The Fast Lane

The idea that mild de­hy­dra­tion will slow you doesn’t hold wa­ter.

Runner's World South Africa - - Contents - BY ALEX HUTCHINSON

De­hy­dra­tion slows you down... or does it?

You’ve heard the warn­ing:

by the time you no­tice you’re thirsty, you’re al­ready (cue omi­nous mu­sic)

de­hy­drated – and that will slow you down. So it’s worth con­sid­er­ing the coun­ter­in­tu­itive re­sults of a 2009 ex­per­i­ment. French re­searchers weighed 643 run­ners be­fore and af­ter a marathon to es­ti­mate how much fluid they’d lost. The fastest run­ners, it turned out, were the most de­hy­drated: sub3:00 fin­ish­ers lost an av­er­age of 3.1 per cent of their start­ing weight, those be­tween 3:00 and 4:00 lost 2.5 per cent, and those slower than 4:00 lost just 1.8 per cent. This ef­fect is even more pro­nounced at the elite level: when Haile Ge­brse­lassie be­came the first sub-2:04 marathoner in 2008, he lost 10 per cent of his start­ing weight – far more than the 2 per cent loss that the Amer­i­can Col­lege of Sports Medicine says “de­grades aer­o­bic ex­er­cise.” So what ex­plains this ap­par­ent con­tra­dic­tion?

The key is to un­der­stand the dif­fer­ence be­tween de­hy­dra­tion, the phys­i­o­log­i­cal state of hav­ing lost fluid, and thirst, the de­sire to drink. For decades, re­searchers lumped the two to­gether, con­duct­ing stud­ies in which vol­un­teers were de­nied wa­ter for hours be­fore un­der­go­ing ex­er­cise tests. Those sub­jects were both thirsty and de­hy­drated – and their en­durance suf­fered even with 2 per cent de­hy­dra­tion.

But it’s also pos­si­ble to be de­hy­drated, at least tem­po­rar­ily, without feel­ing thirsty. Does this mat­ter? In a 2016 study, ath­letes com­pleted a 20-K trail run while ei­ther drink­ing an amount cho­sen to re­place their ex­pected sweat losses or sim­ply drink­ing when they felt like it – a plan that, in the lat­ter case, left them de­hy­drated by 2.6 per cent. The fin­ish­ing times in the two con­di­tions were es­sen­tially iden­ti­cal.

One the­ory is that the dis­con­nect be­tween de­hy­dra­tion and thirst isn’t an evo­lu­tion­ary bug – it’s a fea­ture. As you sweat out wa­ter, you also sweat out elec­trolytes like sodium, which keeps your blood con­cen­tra­tion rel­a­tively con­stant. That dis­con­nect, the the­ory goes, al­lowed our an­ces­tors to keep hunt­ing without con­stantly need­ing to stop for wa­ter.

This re­think has two prac­ti­cal con­se­quences: you can trust your sense of thirst dur­ing a run, but you have to re­pay that fluid debt af­ter you fin­ish – oth­er­wise, the next day, you’ll have noth­ing left to bor­row.

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