Heatstroke continued

Heatstroke part 3: Abnormal heat production, or failure of heat loss?

Yesterday, in our second post on heatstroke, we introduced the concept that the attainment of a body temeprature above 41 degrees celsius is NOT POSSIBLE due solely to environmental conditions, which is how you've probably always been told to think of it.

We explained how body temperature is a function of heat loss and heat production, and provided the potential for heat loss is greater than or equal to the heat production, there is zero chance of heat stroke occuring via purely "normal" physiological means. Therefore, when people do develop this condition, it is not as simple as saying "they didn't drink enough and the conditions were too hot", which was really the take-home message of yesterday's post.

We illustrated this with one example of heatstroke from the published literature, that of a man who hit a body temperature of 40.8 degrees after only 16 minutes of running when the temperature was 22 degrees celsius. There is no "normal" explanation for this, it must be pathology, which is where we continue this discussion today.

Eighteen cases, and not one makes physiological sense

Below is a table showing you 18 documented (that is, published in scientific journals) cases of heat stroke during exercise. There are undoubtedly others (we received two very interesting stories from readers - thank you for those - explaining their own adventures. One had a body temperature of 42 degrees (incredible), the other was at 40.5 degrees, but more on that a little later), but these are the documented cases.

I've highlighted three particularly interesting cases. You'll recognize the one in light green as the example of yesterday's post - a runner developed heatstroke after only 16 minutes of running when the temperature was only 22 degrees celsius, and the runner was only doing 4:30/km - hardly fast enough to overheat in any conditions, let alone the mild conditions, and certainly not in only 16 minutes.

The example in yellow is even more spectacular. This was a runner who collapsed after 16 minutes of a race when the temperature was only 17 degrees celsius. His body temperature was an incredible 42 degrees celsius, and most amazing of all, he was only at 7.4km/h. That is a pace of 8 min/km, which is either a very fast walk, or a very slow jog. I'm sure I don't need to emphasize just how spectacular a failure of physiology it is for this individual to overheat so quickly while exercising that slowly on that cool a day!

And then finally, the example highlighted red is a man who ran a half-marathon in air temperatures of only 4 degrees celsius, and made it to 88 minutes (he was on course for a 1:35 time), and his temperature was 41 degrees. I'm sure you can appreciate just how cold 4 degrees celsius is, and the next time you have to run in those conditions, ask yourself what the chances of overheating are, and you'll have an idea of why this particular case warrants attention!

The problem with heatstroke - a glitch in the balance

These are three cases that punch holes in the normal theory for heatstroke. There are others - 15 of them in fact in that table above, and numerous others, including the two cases we received yesterday from readers. Our approach to these 18 cases of heatstroke is to calculate two things:

1. The heat production as a result of exercise. As we described yesterday, heat is produced thanks to muscle contraction, and we can work out this value with fair accuracy
2. The heat loss through convective, radiative and evaporative means. Again, these concepts were explained yesterday

Now, the key to understanding heatstroke is to recognize that when heat loss potential is greater than heat production, the athlete SHOULD NOT develop heatstroke. I guess the analogy here is that if you are saving more money per month than you spend, you should not ever have to file for bankruptcy!

So let's take those 17 cases and simplify them to illustrate that heatstroke is very rarely a consequence of the environment. What we do is work out the ratio of heat production to heat loss.

• If that ratio is equal to 1, then it means that heat production equals heat loss potential, and the athlete will be safe
• If the ratio is greater than 1, then the athlete has a problem - they are producing MORE heat than they can lose, and therefore their body temperature will rise. They will thus either stop, slow down, or develop heatstroke
• If the ratio is less than 1, then the athlete is safe - they are able to lose more heat than they produce, and so heat stroke should not happen

The graph below shows the result for 16 of the cases - 2 of them do not have precise weather data:


Clearly, every single one of these people had a potential for heat loss that exceeded the amount of heat they would produce from exercise. Therefore, these cases of heatstroke should never have happened, unless our equations are wrong (they aren't!), or unless something else, unaccounted for by the concepts, is causing the problem.

And therein lies the crux. These mathematical models for predicting heatstroke are clearly not "complete" for these individuals. The fact that these 16 runners and cyclists did develop heatstroke means that somewhere, either heat production or heat loss has been incorrectly accounted for. Broadly speaking, there are two possibilities:

1) Heat production is actually a lot higher than is calculated by the equations

This is not because the equations are wrong, because in 99.99% of people, they are relatively accurate, and certainly, the calculation for heat produced during exercise is well-established. However, in these people, something has gone wrong, and it may be that they have produced heat in large quantities from NON-EXERCISE related sources. In our recently published paper in Medicine and Science in Sports and Exercise, we called this "excessive endothermy", which basically means heat production from within.

