Whose position holds water?
An interesting, long-running debate about dehydration has gotten a lot of attention in recent months.
Certain facts are not in dispute. Everyone agrees that exercising for long periods of time without adequate access to fluids is bad for athletic performance and bad for one's health. Conversely, overly aggressive rehydration with dilute drinks can reduce the concentration of sodium in one's blood to dangerously low levels, a condition called hyponatremia.
What's less obvious is whether it is desirable to maintain one's body weight during prolonged exercise in order to ensure optimal performance.
The mainstream viewpoint, as exemplified by the Position Stand of the American College of Sports Medicine, is that athletes should replace enough of their fluid and electrolyte losses to stay within about 2% of their pre-competition body weight. However, Lore of Running author Tim Noakes and Science of Sport bloggers Jonathan Dugas and Ross Tucker believe that athletes can incur more dramatic weight losses without negative consequences.
Much of the relevant experimental evidence is reviewed in a pair of point/counterpoint articles, collectively titled "Does dehydration impair exercise performance?", from the August 2007 issue of the journal Medicine & Science in Sports & Exercise. Michael Sawka presents a "Prevailing View" largely consistent with the ACSM guidelines, while Noakes offers a "Challenging View."
Sawka reviews the usual studies cited by those who share his opinion. In brief, many different investigators using different methods in different labs have collected evidence that dehydration beyond a 2% reduction in body weight impairs aerobic performance because of such factors as an increased body temperature, an increased cardiovascular strain, and a reduced blood flow to the muscles. This evidence has led races such as the Western States 100 to monitor participants' body weights and to force those who've lost more than 5% to regain some weight before continuing.
Noakes and his sympathizers have two main objections to Sawka and his ilk. First, they say, most studies cited by these mainstream scientists have induced dehydration by artificial means such as sauna exposure, ingestion of diuretics, and/or severe restriction of access to fluids, which bear little resemblance to the conditions faced by today's exercising athletes. The studies that have employed more realistic race-like protocols haven't been as supportive of the "maintain your body weight" viewpoint. Second, several additional studies show that the athletes who finish first in long races are often among the most dehydrated ones, with weight losses of up to 5-10%.
Regarding objection #1, it's true that the perfect study has yet to be done. What's known so far is that, on the one hand, if you force people to drink less than they want, their endurance suffers, and, on the other, if you force them to drink more than they want so as to avoid even a 1-2% body weight deficit, their endurance does not improve. But what we really want to know is what they should do in extremely long and/or extremely hot races when ad libitum drinking might not prevent weight losses of 5% or more. Would more aggressive drinking be helpful in that case? We don't have a definitive answer just yet.
The studies underlying objection #2 suggest that some athletes can tolerate a 5-10% drop in body weight without much trouble. Although I find these studies intriguing, they too are imperfect. Even if you can beat your opponents while dehydrated, who's to say that you couldn't post an even better time if you defended your body weight more carefully?
Personally, I'm inclined to avoid large dips in body weight during prolonged exercise. Why put your body through all that when you can keep it close to its usual state by simply ingesting extra fluids and electrolytes? At the same time, I'll admit that the current case for maintaining body weight isn't quite the "slam dunk" that some researchers make it out to be. Along those lines, a final point often noted by Noakes, Dugas, and Tucker is that the now-common recommendation to drink aggressively has sprung out of research funded in part by Gatorade, the sports drink manufacturer. Has Gatorade's money led some researchers to take a "glass-is-half-full" view of the benefits of complete fluid replacement? It's possible.
