If you are in a marathon running race and lose at least 3% of your body weight during the competition, your chances of getting to the end faster are significantly higher, South African researchers reveal in the British Journal of Sports Medicine. Their findings clash with experts and sporting authorities who say that any weight loss over 2% during a competition undermines athletic performance.

The researchers gathered data on 643 competitors, 560 male and 83 female, who completed the Mont Saint Michel Marathon race in France in 2009. They ran 42 kilometers (26.0975 miles).

They were all weighed moments before the race began and straight after it ended. Their aim was to see what impact weight loss might have on their finishing times.

The degree of weight loss ranged from 8% to a gain of 5%, what the writers described as “what would be expected.”

The variations in weight loss occurred even though they all received the same advice – to drink either 250ml of water or an energy drink every 20 minutes during their race.

During the race, temperatures ranged from 9 to 16 degrees Celsius (48.2 to 60.8 Fahrenheit). The authors termed the weather conditions as “not particularly arduous”.

The researchers found that those with the best finishing times were also the individuals who had lost the most weight.

Runners who finished in four hours lost an average of under 2% of their body weight, compared to 2.5% among those who finished within three to four hours.

Those who finished their run within three hours lost at least 3% of their body weight.

Weight loss did not vary according to age or gender, the authors wrote. They added that they found no evidence that higher weight loss undermined the runners’ athletic performance in any way.

In fact, those who gained the most weight by drinking the most had the worst performances.

The authors believe the sports drinks industry has had a key role in conditioning athletes to consume more liquid than they require. The human body does not tell the brain to consume more water than it needs. Drinking too much water is probably the result of behavioral condition.

The authors wrote:

    “Such overdrinking most likely results from specific messaging directed, especially by the sports drink industry. This messaging has promoted the concept that any dehydration that occurs during exercise impairs exercise performance and increases the risk for potentially adverse outcomes.

    As a consequence, athletes may continue to believe that they must drink ‘as much as is tolerable’ during exercise.”

“Inverse relationship between percentage body weight change and finishing time in 643 forty-two-kilometre marathon runners”:
Hassane Zouhal, Carole Groussard, Guenolé Minter, Sophie Vincent, Armel Cretual, Arlette Gratas-Delamarche, Paul Delamarche, Timothy David Noakes
Br J Sports Med doi:10.1136/bjsm.2010.074641

Written by – Christian Nordqvist