Quite where this heat comes from is anyone's guess - perhaps the runner's efficiency is massively reduced for metabolic reasons? Perhaps there is a sudden increase in heat production thanks to unregulated opening of calcium channels? There is a condition known as malignant hyperthermia, where certain chemicals, most notably anesthetics, cause calcium channels in muscle to open. As this calcium is then transferred back into storage, it uses up a great deal of ATP and generates quite enormous quantities of heat.

The malignant hyperthermia linked has been looked at before - there are reviews on the topic. They distinguish between exertional heatstroke and malignant hyperthermia, but don't rule it out, mainly because when we look at people who suffer from heatstroke, they tend, in many cases, to be susceptible to malignant hyperthermia as well! In other words, there is likely a genetic link that predisposes people to these conditions. It has been suggested that heatstroke sufferers have a skeletal muscle abnormality that is similar to malignant hyperthermia.

Is it possible that exercise-induced heatstroke involves a similar "wasteful" use of energy in order to correct some kind of channel disruption? And what are the triggers? Interestingly enough, caffeine is one of the chemicals known to cause calcium channels to open, and so may actually warrant a closer look as a potential "trigger" for heat stroke. I must confess that I don't know the dosage that is required for this effect to exist. Other triggers may be central nervous system stimulants, like ephedrine (common in weight loss products), and the combined use of caffeine and ephedrine may be a real warning sign for this heat producing "spiral". This was in fact reported in Case 1 from the table above - a weight loss supplement contained both caffeine and ephedra.

There is a few other candidate "pathologies" - it may be that there is excessive sympathetic nervous system activation, triggered by a metabolic condition or muscle myopathy. Another possibility is illness - a bacterial infection before exercise may increase the chances of overheating, though this has never been confirmed (for obvious reasons), and relies mostly on retrospective analysis of specific cases (and not all cases either, it's worth noting).

Certainly, hot environmental conditions may be a trigger - we are not dismissing the role of the environment in all this, and it seems feasible that on a hot day, some "trigger" exists that may cause this same excessive endothermy to occur. The point we are making, however, is that the environment is merely the stage for the drama to be played out on - there is a pathlogical process at play here, and environment is not the CAUSE of heatstroke, merely a roleplayer.

A final illustration that heat production may be the cause comes from one case (case 3 above).


This person was admitted to the medical tent after the 56km Two Oceans Marathon with a temperature of 41 degrees celsius. He was placed in an ice-water bath, and wore a cooling vest. His body was then surrounded with ice-packs after that. It took TEN HOURS of cooling to get his body temperature down to normal levels! So how does the human body manage to keep its temperature above 38 degrees celsius when it's surrounded by ice-packs? The only answer can be excessive heat production, so great that it overwhelms the heat loss to the ice water and packs.

2) Heat loss is lower than the calculations suggest

Of course, the other option in these cases is that heat loss fails. If that were to happen, then our scale would tilt to the left, because evaporation and convection would suddenly fail to deal with the heat production (the ratio would then jump above 1).

Of the avenues for heat loss, the most likely to fail is evaporation, and this would of course occur as a result of a failure of sweating. This is difficult to prove, however, because often, people with heatstroke are sweating profusely, and so seems unlikely. Interesting studies of soldiers in Iraq suggests that it can happen when people are exposed to dry heat for a prolonged period (though this study dates back to 1932, and the methods for research may have been limited back then!)

Conclusion

However, it seems more likely that the problem is excessive heat production, and not reduced heat loss. Or, alternatively (to sit on the fence), it is possible that heatstroke is a generic term that actually describes a SYMPTOM, and not a condition. If this is true, then it could be caused by all of the above, or any one of them! There may be no single cause, in fact, it's highly unlikely. What does seem certain is that heatstroke is a failure of "normal physiology", because you do NOT develop this condition simply by running on a hot day and failing to drink enough water.

Therefore, the point of this series on heatstroke has been to debunk some of the myths surrounding the condition, and to explain that it occurs more as a result of a physiological failure than an environmental problem. And it is most definitely not the result of dehydration, which is unfortunately what most people attribute it to! Does anyone seriously believe that our three cases highlighted in the table above were dehydrated within 16 minutes of starting to run in moderate conditions, or after 85 minutes of running at near-zero conditions?

No, heatstroke is a very complex, difficult to predict and even more difficult to explain condition. But hopefully we haven't lost you in the maths of the series, and you now appreciate that heatstroke is pathology, not normal physiology, and does not happen simply because it's hot outside.

Thanks for the emails on your cases and experiences, by the way. We will pursue those further!

