Friday 15 November 2019

The dangerous myth of dehydration

Marathon runners and other fitness fanatics fear dehydration but the facts show that drinking too much during exercise is actually more dangerous than drinking too little

Overhydration is now a problem.
Overhydration is now a problem.

George F Winter

When the SSE Airtricity Dublin Marathon gets underway, some participants will be short of training, many will be short of breath, but none will be short of access to fluids. Indeed, the Dublin streets will be awash with so much water and sports drinks as to resemble the Baths of Caracalla… but with trainers instead of togas.

Along with the rise of big-city marathons that began a generation ago, a fear of dehydration has developed among those who participate in sports; a fear that is fuelled by a sports drinks industry that pours down our throats the message that it's good to drink and bad to be dehydrated.

This relentless promoting and advertising can have unintended fatal consequences. Exercise-associated hyponatraemia (EAH) is a condition caused by the voluntary over-drinking of any fluids, including sports drinks. EAH causes water retention throughout the body, and the organ most affected is the brain since the skull limits its capacity to swell.

In EAH with encephalopathy (EAHE), if the brain swells excessively, the pressure rises, first causing a loss of consciousness, often associated with seizures, leading ultimately to death.

To date, there have been 16 documented deaths from EAH/EAHE worldwide in sportspeople. By contrast, no competing athlete has ever died from dehydration.

Former Northern Ireland marathon champion John McLaughlin recalls: "I ran several AAA marathons in the 1970s and back then, the rule was that the first feeding station was at six miles and the rest at five mile intervals thereafter.

"You were liable to be disqualified if you drank before six miles. I was more interested in sponging down with water in marathons than drinking.'

So what happened between the races of the 1970s, when some drinking was tolerated, and today's events where there is water - and sports drinks - everywhere, and all of it to drink?

Professor Tim Noakes (now retired) of the Sports Science Institute of South Africa in Cape Town reported the first cases of EAH and EAHE in 1985, and by 1991, he and his colleagues had found the cause: voluntary over-drinking.

In September 2009, in a speech to the Royal College of Surgeons of Edinburgh, he noted that the relatively austere approach to drinking, such as that described by John McLaughlin, lasted until the emergence of the world's first sports drink, Gatorade, whose sales began to flourish in the late 1970s.

Its commercial success, said Noakes, depended on three physiological dogmas "that are the opposite of our biological design forged in our evolutionary past".

These dogmas are that we are poorly adapted for exercising in the heat; that we must not develop any weight loss during exercise; and that we need to ingest additional salt to survive as athletes.

Noakes was unequivocal: "There is absolutely no scientific support for any of these ideas. In fact, they are the opposite of the truth."

For example, Emil Zatopek won the 1952 Olympic marathon and drank nothing; nor did Abebe Bikila in winning the 1960 and 1964 Olympic marathons; nor Mike Gratton when he won the 1983 London Marathon.

When Ron Hill won the 1970 Edinburgh Commonwealth Games marathon in 2:09:28, he drank nothing and lost 3.9pc of his body weight; when Jim Alder won the 1966 Jamaica Commonwealth Games marathon in 2:22:08, he lost 5.6pc of his body weight; and when Haile Gebrsellassie won the 2009 Dubai Marathon in 2:06:08, he lost 9.8pc of his body weight.

To further ram home the point that the fastest finishers in marathons are often the most dehydrated, in 2010 the British Journal of Sports Medicine published a study of 643 marathon runners who competed in the 2009 Mont Saint-Michel Marathon in France, reporting that body weight loss was greater than 3pc in runners completing the race in less than three hours. They stated that their results were "…not compatible with laboratory-derived data suggesting that body weight loss greater than 2pc during exercise impairs athletic performance".

Prof Noakes's book Waterlogged: The Serious Problem of Overhydration in Endurance Sports (2012) is an elegantly written 420-page demolition job of many flawed scientific studies that the sports drink industry cleaves to, and he presents overwhelming proof to support his contention that "[t]here is no published scientific evidence to support the hypothesis that heatstroke is caused by dehydration and that it can be prevented by drinking as much as tolerable during exercise".


Someone who knows about running in hot conditions is ultrarunner Dr Andrew Murray, sports medicine consultant at the University of Edinburgh and a member of the International Marathon Medical Director's Association. Earlier this year - in an epic nine-day adventure, together with running partner Donnie Campbell - he became the first to run more than 500km across the Namib Desert, home to the world's highest sand dunes.

Dr Murray said: "It used to be the perceived wisdom that being dehydrated led to poor performance, but recent evidence has not shown a decrease in performance, but has shown that drinking too much can actually be a problem.

"My advice is to ensure that you are reasonably hydrated before a race, without going overboard. During the race, drink to thirst. If you are thirsty, drink, if you are not, then, all other things being equal, you don't need to.

"Personally, I will often finish a race 4-5pc dehydrated, as it is a balance between drinking to feel comfortable, and avoiding the stomach discomfort I get if I try and replace all the fluid I have lost."

Around the same time as Dr Murray was running across the Namib Desert in February 2015, the Third International Exercise-Associated Hyponatraemia Consensus Development Conference convened in California.

The main message of the conference was that as far as EAH is concerned, "…prevention is dependent on drinking less.

Thirst should provide adequate stimulus for preventing excess dehydration and markedly reduce the risk of developing EAH in all sports."

Enjoy the marathon, but drink according to the dictates of your thirst, not to the dictates of the sports drink industry.

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