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A nice, simple little study was posted online at the British Journal of Sports Medicine a few weeks ago that adds some data to the big debate about hydration that I wrote about in November.
The details: 643 runners at the Mont Saint-Michel Marathon were weighed before and after the race. Sure enough, the fastest finishers lost the most weight, with those under 3:00 averaging 3.1% weight loss, compared to 2.5% for 3:00-4:00 and 1.8% for greater than 4:00. If you break it down the opposite way, those who gained weight averaged 3:58; those who lost 0-2% averaged 3:54; those who lost 2-4% average 3:47; and those who lost more than 4% average 3:40. Here’s the distribution of weight loss:
And here’s the graph of finish time versus weight change:
So what does the second graph tell us? “There was a significant linear relationship (p<0.0000001) between the degree of BW loss and race finishing time so that lesser degrees of BW loss were associated with longer finishing times. However, the predictive value of this relationship was small.” In other words, don’t head out and try to lose weight in your next marathon!
Seriously, as one just-published response has already noted, there are limits to what you can infer from a field study like this. It’s correlation, not causality, and I’m sure we can all think of many other reasons that back-of-the-packers are drinking more than the race leaders. So take it for what it is: a data point, and one that suggests that the old chestnut about greater-than-2% dehydration impairing performance needs to be rethought.