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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."

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 atheltes 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 Institue?"


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!



Theories and Fallacies of muscle cramps

As promised in yesterday's post, today we kick off our latest series - Muscle Cramps. We hope that none of you did cramp in the middle of the night, as we mentioned yesterday! Though if you did, we're sure you stretched your calf and avoided the temptation to point your toe!

This is a follow-on from our series on Fluid Intake and Dehydration, and as we were preparing to write this series, we realised that there may actually be even more nonsense and blatant lies in the media than there were for dehydration!

Conflicts of interest revisited

In the dehydration series, we dealt with the very obvious conflict of interests that arise when a company which manufactures and sells sports drinks become the company who are funding and then performing much of the research on fluid and exercise. This is what happened when Gatorade created the Gatorade Sports Science Institute, and began funding research studies all over the USA, that rather unsurprisingly told the world that thirst was not enough, and you just had to drink as much as you could.

Can you imagine Gatorade issuing the results from those first studies saying to people "Folks, we've tested the sports drinks, and we don't have much evidence that you really NEED them. You'd most likely be fine without them, but hand over your money and buy your Gatorade at the counter anyway". An unlikely scenario. Of course, it was never as simple as that, and as we tried to explain previously, some of the early lab-based science was actually sound, but its application became the problem. More than this, the manner in which the research was compromised, becoming a form of shameless endorsement for the sake of sales in subsequent years was the ultimate problem. But that was all covered in our previous series, for those who are interested...

Muscle cramps - even more pervasive mis-marketing, but a complex issue

The same marketing vs scientific integrity debate exists for muscle cramps. The industry that has sprung up around the muscle cramp issue has spread far and wide. It includes Gatorade, who advocate the use of their drinks to replace the loss of salt which is, according to their research, responsible for the cramp in the first place! But more than this, there are dozens of products that claim to prevent cramp - next time you are in a pharmacy, take a look at the range - everything from gels, to creams, to pills, to effervescent tablets.

The two broad theories for muscle cramps

All these products work off the same premise - the put back the electrolytes that exercise will take out. And it's the loss of those serum electrolytes, the theory goes, that are responsible for the cramps during exercise. This theory, over 100 years old, is one broad category of theories for muscle cramps.

The second theory is that muscle cramps are caused by a 'malfunction' in the control of the muscle by the nerves - an abnormality of neuromuscular control which is caused by fatigue.

Our objective in this series is to look at these two theories, beginning with a bit of groundwork and history...

Defining cramp

Perhaps one of the first things to do is provide a definition for cramp, as well the usual disclaimer that we cannot possibly cover all the possibilities and scenarios in this series. Firstly, cramp has been defined as a "spasmodic, painful, involuntary contraction of the skeletal muscle that occurs during or immediately after exercise".

Note that this definition applies to exercise-related cramps only, and therefore, it excludes a whole host of other possible cramps. We must point out that if you do suffer from very regular cramping, there are some conditions that can cause this - endocrinologic, neurologic, and vascular disorders, treatment with certain drugs, and occupational factors. Then of course, some cramps are what the experts call "idiopathic", which means they have no cause (but actually means we don't know what causes them, but it sounds better to say "idiopathic"!). If you are a regular cramper, it's probably worth seeing a doctor and just having an exam to determine whether any of these broad factors might be responsible.

But returning to muscle cramps, the lifetime prevalence of cramping is reported to be as high as 50%, which is remarkably high. Some people are also quite clearly more susceptible, and you can actually predict with a fair degree of accuracy who will cramp during a marathon based on their history and their racing strategies (more on this later).

The history of cramping - the electrolyte depletion theory

The earliest reports of muscle cramps come from 100 years ago, when labourers in hot and humid conditions of the mines and shipyards suffered from cramps. Even that far back, the sweat could be analysed, and it was noticed that the builders had a high chloride level in their sweat (chloride, incidentally, is one half of the salt in your sweat). The conclusion that was made was that the labourers were sweating out valuable electrolytes, causing their muscles (and nerves) to malfunction. The heat and humidity were key factors that caused this situation. It must be pointed out that no one prospectively measured the sweat of the labourers who DID NOT CRAMP, something that we'll look at in our next post.

