Fluid intake, dehydration and exercise: Part II

17 Oct 2007 Posted by

Today sees the second post in our series on fluid intake, dehydration and exercise. Yesterday we looked at the history of fluid intake and how radically our beliefs on the subject had changed. Today we turn our attention to the evidence that has accompanied this shift, beginning with the contention that runners who become dehydrated are likely to develop heat stroke. It is the metabolic rate, not dehydration, that predicts core temperature

If you examine the scientific literature before the 1960′s, you will not find much research on how fluid ingestion affects temperature regulation. The earliest thermoregulation study we know of is from 1938 and was performed by Marius Nielsen in Copenhagen. The title was, “Die Regulation der Korpertemperatur bei Muskelarbiet,” which in English translates as “The regulation of the body temperature during muscular work.” Nielsen performed an exhaustive series of experiments on several men, in which he demonstrated 1) the core temperature goes up as you exercise at higher power outputs and therefore exercise intensities; and 2) the core temperature is regulated at a higher level during exercise. In fact one figure shows that during a four-hour exercise bout at 180 Watts, the rectal temperature is maintained at about 38°C for the duration of the exercise.

Fast forward to 1960, when Sid Robinson published an article titled, “Temperature regulation in exercise.” He began his summary of that paper with this:

The central body temperature of a man rises gradually during the first half hour of a period of work to a higher level and this level is precisely maintained until the work is stopped…During prolonged work the temperature regulatory center in the hypothalamus appears to be reset at a level which is proportional to the intensity of the work and this setting is independent of environmental temperature changes ranging from cold to moderately warm.

Robinson’s paper agreed with the findings of Nielsen—namely, that the core temperature during exercise is regulated at a higher level during exercise. This was precisely our point in our “When abnormal is normal” post: the body is quite happy with change during exercise, and many variables are regulated at different (higher) levels during exercise without any problems. In addition, these scientists showed that the metabolic rate is the thing that determines your temperature. In other words, the harder you exercise the hotter you get.


Introducing fluids – dehydration takes over

Up until this point, little mention was made of fluid – it was all about work rate. In 1970, Professor David Costill published the first lab study that investigated the effects of fluid ingestion on temperature regulation. In that experiment the main finding was that when the runners drank no fluid, their temperatures were higher at the end of the two-hour run, and therefore a relationship between the volume of fluid ingested and the rise in core temperature was established. Several other studies together with this one, then, appear to have shifted the paradigm. Gone was the idea that metabolic rate predicted the rectal temperature, and in its place came the concept that the level of dehydration was responsible for driving the temperature higher during exercise.

The paradigm shifts Dehydration as the cause of the rise core temperature

We now move into the 1990′s, which saw a more robust and detailed repeat of Costill’s 1970 study. In this one, published in 1992, the cyclists were made to cycle on a bicycle in a laboratory for two hours, while drinking different volumes of fluid. The title of this study says it all: “The influence of graded dehydration on hyperthermia and cardiovascular drift during exercise.”

Montain and Coyle 1992 graph - Science of Sport

The main finding, which is shown in the graph above (click to enlarge it) is that when the subjects drank no fluid (NF – the open circles) they had the highest rectal temperature, and when they drank more (LF – the solid circles), their temperatures were lower.

So then it is case closed? Two very good lab studies, published by very well known and respected Exercise Physiologists, seem to show very clearly that ingesting more fluid keeps your body cooler. The studies were well-controlled and the data are robust, right?


Good science – but can it be applied to YOU?

Well, yes, they were well-controlled and experimentally sound. However both of these studies have major limitations to the manner in which they can be applied, and here is why. First, the wind speeds that were moving over the runners and cyclists were not anything like what they would experience when exercising outside.

For example, the Costill study used elite level runners (average VO2max of 74 mL/kg!) exercising at 70% VO2max. This running speed corresponds to more than 15 km/hour. However the air blowing on them from a fan was only moving at 5.7km/h, which is no more like the speed of air when you’re walking. Likewise, in the Coyle study, the cyclists were riding at power outputs corresponding to speeds of 30km/h, yet the air moving over them was only a mere 9km/h. Therefore:

  • These athletes were being made to exercise at a high intensity, which means that they are PRODUCING substantial heat, BUT…
  • They were denied the opportunity to lose this heat because they were not given appropriate wind speeds

The importance of wind speed

Far more significant than fluid intake

The effect of convective cooling is substantial. We won’t go into the physics of the equations, but we can tell you that if you do a mathematical model, you discover that a change in wind speed of only 1km/h can change body temperature by more than 2 degrees Celsius over the course of a two hour trial. Now, if you take a study like that of Coyle’s, where the small fan provided wind speeds of 9km/h, then you can see how vast the difference would be if the windspeed was just a little higher, let alone the realistic 30km/h!

Therefore, while these studies showed clearly an effect, it is arguable that this small effect was amplified by the lack of wind speed in these trials. Even in these two trials the differences in core temperature at the end of the two hours of exercise was less than 1°C. Even more important, the peak temperature were what we would call “normal” as they were only ~39°C. Even more still, the authors of these studies, although claiming the effects of dehydration on temperature, did not report that any of their subjects suffered any ill effects from not ingesting fluid, and they reported no signs or symptoms of any kind of “heat illness” after the exercise trials.

So does it come as any surprise that the 1992 study above was, and we quote, “supported by a grant from the Gatorade Sports Science Institute?”



To summarize this post, the important observations here are the following:

  1. Metabolic rate is the best indicator of the core temperature
  2. Any affect of fluid ingestion on the ability to regulate the core temperature is small (less than 1°C)
  3. A major lack of convective cooling might have amplified this small effect so that it is even much less than 1°C
  4. The workload in these studies was fixed, and the subjects were not allowed to pace themselves as they are in a real race situation.

The last point above is yet another limitation to how these studies are applied… however we will leave it at that for now as this post already is very lengthy—remember that we wrote theses on these topics! So to condense the concepts down into one post is a real challenge. Be sure to keep coming back for the next post in this series, where we will show you the real effects of dehydration on cycling performance and temperature regulation!

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