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From "Hydrate of Die? Hardly":
In an odd turn of events, the same people who don't drink enough on a daily basis will over-hydrate during the simplest athletic effort. The following article concerns drinking during prolonged physical effort, examining how we have come to a place where recreational athletes drink too much due to the fear-driven marketing of sports hydration products.
This current thesis contradicts the recommendations I wrote in "Extreme Alpinism" (1999) when I too was under the influence of the so-called science of the time. I wish I had been more skeptical back then.
Hydrate Or Die? Hardly. Except in extreme cases, dehydration during most efforts will not kill you. However, over-hydration can have fatal consequences. It is imperative to understand how our bodies lose fluid, to replace it intelligently, with the objective being to maintain performance and to do so with as little risk as possible.