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As of September 2017, new Sweat Science columns are being published at www.outsideonline.com/sweatscience. Check out my bestselling new book on the science of endurance, ENDURE: Mind, Body, and the Curiously Elastic Limits of Human Performance, published in February 2018 with a foreword by Malcolm Gladwell.

- Alex Hutchinson (@sweatscience)

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It’s official: drinking to perfectly maintain fluid levels makes you slower, not faster. At least that’s the message in a new British Journal of Sports Medicine meta-analysis, from Eric Goulet at the University of Sherbrooke. What’s new here is the recognition that there’s a fundamental difference between treadmill or bike tests where you continue at a set pace until exhaustion, and “real world” time trials or races where you cover a given distance as fast as possible. Many of the classic studies underlying the orthodox view (that losing more than about 2% of your body mass hurts endurance performance) are time-to-exhaustion tests:

These tests with no known end points are unrepresentative of out-of-door exercise conditions faced by field athletes and therefore possess a very low ecological validity [Goulet writes], which therefore question their value and relevance in the design of fluid intake guidelines aimed at maximising endurance athlete performances.

To address this point, Goulet reviewed five studies that looked at dehydration in self-paced cycling time trials — and his conclusions were starkly different from the conventional wisdom. On average, subjects in these studies with an average dehydration of 2.2% of body mass actually had a non-significant improvement in performance of 0.06% compared to subjects who drank enough to roughly maintain body mass (losing an average of 0.44% body mass).

Drinking according to the dictate of thirst was associated with an increase in [time trial] performance compared with a rate of drinking below (+5.2±4.6%, p=0.01) or above (+2.4±5.0%, p=0.40) thirst.

Is this shocking? Well, it agrees with what Tim Noakes has been arguing for several years. It also agrees with real-world data like this study from a marathon in France, which found that sub-3:00 finishers lost 3.1% of their body weight, 3:00-4:00 finishers lost 2.5%, and slower-than-4:00 finishers lost only 1.8%. Will it change anyone’s mind? Goulet is hopeful: he says the findings should “contradict and abolish the old and much-believed dogma stating that ‘during prolonged exercise it is of capital importance to drink ahead of thirst, otherwise it is already too late.'” I’d like to think that’s true, but I suspect it will take quite a few new studies before opinions start to shift.