Ross

Heat stroke dissected

Heat stroke: A problem of physiology, not fluid or environment

Continuing on from our post two days ago, we are looking at heatstroke, a condition where the body temperature rises above 41 degrees celsius (this cut-off is somewhat arbitrary, it has to be said, at least in the exercise literature).

In that post, we introduced some of the paradoxes of heatstroke. The classic teaching on heatstroke is that body temperature rises excessively thanks to excess heat production which cannot be matched by heat loss. Heat production is thus a result of high exercise intensity, which means that this theory holds that you quite literally exercise yourself to death by generating so much heat that you overwhelm your body's capacity for heat loss. What it fails to account for is that humans usually slow down long before this limit is reached, or they stop exercise altogether once they hit a certain temperature, and so it's difficult to explain why they run themselves into heat stroke unless there is some "malfunction", which is where we're ultimately headed with all this.

Of course, the fluid-pundits climbed on board and advocated that the biggest problem would happen if you failed to drink enough water, because then your body temperature would rise even more rapidly and heat stroke would be a very real possibility. This particular post is not about the fluid-hyperthermia myth - we covered that in great detail in our series on dehydration, for those who are interested. Instead, we're interested in the physiology of body temperature regulation (and fluid, quite frankly, is barely involved).

It does get quite technical, but we'll do our best to speak logically, rather than mathematically! As a result, we will skim over the specifics of the equations, but I'd encourage you to check out this paper (which inspired this series, it was published earlier this year), where the equations are presented and discussed in more detail. As always, if you can't get the paper, drop us an email request and we'll send it along...!

Body temperature balance

The figure below is a (very) oversimplified schematic of the two halves of heat balance. It says that heat storage (which can be negative/heat loss), is equal to heat production minus heat loss. We can quite easily calculate and predict the two sides of the scale using mathematic formulae because we know what factors affect the heat production and heat loss components. These are convective, evaporative and radiative heat loss/heat production.


So for example, we know that heat production is a function of exercise intensity (cycling or running speed), body mass and a constant, which varies depending on whether you assume that the person has a high or low level of efficiency. The larger you are, and the faster you run, the more heat you will produce, which is why smaller runners have an advantage in hotter conditions. For example, the equation for an inefficient runner reads:

Heat Production (Watts) = mass x [(5.89 x speed) - 4.69]

On the right side, we have heat loss, which is largely influenced by the environment. Here, it's convection and evaporation that are mostly responsible, which is why air velocity and sweating are so important. Note that sweating by itself does not remove heat, only evaporation, which is why humidity is so vital - if sweat drips off, it does nothing for temperature, as our readers in the East and tropical regions will testify! (Also, note that body surface area, which is a function of mass and height. The larger the athlete, the greater their capacity to lose heat, but it doesn't quite manage to offset the fact that they also produce more heat)

Introducing the mathematical equations - conceptualizing the limits of exercise

Because we know how these factors interact and influence heat production and heat gain, it's possible to take that basic equation and refine it a little more. It now becomes:

Heat storage = Heat production - Convective heat loss/gain - radiative heat loss/gain - evaporative heat loss

Note that in all cases, the option exists to either gain heat or lose heat. For example, convective heat LOSS happens when the skin is warmer than the surrounding air, but as soon as the air becomes hotter than the skin (at about 35 degrees celsius), then convective heat loss falls to zero, and then eventually switches around - you start GAINING heat from the environment.

Now, we are in a position to make some interesting calculations regarding heat stroke, because we know that the body temperature will rise when heat is stored. And if we know how much heat is stored, we can calculate how much body temperature will rise. That is, we know that every 3.47 kJ per kilogram will raise body temperature by 1 degree celsius, and so if a man weighing 80 kg gains 278 kJ in one hour, his temperature will increase by one degree celsius in that time. To extend this further, if he wants to run into heat stroke, he'd have to raise his temperature by 4 degrees celsius, which would require him to store 1111 kJ.

So the approach we can now take is the following:

• We can calculate the rate of heat production (thanks to knowing the running speed and mass of the person);
• We can calculate the rate of convective cooling if we know the air temperature
• We can calculate the rate of radiative heat gain if we know cloud cover
• We can calculate the maximum capacity for evaporative heat loss if we know the humidity

These four variables are all we need to be able to say whether the possibility of heat stroke exists, because:

• If the capacity for heat loss is greater than the calculated heat gain, then heat stroke is not possible (mathematically, anyway. More on this a little later)
• If the capacity for heat loss is lower than the calculated heat gain, then our scale tilts towards heat storage, and the result is that our athlete will gain heat, his temperature will rise, and in theory, heat stroke is possible.

Example: Why heatstroke is not an environmental problem

This is best illustrated with an example:

Note that we're making "worst case scenarios" here - we assume he's inefficient, that there is no wind other than the wind he generates by running and that he is also running on a bright sunny day. We do this to make a point - by taking the "extremes", we want to see just how bad things need to be in order for him to develop heat stroke.

