Runners who shed 3% or more of their body weight during a marathon will finish quicker, reveals research published online in the British Journal of Sports Medicine.

There is considerable debate about the issue, say the authors, but sports bodies continue to caution that a weight loss in excess of 2% impairs athletic performance.

The authors included 643 contestants who completed the 2009 Mont Saint Michel Marathon in France - a distance of 42 kilometres (just over 26 miles).

The runners, of whom 560 were men, were weighed right before the start, and immediately after the end of the race, to assess weight loss, and see if this had any bearing on finishing times.

The degree of weight loss among the runners was what would be expected, ranging from a loss of 8% of body weight to a gain of 5%.

This occurred even though all runners were given exactly the same advice on how much to drink to avoid dehydration throughout the race. This was to drink either 250 ml of water or an energy drink every 20 minutes.

Weather conditions were not particularly arduous. The temperature ranged from 9 to 16 degrees C, with moderate humidity, and a strong wind.

The fastest runners were those who lost the most weight.

Those who completed in four hours or more lost an average of under 2% of their body weight; those who took between three to four hours lost an average of 2.5%.

But the competitors who completed the course in under three hours lost 3% or more of their total body weight.

Neither age nor gender had any impact on weight loss during the race, and there was no evidence that higher levels of weight loss impaired these runners’ athletic performance.

If anything, the results suggest that those who gained the most weight by drinking the most (9.5% of competitors), performed worse.

The body does not signal the intake of more water than it requires, and so overdrinking is likely to be the result of behavioural conditioning, and prompted in no small part by “messaging” from the sports drinks industry, say the authors.

“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,” they say.

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

Source: British Journal of Sports Medicine