Fluid intake, dehydration, and exercise: Part III

Welcome back for Part III in this series on fluid intake and dehydration during exercise! Thus far we have examined a brief history of fluid replacement during endurance exercise in Post I, and in Post II we tried to explain how some of the lab research has perhaps been over-interpreted, and how that has lead to a false belief that ingesting fluid during exercise will keep you cool. In that post we reported the findings of earlier researchers who concluded the following:

• The core temperature is maintained at a higher level during exercise
• It is the metabolic rate (or in other words, how hard you are exercising) that predicts the core temperature

Now, in Part III, we will compare the research that has been done in the field versus that from the lab, and show you the evidence of what really happens when people exercise in a variety of conditions.

The field study - Is it really science?

Many scientists will downplay the importance of field studies as they are largely uncontrolled studies from which we cannot assign causal relationships. So in other words, from a field study alone, we cannot say that dehydration causes one's temperature to rise, or that ingesting fluid will keep one cool. However, field studies do play a very important role as they represent what is actually happening when people exercise in real conditions. Also, they represent a portion of the scientific evidence that we accumulate and therefore these studies contribute to the available data from which we form models to explain physiology.

The very first field studies in Exercise Physiology were published by E.F. Adolph in the 1940's. Adolph and his team performed numerous experiments on American soldiers marching in the desert, and wrote a book on all of this work: "The Physiology of Man in the Desert." We won't try to explain all of his work here, but two of the take home messages from Adolph are the following:

1. Ad libitum access to fluid is sufficient to enhance performance (as measured by hiking in the desert)
   
2. Fluid restriction will affect performance---11 soldiers in a fluid restricted group could not finish the hike, whereas only one in the fluid ingestion group could not finish

It is normal to lose fluid and decrease body weight during exercise

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.

So what does happen during exercise? Data from running and cycling studies

Two studies from Jonathan's doctoral work measured the rectal temperatures of runners during a 56 km road race and cyclists during a 109 km race. The main finding of these two studies was that the rectal temperature rises for approximately one hour and then levels off, after which time it remains within a very narrow range (less than 0.5 C). The data from the runners are below:

And the data from the cyclists are here:
What both these graphs show is that the core temperature is maintained within a very narrow range during the event. Secondly, most of the changes in the core temperature occur at the beginning of exercise, and not at the end. Thirdly, although the two groups were different since the runners were going simply for race completion and the cyclists were highly-trained and racing, the temperature responses are similar. In addition, the environmental conditions were quite different between the two races: the marathon was cool and wet, and the cycle race was warm and dry. Yet, again the temperature response was similar.

The body temperature response - higher is normal

Another important aspect of all of the field studies that have measured the post-race rectal temperature is that 39-41 C is quite a regular (and normal) finding. In 13 different field studies we have reviewed, the range of post-race core temperatures was 37-41.7 C. The one study that included untrained or lesser trained runners was the one that reported the lowest temperature---37 C in a group of 63 marathon finishers. Nearly all the other studies measured highly-trained runners finishing marathons in 2:30 - 2:45.

Their finding is that in a variety of environmental conditions, it is the metabolic rate that determines the core temperature, and not the body weight losses and fluid replaement! Therefore it is those athletes who exercise at higher intensities that have higher core temperatures. Furthermore, although these athletes reach "high" temperatures, they do not exhibit signs or symptoms of "heat illness," and recover quickly. So in fact we would say that a post-race core between 39-41 C is quite normal and well within the limits of the body's coping mechanisms.

The lab vs. the field - assigning importance

We've described that when we exercise out doors, we lose weight without any apparent impairment of performance or risk of hyperthermia. Yet the lab studies, conflict of interests aside, show that when you ingest more fluid you stay cooler, which is a complete contradiction of what is actually observed during your exercise. So which is right?

One very important difference between lab and field studies, in addition to providing insufficient air velocities, is that the subjects in the lab studies are not allowed to pace themselves, and therefore they cannot change their metabolic rate (their running or cycling speed) as they wish. Note that people will slow down based on how they are feeling, and one such drive for slowing down is the feeling of getting too hot. But in the lab, this cannot happen and so an artificial situation is created where the subject simply has to keep going without slowing down, and a vital part of the regulation of physiology is removed.

In addition, it does not mean that there is no effect of fluid ingestion on one's ability to regulate core temperature. When the exercise intensity is fixed, there is most certainly an effect. However, as we have emphasized in Post II, that effect is 1) very small, and 2) is likely amplified by the lack of air velocity in those studies, meaning that it is probabbly even smaller than it has been measured as.

However what this aspect does represent is a limitation to how these studies can be applied. In other words, we cannot take those findings and apply them to the normal exercising population because running at exactly the same running speed for 2-3 hours is simply not how people complete marathons. Instead, they alter their running speed---that is, they slow down--- as they become fatigued or as their brain senses that they might get too hot. And this has a profound effect on their core temperature. Namely, it ceases to rise or even falls if they slow down enough.

So the importance of the lab studies is that they add to our knowledge by controlling for specific variables and measuring how one thing affects another. Without these studies we can never know the exact relationship between different physiological systems and characteristics. But at the same time, we must be extremely careful how we apply the data from these studies, because it does not represent what people are actually doing. Therefore we must rely on both field studies and lab studies to draw our final conclusions. Both types of experiment play an important, but different, role in helping us understand how the body regulates its temperature during endurance exercise.

In the end the message is that yes, fluid ingestion can indeed affect your ability to regulate core temperature. However, this effect is overstated and very small, and your body will always protect you by making you slow down before you suffer and major physiological consequences from this. The evidence to support that is not apparent in this post, but is something we will present here in the future. What we can say now is that when performance is a desirable outcome, you must drink to thirst, for it will optimize the amount of fluid ingestion.

Should you choose to ignore your thirst, you will not collapse from "heat illness," and nor will you die from heatstroke. However, you will be miserable and you will run slower than you would like. So listen to your body and join us later in the week for Part IV in this series!


See also:
Part I: History of fluid intake and a conflict of interest Part II: Fluid intake, dehydration, and exercise

Part II: Fluid intake, dehydration, and exercise