So, our calculations reveal the following:

To empahsize this further, we can work out that for our runner to keep his body temperature EXACTLY the same, he would have to evaporate 1.5 L of sweat per hour. But our calculations also reveal that it would be POSSIBLE to evaporate 1.6 L of sweat per hour. This means that he has no problem losing the heat he produces, and should NOT develop heatstroke (once again, for more detail on the calculations, refer to this paper)

Here's the catch: He did get heatstroke, in only 16 minutes!

Ah, but now, what if I told you that this man is one of the 18 cases reported in the literature. In fact, this runner, running in these conditions, was pulled out of the race after ONLY 16 minutes, with a rectal temperature of 40.8 degrees celsius! Therefore, despite the fact that there were no limitations in the environment, and the fact that he COULD have lost all the heat he produced, he failed. And the result was that he developed heat stroke after less than 4km of running!

If that does not strike you as extra-ordinary, nothing will. Your first thought might be that our maths is dodgy (and you have a reasonable case, as I'll explain at the bottom of the post), but really, consider those conditions: 22 degrees, and the humidity was high, sure, but they're not difficult running conditions. If you stood on the start line of a 10km race in those conditions, the thought of heat stroke would not cross your mind. How about after 16 minutes? You should be thinking that something serious went wrong with this runner. And you'd be right. The problem is that we don't quite know what it is!

Some pointed questions about heatstroke

In the interests of time, we'll tackle that question in the next post in this series. But what I want to leave you with are the following questions, which will hopefully give you reason to challenge what you know about heatstroke:

1. If heatstroke is purely due to the environmental conditions, then why is it so rare? In SA, for example, we have a cycle race with about 30,000 participants per year, and only 5 cases in the last 6 years have been reported. That's 1 in 30,000. And the prevalence seems about that low. Now, consider that 29,999 people will be exposed to the SAME conditions, and NOT develop heatstroke, and suddenly you realise that the environment is NOT the crucial variable. Obviously, it contributes, as we've shown above, but it's not the driver. Something else is...
2. Heatstroke cannot simply be a function of exercising so hard that you overwhelm your body's capacity for heat loss. The cases we showed in yesterday's post are representative of this, and that's what I'll write about tomorrow. But the point is, as we saw in the example above, heatstroke occurs even when the theoretical limit doesn't exist. It's not a function of heat production through any normal means.
3. Third, why is heatstroke more common in back-of-the-pack runners? According to every theory, heatstroke should be most likely in faster runners (especially larger ones). Yet this is not consistent with what is observed. We had an email from someone who is involved with the marines (which is where heatstroke does seem to occur, though it's rarely documented in scientific journals), and I dare say (with respect to the marines), they're not exactly exercising that hard when they develop heat stroke. So something else must go wrong.

In conclusion, heat stroke doesn't seem to be driven by the environment, though it's a contributing factor. It's also not explainable by the athlete's "high" workrate, because they are rarely actually producing that much heat. So the quest begins for the answer. Join us next time!

Ross

Disclaimer:

I've made use of mathematical equations in this post to illustrate the concepts. That's certainly a point of contention, because the human body is more complex than an Excel spreadsheet. So I don't mean to oversimplify or rely too heavily on the maths and calculations. However, what these equations do allow is a demonstration of the conceptual issues around heatstroke. We assume the worst (no wind, direct sun, poor efficiency) and then show that despite everything being "worst case", the capacity for heat loss exceeds heat gain. The equations are therefore useful to demonstrates concepts.

Where they would fail is if we tried to use them predictively or prescriptively. In other words, we can't say definitively that a 60kg man running at 15km/hour at 25 degrees with humidty of 60% will have a body temperature of 39 degrees after 45 minutes. That would be reckless use of the tool. So please, understand that we've illustrated concepts here, and hopefully made a strong point that the environment is rarely a key, and that actually developing heat stroke is extremely difficult according to "normal physiology".

We'll pick up on this point again tomorrow.

Ross

References:

For those who feel like sinking their teeth into the cases and the equations a little more, check out:

Rae, Knobel, Mann, Swart, Tucker and Noakes. Med Sci Sports Exerc, 40: 1193 - 1204, 2008

Heatstroke: Some interesting observations

Heatstroke: The reality doesn't fit with the perception

One of the more interesting ways in which we can study physiology (especially during exercise), is to observe it when it fails. Take for example the Calvin and Hobbes equivalent of "failure physiology":


Calvin (the young boy, for those who haven't discovered Calvin and Hobbes) asks his father a seemingly simple question, and gets an absurd answer. Yet incredibly, this is how exercise scientists have approached certain problems for many years - fatigue and temperature is the most obvious of them! So we study what happens at failure (exhaustion) and then infer the cause backwards from there! For example, when studying fatigue, many exercise physiology studies make runners or cyclists exercise at a fixed workload until they are absolutely exhausted and then measure things at the point at which they stop, assuming them to be the cause.

