Do you really need to drink enough to replace sweat losses?

My column in today’s Globe takes on a pretty controversial topic: how much water can you afford to lose as sweat before your performance suffers? I gave a little preview of one of the studies in a blog entry a few weeks ago, but this article is a much more detailed look at some recent research suggesting that the amount of weight you lose during exercise doesn’t necessarily correspond to the amount of water you lose:

Here’s a riddle posed recently by South African scientists: A group of soldiers undertook a gruelling 14.6-kilometre march during which they lost an average of 1.3 kilograms. But sophisticated measurements with isotope tracers showed the total amount in water in their bodies actually increased by 0.53 kilograms. Where did this “extra” water appear from?

Groups such as the American College of Sports Medicine have long advocated weighing yourself before and after exercise to determine how much fluid you lost. A litre of water weighs exactly one kilogram – so by this calculation, if you’re a kilogram lighter, that means you sweated out one litre more fluid than you replaced by drinking. Lose more than about 2 per cent of your starting weight, the ACSM warns, and your performance will suffer due to dehydration.

But the South African study, published in the British Journal of Sports Medicine by researchers at the University of Pretoria, adds fuel to a simmering debate about whether weight loss during exercise corresponds to water loss. They argue that some of the weight loss is from the energy stores you burn, and that your body has “hidden” stores of water that are released during exercise – which may mean we need to rethink how we approach hydration. [READ THE REST OF THE ARTICLE…]

The print version of the article is accompanied by a fantastic graphic by Trish McAlaster that breaks down the various ways your body gains and loses water during a marathon. So far it’s not available online (I’m hoping it will be posted later), but it you have a copy of the paper around, check it out.