A study looking at athletes' body weight after a marathon suggests some runners are drinking too much during races and impairing their performance.
Research in the British Journal of Sports Medicine showed those who took the longest to complete the race lost the least weight.
The study suggested these competitors drank more fluids, which increased their weight and slowed them down.
Runners who lost more than 3% of their weight during the run finished fastest.
The amount of weight lost among the runners ranged from a weight loss of 8% to a gain of 5%.
This occurred even though all runners were given the same advice on how much to drink to avoid dehydration throughout the race.
The study compared the weights of 643 contestants who completed the 2009 Mont Saint Michel Marathon in France.
The runners were weighed right before the start and immediately after the end of the race to assess weight loss.
The fastest runners were those who lost the most weight.
Those who finished the race in four or more hours lost an average of 2% of their body weight.
Those who took three to four hours lost an average of 2.5% of their body weight.
The fastest runners, who finished the race in under three hours, lost 3% or more of their body weight.
Age or gender had no impact on weight loss during the race.
The researchers said that those who gained weight during the race, performed the worst.
The researchers split the runners into four groups according to weight loss.
Around one in ten (9.5%) of the runners actually put on weight during the race.
Those were the athletes who finished the race in the slowest time.
The researchers said that one of the factors contributing to weight gain was the tendency of some athletes to drink too much during the race.
Professor Timothy Noakes, one of the lead researchers, said overdrinking was likely to be the result of behavioural conditioning and prompted by advertising:
"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."
He stressed that the body does not signal the intake of more water than it requires.