Whose position holds water?

January 21, 2008

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.


  1. Hello GregRoss Tucker here. Interesting article, well put together.If I may add one or two points:The first is that if you drink to thirst, which is what we are advocating, then you won't incur large drops in body weight or fluid. All the evidence, from thousands of athletes in field studies and in the lab research (even that funded by Gatorade), suggests that drinking to thirst will see body weight changes of 2 to 3% during a race. That is hardly substantial, and even the ACSM evidence fails to find any problem with 2%. Your explanation for the "imperfect" studies in regard is very good.Secondly, to suggest that an athlete will run faster if they simply replaced more fluid is an erroneous assumption – when the elite athlete finishes a marathon they have lost 3% of their body weight. To now suggest that simply by drinking more they might have run 1 minute faster is a bizarre notion. To suggest that Haile Gebrselassie could run 30 seconds faster had he just taken in more fluid is quite ridiculous. I recall that when Armstrong rode in the Tour one year, he lost about 3% of his weight, and one scientist suggested that he might have lost 5% as a result. When you then did the math, he would have beaten the second place rider by about 5 minutes in a time-trial had this been true! Clearly this never happens.So in this case, one might have to ask the relatively "unscientific" question of What do the elite athletes do? And it's quite clear that they don't drink to replace fluid loss. Now, if it costs any time, then it costs these guys huge money and prestige – they'd have figured it out by now.So the bottom line is that we are saying "Drink to thirst". We're not saying lose 5% of your body weight. We are not saying don't drink anything, you don't need it. That is how the argument has been portrayed by Gatorade and the scientists whose funding they provide.Of course you need fluid! But you only need to drink to thirst, and you'll be absolutely fine.

  2. Ross:Thanks for your comment.I assume that, when you refer to "all the evidence" showing body weight changes of 2-3%, you are thinking of studies such as that of Noakes et al., PNAS 102: 18550-5, 2005 (www.pnas.org/cgi/content/full/102/51/18550). This study, as you are probably aware, compiled body-weight data (and many other data) from long-distance running, cycling, and triathlon events. Presumably most of these athletes were drinking to thirst; it's hard to imagine that they'd drink LESS than desired, given the availability of fluids at most modern-day events. Yet approximately half of these athletes lost more than 3% of their body weight. (The spread of body weight changes is shown very nicely in their Figure 1.) In other words, drinking to thirst does NOT limit weight loss to 2-3% in a substantial fraction of athletes if the exercise lasts long enough. In light of this, we should be concerned about whether larger weight losses are detrimental.I agree with you that the Lance Armstrong calculation is ridiculous. At the same time, I don't think we can assume that elite athletes always behave optimally, especially in sports like ultramarathoning where there is much less money at stake.

  3. Hi Greg,Nice article; many runners must find this sort of summary very useful. Certainly, from my perspective as a non-running scientist, I largely agree with your assessment of the research you reviewed.What you don't talk much about are the sorts of experiments that (to my mind) would directly address the question "How much should you drink during a race?" I have in mind randomly assigning runners in a race to different in-race fluid replacement regimes (probably including "drink to thirst", plus various set rates of in-race fluid consumption that bracket the amounts most runners consume when "drinking to thirst). It would of course be of interest to do the experiment on runners of various ability levels, so as to test whether the optimal in-race fluid replacement regime depends on ability. This sort of experiment essentially ignores the underlying physiological mechanisms affecting performance in order to focus on the phenomenological effect of in-race fluid replacement regime on performance under actual race conditions. I take it this is the sort of experiment you mean when you refer to experiments run with "race-like protocols"?This sort of black box experiment ("we just want to discover what works, we don't care about why") was the basis for most progress in most areas of medicine, at least until fairly recently. I can see why it would be much less easy to do this sort of experiment than the "put the athletes in a sauna and then run them on a treadmill"-type of experiment you focused on. But it seems to me that the information gained would be worth it (at least to the runners who want such information). Can you elaborate on whether this sort of experiment has been done, and if so, what the conclusions have been?

  4. Jem:In referring to experiments with "race-like protocols," I was thinking of those summarized by Noakes in Table 2 of his May 2007 review article in the Journal of Sports Sciences (volume 25, pages 781-96). These studies sort of follow the methods you suggest (although they were conducted in laboratory settings rather than during actual races, which probably maximized subject compliance).A major limitation of those studies, in my opinion, is that the differences in final body weight between the "some fluids"/"ad libitum" groups and the "full fluid replacement" groups were small — typically the first group lost 1-2% and the second group lost 0.5% or less. There were no major performance differences between these groups, but that's unsurprising given the tiny amounts of weight being lost.The experiment that I'd like to see but has not (to my knowledge) been published would employ a similar design but would extend the exercise period long enough to see larger weight losses (say 5-7%) in athletes drinking "ad libitum." On a separate day, these same athletes would be encouraged to drink more aggressively so as to keep body weight within 2-3% of its initial value. (The order of the "ad libitum" and "aggressive drinking" trials would be randomized, of course.) Only then, I think, could truly useful performance comparisons be made between the "ad libitum" rehydration strategy and a more aggressive rehydration strategy.

  5. You guys are getting too technical! Sometimes not thinking about all this stuff works better. Speedgoatkarl

  6. Greg: Just a short note about those Lab experiments. I believe one of the hotly debated issues is often the effect of ambient air flow on cooling, or not cooling, subjects who aren't moving, because they're sitting on a lab bike or running on a treadmill. Many labs I've been in use portable fans of some kind to move air around subjects and get a little closer to "the real world." But I was amazed when I visited Noakes's lab in Cape Town. One entire wall–like maybe 10' by 15'–is a fan, so air is moving over every inch of subjects' bodies. This certainly seems closer to the real world than other Lab arrangements I've seen. Although maybe these kinds of fan setups are becoming more common in serious ex phys laboratories.

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