” he says. “If you drink when thirsty, you will not become hyponatremic and you will not suffer from significant dehydration.” 

LISTEN TO YOUR BODY

Ron Hill drank nothing when he won the 1970 Commonwealth Games marathon with a time of 2:09:28. Neither did Mike Gratton when he won the 1983 London Marathon, and so it has continued. 

Kenyan athletes adopt what researchers described in the Medicine & Science in Sports & Exercise journal as an “ad libitum fluid intake”, drinking what and when they feel like during the day. 

Likewise, a 2011 study published in the Journal of the International Society of Sports Nutrition found that, among the top Ethiopian runners, no fluids were drunk before or during training, with only modest amounts imbibed afterwards. 

The message is clear. If your body needs more fluid, take it. If it performs well with less than the general recommendation, then adjust accordingly.