Later, the builders of the Hoover Dam famously recovered from cramp when they were made to drink salty milk, entrenching the theory that salt loss was the cause of cramp.

And perhaps rather surprisingly, that was it - based on those anecdotal observations, the theory which you probably hold true today, was born. That is, cramp is caused by a loss of sodium, chloride, and later calcium and magnesium were added to the mix. Heat and high humidity were implicated as "accessories", and the term "Heat-Cramps" was even conceived. According to this theory (as seen by this article and the "expert" testimony) , cramps happen because athletes exercise in the heat, lose electrolytes in their sweat, and the depletion combined with high body temperatures cause muscle cramp.

For example, take these testimonies:

"When a young athlete experiences heat cramps, pull him or her off the field into a cool area and gently stretch the affected muscle. "Have them drink, drink, drink, and then drink more," says Albert C. Hergenroeder, professor of pediatrics at Baylor College of Medicine and chief of the sports medicine clinic at Texas Children's Hospital.

"High-sodium drinks will prevent children from getting heat cramps," says Jackie Berning, PhD, with the National Alliance for Youth Sports. "Gatorade has just enough sodium to prevent those cramps. But if you're a heavy sweater, and you're still getting cramps after drinking Gatorade, eat some salted pretzels or salted nuts. Those work fine.""

There is of course more to it than this, but the essence is that the serum electrolyte depletion theory was created without any controlled, clinical studies to establish whether the depletion of salt through excessive sweating was to blame. Rather, the theory was picked up on and used to spawn the numerous products you can purchase today. But, as I'm sure you've guessed, there are some holes in it.

The problems with the serum electrolyte depletion theory

First of all, there is a key conceptual problem here, and that is that when you sweat, you don't actually reduce electrolyte concentration. That is, there are certainly electrolytes in the sweat, but the concentration of these electrolytes is so low, that sweating is likely to make you HYPERTONIC, not hypotonic. We looked at this in our posts on fluid - when you sweat, you lose more water than electrolytes, because the sweat is HYPOTONIC. Therefore, sweating cannot lead to a fall in electrolyte concentration.

What transpired was that Gatorade (and the rest of the 'industry', it must be said) developed the theory of "salty sweaters", which is the term they gave to people who they said have abnormally high salt levels in their sweat. Small problem - no one actually knows what a salty sweater is. How much salt does there need to be in the sweat before you are placed in this group? No one knows. Recently, Professor Martin Schwellnus, widely published in this area, posed this question to scientists at the Gatorade Sports Science Institute at a conference on cramping - he received no answer.

The truth is, even the saltiest sweaters around still have hypotonic sweat, and so the more they sweat, the more they will cause their electrolyte levels to rise, not to decrease. This is a very obvious problem that is overlooked by the electrolyte replacement advocates.

Of course, those of you who read our fluid series might be thinking that if you then drink a lot of sports drink, you can reduce the electrolyte content, but that's yet another reason why drinking too much is not a good idea...

The cramping paradox - why specific muscles?

The second problem is something we asked you in yesterday's post. We asked whether the depletion of serum electrolytes would be expected to cause cramps in specific muscles, or all over? Hopefully it is evident that if a cramp was caused by a loss of serum electrolytes, there is no reason for the cramp to be limited to one muscle only. Rather, you would cramp everywhere. In fact, in people who have lost a great deal of salt and have become hyponatremic (not during exercise, but clinically), we know that they cramp in ALL their muscles.

But somewhat surprisingly, exercise-associated muscle cramps ONLY happen in the muscles that have been used extensively for exercise. The afore-mentioned Prof Schwellnus found in 2004 that the quadriceps, hamstrings and calves made up 95% of cramps in the 56km Two Oceans race in Cape Town.

Leading onto the next post - further evaluation of the electrolyte depletion theory

In the interest of time, we'll call it on this post for today, and say that in our next post, we'll tackle the electrolyte theory in more detail and look at some of the studies that have looked at people who cramp and those who don't and compare their values.

Join us then!

Further reading:

Schwellnus, M. (2007) Sports Medicine, vol 37, 2007

Schwellnus et al., British Journal of Sports Medicine, vol 37, 2004

Posted by Ross Tucker and Jonathan Dugas on 11/20/2007 09:54:00 AM  
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