"We happened to notice that the runners all stopped when they had low glycogen levels/high body temperature/high lactate levels. Therefore, they stopped because of it!" is the logic applied (we covered this in our fatigue series earlier this year).

When the body overheats (i.e. heatstroke), it's the classic illustration of this "load limit" concept for humans during exercise. We know from these "run until you drop" studies that humans almost always stop exercise when their body temperature hits 40 degrees. This has been called the "critical limiting temperature" (Calvin's dad would be proud), and the theory was that muscle activation fails at this point. It was subsequently shown that if you allow the athlete to choose their own speed (like you do in every exercise situation), they slow down long before they overheat - we called this Anticipatory Regulation of exercise by the brain.

The problem with this theory - physiology does fail

This anticipatory theory, and even the "load-limit" theory, do pose some problems for us, however, because they imply that humans will never exercise so hard that they drive their body temperature to even higher levels. Recall that heatstroke is defined as an increase in body temperature above 41 degrees celsius (or 104F), which would be very difficult to explain if your brain "fails" at 40 degrees, or regulates you so that you never get there.

Of course, you may argue that some people can "over-ride" this physiological regulation, but I would argue that this is akin to suggesting that someone can commit suicide by holding their breath. It doesn't matter how badly you want it, but physiology always wins the day! I will concede that some people have exceptional "mental toughness" and approach the limit much more than others, but no one breaks right through it. And now, consider that heatstroke very rarely happens in the elite athletes, who would surely be the ones most likely to have this "mental override" capacity. So it seems an oversimplification to say that heatstroke happens because people "push themselves" too hard in the heat.

Similarly, when you actually look at cases of heatstroke, something even more intriguing jumps out at you - heatstroke has almost never been reported on hot days! Look at the following table, which shows 18 documented cases of heatstroke.

Hopefully, you'll be struck by the relatively low temperatures at which many of them have occurred. There are races, for example (cases 3 to 9), where temperatures are below 20 degrees, sometimes only 4 degrees celsius! That should set off an alarm in your mind about how heatstroke occurs - is it simply a case of exercising in the heat until you overheat? It seems more complex than this. And what I'll do in our next post is look specifically at two or three of these cases and show how it's physiologically "impossible" to explain these cases according the classic physiology that you run yourself into trouble. Basically, it's impossible to run fast enough to develop heatstroke unless you're very heavy and run very fast...(but more on that in the next post of this short heatstroke series)

Diagnosing heatstroke

These 18 cases are among the ONLY documented cases of heatstroke, which is itself incredible. You probably think that heatstroke is very common, because every time there's a race in vaguely hot conditions, you're warned to drink lots of fluid and take every precaution to avoid heatstroke (especially in the USA, I must say. That's a fact). But incredibly, exercise science and sports medicine often even fail to measure that load limit before they attribute just about any "failure" in the heat to heatstroke! So, returning again to the definition (a body temperature that exceeds 41 degrees celsius), you'd be amazed at how often heatstroke is "diagnosed" without ever measuring the body temperature. Instead, the default response to a case where an athlete can't exercise and it happens to be a relatively warm day outside is "heatstroke" regardless of what the person's body temperature is.

So I must stress that there is a big difference between heatstroke and what one might term "exercise intolerance due to feeling terrible in unaccustomed heat," which is what I think accounts for most of the problems people experience during running. And in our discussion of the Chicago Marathon, it was mildly amusing to read some responses that were indignant at my suggestion that some people expected marathons to be "easy", whereas others (those in the more demanding climates around the world and thus used to the heat) have a completely different view of running in different conditions. In some places, a marathon at 24 degrees with moderate humidity is a pleasure. For others, it's a cauldron of heat and danger...!

The physiological abnormality of exercise

Returning to the heat issue, what may surprise you is that during exercise, almost regardless of the air temperature, humidity and windspeed, your body temperature will regularly hit about 39 degrees celsius, with no ill effects whatsoever - it's a controlled "hyperthermia", and you're halfway to heat stroke without ever even realising it! It's actually amazing to consider how exercise makes the "abnormal" feel normal. Take a physiological snapshot of yourself during a 10-mile tempo run and your heart rate is 175 beats per minute, your breathing rate 54 breaths per minute, your body temperature is 39 degrees celsius. A doctor presented with those statistics would likely admit you to an ICU, yet you feel absolutely perfect during exercise!

