Water Stations in the Marathon-how much is too much fluid?

Marathon season: London this weekend, Edinburgh next month. 
Many more take place in Europe & North America over the next few months as the temperatures increase.
So it makes sense to be hydrated? But how much is too much of that good thing-water?

Despite years of advice to the contrary, it appears that thirst may, after all, be the key indicator as to when runners should take fluids on board: including the International Marathon Medical Directors Association (IMMDA).


Ross Tucker, exercise physiologist & consultant scientist to the Sport Science Institute of South Africa states that the thirst mechanism is controlled primarily by the sodium level in the blood. With almost half of runners drinking to a preset schedule or consuming as much as possible according to Loyola Chicago University research (Winger, Dugas & Dugas Br J Sports Med. 2011 Jun;45(8):646-9. Epub 2010 Sep 28), it would appear that this advice to listen to the body is falling partly on deaf or misguided ears.


But does this potential excess fluid lead to impaired performance, or real danger?


in 2002, the Boston marathon was exposed as having 13% of its runners may have suffered from hyponatraemia, a condition where over-hydration dilutes the sodium levels, and in serious cases this leads to brain swelling, seizures and life-threatening complications-or worse as four female runners have died of this condition after participating in marathons. This was not hearsay-it was published data in the New England Journal of Medicine of April 2003!

However the cases of runners overdoing the water on marathons had actually been documented firstly in 1985 by Dr Tim Noakes, whom published  "Water Intoxication: A Possible Complication During Endurance Exercise Med Sci Sports Exerc. 1985 Jun;17(3):370-5 ." in which he and his co-authors  Goodwin, Rayner, Branken & Taylor wrote that the condition appears to be caused "by voluntary hyperhydration." 


In 2001, Dr. Noakes was invited by the International Marathon Medical Directors Association to write an "Advisory Statement on Fluid Replacement During Marathon Running" . In the paper, he recommended that marathoners drink 400 to 800 milliliters/hour (13.5 to 27 fluid ounces). 


Dr. Noakes's advice touched off a firestorm, as it was basically 50 percent lower than the widely quoted recommendations of the American College of Sports Medicine, the National Association of Athletic Trainers, and other sports medicine groups, which have generally advised endurance athletes to drink 600 to 1200 ml/hour (20 to 40 ounces).


Chris Almond MD was the main author of the NJEM article in 2003, and his conclusions noted that out of 488 runners, 63 werre clinically hyponatramic after the race. 
His team also identified three triggers:  

• 1) weight gain during the marathon from excessive fluid consumption
• 2) a finishing time slower than four hours
• 3) very small or very large body size

Of these, the first was the most important. "The strongest single predictor of hyponatremia was considerable weight gain during the race," the study concluded. 


Another significant finding: A sports drink doesn't protect you from hyponatremia.

 "In our subjects, we didn't find that consuming sports drinks was any different than consuming water," Dr. Almond said. "Sports drinks are mostly water themselves, and contain only small amounts of sodium." This is backed up still further by Tucker. He states that 


"The sodium content of a sports drink helps attenuate the drop in blood sodium levels, but not by much. People who drink too much could probably get away with drinking 1.4 litre/hour of sports drink, but only 1.2 litre/hour of water. But it's a moot point - the root cause is still overdrinking, and that's what should be avoided."


The American College of Sports' Medicine Guidelines were updated in 2007, and they do not recommend any specific volumes of fluid, and certainly not taking on as much fluid as is possible. They do still adhere to the oft-quoted definition of 'excessive dehydration' as being greater than 2% body weight loss from water deficit-despite the evdence not always showing this as the threshold for problems.


Yet some athletes appear to tolerate these levels of dehydration better than other, and often the athletes at the front in marathon races will lose 3-4 kilograms according to Ross Tucker. 


He suggests that the athletes who"lose the most fluid are often the winners – which makes sense because they're running the fastest, and hence have the highest sweat rate. And when you are running along at 3 min/km, you don't have the time to drink all that much either- you'll see they sip water or sports drink every few kilometres, but that's it. There's none of this 'one litre per hour' doctrine that others are following."


Some form of predictive monitoring may be of some benefit, such as weighing-in prior to runs in advance to establish some form of predicted losses due to fluid. This form of 'customised sweat-test' is however, oversimplified, and also too dependent upon environmental onditions which may differ completely from race conditions.


NOW  when we live in an age when runners keep right on to the end of the road for their charity or friends & family, they do so increasing the risks the longer they take-if they keep taking the fluids.