Despite the huge number of people running long distances these days, there is no clear advice on how much water or other fluids you should drink during the event.
Does too little harm your performance?
Does too much slow you down or worse threaten to harm you or even kill you?
So, what is a right amount to drink during a long running event such as a marathon?
What strategy should you use to keep hydrated and maximize you performance?
There has been considerable variation in attitudes and expert advice and the thinking has changed over the last 15 years.
This article reviews the latest research and provides the answers to these questions.
In 1995-1999 the general advice was that runners should begin drinking early in the race, and continue to drink at regular intervals (at all the drinking stations; as much as you can).
The idea was to avoid dehydration at all cost by drinking large amounts frequently.
Runners were advised of the need to replace water lost through sweating, by drinking the maximal amount of water that they could.
The concept was to stay ahead of your thirst to avoid dehydration sneaking up on you.
By the time you started to feel thirsty you may have already started to get dehydrated.
Your thirst response lagged behind your actual need for water to avoid dehydration.
So the advice was to drink early and regularly as much as you could tolerate.
But recently, several marathoners have actually died as a result of complications from drinking too much fluid.
This has occurred because of a dangerous condition referred to as hyponatremia, or water intoxication.
The danger in this condition is due to low blood sodium levels triggered by consuming too much water, too quickly.
If you consume more water than you lose through sweating and breathing, and urinating, you can dilute your blood’s sodium levels.
The process of osmosis then transfers water from the blood into the cells of the body to maintain sodium levels.
The extra water causes the cells to swell, and if the brain cells are affected, it can be fatal.
Many experts have seen the warning signs and marathon runners are now being advised to drink less and to drink only when thirsty.
However recent research has shown that too many runners are not heeding this advice which has been poorly communicated to runners everywhere.
Research published in the journal Sports Health, showed that most of the 419 runners had no plan or strategy about fluid consumption during the race.
About two-thirds of the runners said that they were not worried about the possibility of developing hyponatremia.
A second survey, published in the British Journal of Sports Medicine, showed that 50% of runners appeared to be drinking too much fluid during the race.
A research study on the exercise-associated hyponatraemia in the London marathon was initiated after 14 runners were treated at hospital with symptoms of hyponatraemia several hours after the finish the 2003 London Marathon.
One young male runner died from severe hyponatraemia complications after finishing a London Marathon event.
The study of 88 volunteers showed that about 12% developed low sodium levels in their blood and symptoms of hyponatraemia.
The affected runners had consumed more fluid that the other runners and gained more weight than did non-hyponatraemic runners.