The point is that the perceptions you have of your own physiology are strongly influenced by set-points. That is, your brain can adjust what is "acceptable", the set-points, to create a context to interpret incoming physiological signals. Think, for example, of when you have a fever - you are shivering even though your body temperature is 39 degrees celsius and you've overheating. That's because your body's set-point has been reset and you are kicking in heat GAIN mechanisms even though you're hot - how you FEEL is not necessarily the same as how you are ARE. A similar concept applies to exercise.

And the point of all this is to introduce the issue of heatstroke to you. Your body is a remarkably designed machine, capable of losing far more heat than you might realise. Yet it CHOOSES to allow you to gain heat and you become "hyperthermic" during exercise even on cold days. Where this is leading me is to the next post on heat stroke, where I'll look at the physiology of heat gain and heat loss and explain how actually running yourself into overheating is a pretty tall order. Instead, there is something else that accounts for heat stroke, and we'll put forward a few suggestions about what it might be.

Join us then.

Ross

Chicago Marathon 2008

The data do not lie: The actual environmental conditions from the course

Apologies for the absence lately, it has been a hectic time at work and for other endeavors lately, but our deadlines have now passed and we can return to a more regular posting routine that we are accustomed to. Just in time, too, as the NYC Marathon is just over two weeks away, so watch for our previews of that one as Paul Tergat and Paula Radcliffe try to add to their previous victories in that race.

But for now let's look back to the Chicago Marathon, where much was said about the weather conditions on the day. All the fuss was due to last year's oppressive conditions which forced the organizers to close the course early and send people back to the finish. It was a cooking day in Chitown last year, to be sure, but this year it was a stunning day---and we have the data to prove it!

The historical record

A look at the past three runnings of the race reveal three vastly different days. In 2006 it was a miserable day---cold, in the 30s or 40s F, if I recall, and overcast the entire time. Generally a dreadful day to run a marathon! 2007 was quite literally burned into our memories - it was already in the 70s F at the start, with no wind, and glaring sun that baked an already hot city into the 90s F by day's end.

This year was cooler at the start, and much less humid as the day went on, producing a warm but dry day. Here is how the conditions stacked up from last year and this year, according to the data from the weather website Weather Underground:

These graphs show the "official" historical data on the Weather Underground site. Temperature data are on top, while relative humidity data are at the bottom. We took values from the same station, so there is no bias in that sense. The big difference is the starting temperature---it was over 10 F lower this year, and so even though the humidity was similar at the start I can tell you it was a vastly different day. And from 11:00 the humidity was consistently 10% lower than last year.

This year's weather was something runners know about. It was one of those days where you step outside in your running kit and think, "Hmmm. . .maybe I should bring a long-sleeved shirt with me to the start because I might get cold standing around before the gun." Last year, on the other hand, was muggy, hot, and oppressive. The preceding month was unusually warm for fall in Chicago, and every morning was hot and humid even before the sun came up. You never felt cool last year in September during your morning run or ride, I can tell you that much.

The "official" vs. the "actual"

But the official data tell only half the story, and after last year the race organizers knew they needed something more to inform them about the conditions on the course. The problem last year was that around 11:00 it was clear to the officials that they were going to have problems if they kept the course open as their "peripheral" resources, i.e. ambulances on the course, were all deployed and transporting runners to local hospitals. Therefore it was the best decision they could have made to close the course, because had anything else happened they would not have been able to respond to it.

Part of the solution was to invest in technology and purchase four portable devices to measure the temperature and relative humidity on the course, rather than relying on the data from weather stations. We placed these devices (together with faithful graduate students!) at strategic points on the course, namely the northern-most, western-most, and southern-most points and the finish area. Then we took readings every 15 min, although for simplicity we have included only hourly measurements here:

Again, temperature data is on top and humidity data at the bottom.

The first noticeable detail is the the readings we took on the course are quite different from the "official" data. This might not be a surprise as that weather station is likely distant to the race course, and weather is a very local phenomena. But still, as far as we know this is the first this this kind of discrepancy has been shown, and the important implication is that if you organize an event and want to know what is happening on your course, you must collect the data yourself and analyze in real time to see how things are changing and exactly what is happening.

During portions of the race the difference between the "official" values and the data we measured was as much as 40% for the humidity and 11 F for the temperature. In addition, the difference between the northern-most station, located near mile eight, and the finish was 6-7 F. The reason is probably because that part of Chicago, the neighborhood of Lakeview, consists of heavily shaded narrow streets. Compared that to the finish in Grant Park, which is totally exposed to the sun, especially for the first half of the day as it is eastern border is Lake Michigan.

