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Drinking too much during marathons (hyponatraemia): an update

September 9th, 2009

Just noticed a University of London preprint that has been accepted for future publication in the British Journal of Sports Medicine that looks at incidence of hyponatraemia in the 2006 London marathon. This topic has received a fair amount of attention in the past few years (justifiably, since at least five people have died recently in the U.S. and Britain, according to the paper), but there are a couple of new wrinkles in this paper.

First of all, this wasn’t your typical hot marathon where people are pouring fluids down their throat with abandon — the 2006 London race was held in “wet, rainy conditions with air temperature 9-12 [degrees] C.” Still, 11 of the 88 runners studied developed “asymptomatic hyponatraemia,” as diagnosed by low sodium levels. They didn’t have any negative effects — or any symptoms at all, actually — but they were on the border, supporting the contention (the authors claim) that hyponatraemia is underdiagnosed.

As expected, the hyponatraemia sufferers drank more (every mile, most commonly, compared to every second mile for the non-sufferers), and they put on weight during the marathon on average, while everyone else lost weight. But there were some anomalies: four of the hyponatraemics actually lost weight, but still somehow ended up overhydrated. It’s not clear how this happened, though the researchers speculate about “inappropriate antidiuretic hormone (ADH) release during exercise causing altered renal function and secondary fluid retention.”

So what do we take from this? Well, it’s hard to get too worried about an asymptomatic condition that doesn’t cause any problems (though of course if they persist into the symptomatic regime, they risk serious problems). On the other hand, these results tell us that quite a few people are still chugging water well beyond their needs. So maybe it’s worth bearing in mind the words of Tim Noakes, the respected South African sports scientist who has been stirring up dissent about our current obsession with proper hydration: “If you are thirsty, drink; if not, do not,” he wrote in 2007. “All the rest is detail.”

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