Hammer Nutrition

For instance, by the time you feel thirsty, you could have a 2% body-weight water loss, already into the impairment zone.

Authors from Science of Sport blog, both PHDs

Since then a number of studies have been performed at races and other endurance events, and the one main finding of all of these studies is that athletes replace only between 40-60% of their weight losses, and complete the race 2-5% "dehydrated."

Despite this fact, from real athletes competing in real events, many scientific articles and lay magazines continue to emphasize that "dehydration" of this magnitude (2-5% of the pre-race weight) is detrimental to health and or performance. This is the basis for the many advertisements proclaiming the importance of drinking to runners, as we discussed in Post I of this series.

The evidence from all the field studies, however, shows rather that changes in body weight of this magnitude are not associated with collapse and high core temperatures. One reason for this is likely because, as we stated in a comment to another post, the body weight is not the regulated variable, and so even if you lose some weight the body is fine, and is in fact responding normally to that exercise. We will examine that concept in the next post in this series.

http://www.sportsscientists.com/2007...ercise_21.html