So based on the data, one has to ask, "Why not run the course in the opposite direction?" After all, temps in Lakeview were 5 F lower when we stopped collecting data at 11:00 there! Of course we did not measure into the afternoon, and by then the temps on the northside might have been similar to the finish. But this is a good example of how science and data can drive critical decisions that might improve future events.

Was the weather a factor?

This question is being debated in the comments section for the race report, and we have people weighing in on both sides. Was it hot? Yes, it was, and the air temperature was close to what it was last year, in fact. The humidity was much lower, though, and the real difference was probably the starting temperature and the fact that the weeks prior to the race this year were generally much cooler. Overnight lows have been solidly in the 50s F since the beginning of September, and so the city never really heated up like it did last year.

The conditions were far from "ideal," but then again they were not dreadful by any stretch. It was a stunning day in Chicago, and normally wuhen the environmental conditions are considered "hot," it shows in the winning time. Evans Cheruiyot's 2:06:25 was incredibly fast, and not indicative of a winning time in "hot" conditions. That said, the conditions were not ideal for world record-type performances. On the day, in 'real-time', we wrote somewhat arbitrarily that the elites were probably slowed by about a minute as a result of the temperature, which seems reasonable, even now.

The early pace in the elite race was too fast, given the conditions. They paid for it in the second half, with the exception of Cheruiyot (who did also slow down, it has to be said), and large time gaps were the result of the super fast early pace, combined with the conditions. However, for the vast majority of the field, the conditions were not harmful, and the athletes running anything slower than...oh, about 2:08, were never, ever in danger of heating up too much, or becoming critically dehydrated.

The medical tent

I was sitting at the entrance to the medical tent, and in time we will analyze the data regarding symptoms, number of admissions, time to discharge, etc. One anecdote I can share now, though, is about a runner who must have run about a 3:15 or so, judging by what time he came to the tent. He was on the back of one of the golf carts used to get collapsed runners from the finish to the tent, and was sitting up with a Gatorade cup in his hand. As he passed us, he recoiled and spewed what must have been a beaker full of vomit. That in itself was not remarkable, but the volume was---the poor guy must have puked 500+ mL of Gatorade/water! Yet there he was, trying to drink MORE, probably because he'd been told he felt so terrible as a result of dehydration! Dehydrated, with maybe a liter of fluid sitting in his stomach... if something doesn't strike you as being "wrong" with that picture, then nothing will He had no business with that kind of volume in his stomach, and in case you missed it last year you should read our series on dehydration and fluid intake. Just click the "Featured Series" tab above for the links.

In the meantime feel free to weigh in, especially if you ran the race, and watch for more data from the medical tent as we analyze that!

Jonathan

Fluid intake, dehydration and exercise: Part IV

Why waiting until you are thirsty is NOT too late

We really hope everyone is enjoying this series so far. It is proving fun and challenging to write, and we hope that is coming across in the posts. So far we investigated the history of fluid ingestion in Part I, demonstrated why it is the metabolic rate that predicts temperature in Part II, and weighed up the strengths and weaknesses of the lab-based and field studies in Part III. For Part IV we will look at the thirst mechanism and why waiting until you are thirsty is not "too late."Myth busting: If you wait until you are thirsty, it is too lateHow often have you heard this? This is an oft stated mantra of athletes, coaches, and arm-chair quarterbacks everywhere. But where did this concept originate? In 1965 John Greenleaf did a study on four well-trained men to examine how much water they would ingest during exercise in the heat. The title was "Voluntary dehydration in man," and is the first reference to the finding that when given ad libitum access to fluids---that is, when we drink to thirst---humans do not replace 100% of their weight losses. For those of you who have read Part II and Part III, this should be no surprise, since in those posts we introduced the concept that weight is not the regulated variable, and therefore your body does not care how much weight you lose during exercise. This "thirst is bad" guide stuck, however, and some time later you were introduced to the mantra above: "If you wait until you are thirsty, it is too late."

What is it too late for?The argument is that by waiting until you are thirsty, you are already dehydrated. This argument has been perpetuated because you have been led to believe that weight losses equal body water losses. However, even in a class lab we performed recently, our volunteer cycled for just over two hours. During that time he burned nearly 300 g of carbohydrate and fat while ingesting water ad libitum. His weight losses, or "dehydration," were 1 kg. Yet a whole 30% of that "dehydration" was not water at all and instead represented fuel that he burned. Let us say that again---the weight loss method overestimated his "dehydration" by 30%. So the take home message here is that the body weight losses grossly overestimate the fluid losses, and when someone is said to have lost 4% of his or her body weight, at least 10% of that or more will be fuel that has been burned during the exercise.

The thirst mechanism - a well-oiled physiological machine

The reality of the situation is that humans (and mammals) have very well-developed and successful mechanisms in place to help conserve and maintain their fluid balance, although the sports drinks companies have informed you otherwise. As we have said, the body is not concerned about body weight, but rather the concentration of the body fluids---otherwise known as the osmolality, and here is how it works.

Incredibly small increases (1%) above the resting value (280-300) first will trigger the release of anti-diuretic hormone, or ADH. Its job is to keep you from losing any more water in the urine. It has a profound effect so that even small amounts of ADH produce a maximal effect---that is, it is not possible for you to produce any less urine. Next, if ADH does not do the trick, as is the case when you are exercising and sweating, your thirst kicks in. Again, this occurs at a very marginal (4% or less) elevation of the osmolality. The effect is that we seek fluid, drink, and some time later the fluid gets in to the blood and dilutes it back down below the thirst threshold. This cycle continues indefinitely until you stop excreting fluid (i.e., sweating) and restore your osmolality once and for all.

So in fact humans have a very acute sense of when it is important to drink fluid, and it does not take much to stimulate us to seek water. Thirst is a very deep-seated, physiological desire for water, and it has been shown again and again in lab studies to effectively defend the osmolality.Why is the osmolality so important?The reason the body does not care about weight losses and instead "defends" the osmolality is that this concentration of the body fluids is what keeps the fluid balance between the cells. We have fluid both inside and outside the cells, and under normal conditions, the osmolality maintains this balance. The following two changes are possible:

• The osmolality can increase outside the cells. This will cause the fluid to leave the cells. Because this is undesirable, the ADH and thirst mechanisms explained above kick in and we correct the change to restore balance (homeostasis, in physiology-speak!)
• The osmolality can decrease outside the cells. If this happens, then fluid will move into the cells. Similarly, the body will initiate a sequence of responses, including the release of other hormones (aldosterone, for example) that we won't go into here.

As our bodies are mostly water, you can imagine why keeping these fluid volumes balanced is so important, and that is precisely why the body defends the osmolality and not the body weight.

"My sweat tastes salty"

Yes, it certainly does, and that is because it does contain some sodium. However it contains profoundly less than the fluids in your body, and is still mostly water---body fluids have a sodium concentration of 140mM while sweat has a value of 20-60mM. Therefore when you remove a liter of sweat from your blood, it has much more of an effect on the volume compared to the solutes (sodium), and what happens is that the osmolality rises in response to sweat losses. This is absolutely crucial to realise - you cannot lose sodium, even if you are a "salty sweater", as Gatorade are now claiming. If the sodium content of the blood is dropping, it's because you're drinking too much water, not because you're sweating sodium!In fact, a very interesting study was published in 1992 by Robert Cade, the man who invented Gatorade. His experiment took place during a marathon, and the groups of runners were given Gatorade, 1/2 Gatorade (half water, half Gatorade), or water. The really interesting finding was that the water group maintained their sodium concentration (a surrogate for the total osmolality) just fine, while the Gatorade group actually increaesed its concentration. In fact this explains why people drink more of a sports drink compared to water---the sports drinks keep your osmolality higher and therefore makes you thirstier. So instead of lowering osmolality, which is what your body wants you to do, the sports drinks raise it. Seems kind of counter-intuitive, doesn't it?

The final word - Drinking to thirst optimizes your fluid intake

We hope it has become clear that, for a number of reasons, it is not necessary to drink so much during exercise, and in furthermore no one needs to tell you how much to drink. As we have shown you here, the thirst mechanism is highly sensitive and very successful at what it is meant to do: maintain your osmolality, not your weight. But the final message here is that when you drink to thirst, you optimize your fluid intake, and by that we mean your thirst will always keep you from drinking too much or too little. There is such a thing as both of those, but drinking to thirst will always prevent you from straying too far in one direction or the other.

In addition, who wants to carry around three Liters of fluid in a backpack when half that volume will be just plenty? And when there is no scientific evidence to support the claims that dehydration increases your core temperature or elevates your risk for heat stroke, it seems quite unnecessary. In fact, the concept that people are "dehydrated" while losing a few kg's is now debatable.

One last thing, is that as humans, we are regarded (by most, anyway) as the smartest animals, right? Yet for some reason, companies making fluids deem it necessary to inform you how much you should drink. Have you ever had to force your pet cat or dog to the water bowl? Have you ever seens signs in the wild pointing animals to the watering hole with instructions to drink before they're thirsty? Yet somehow, the Gatorades of the world have "discovered" the NEED to educate us all about fluid. It does strike one as patently ridiculous - thirst is good enough for every animal in the world, it's good enough for us...!

Looking ahead to next week

We really hope you have enjoyed this series! Next week we will focus more on running again as we preview the USA Men's Olympic Marathon trials and the NYC Marathon. It will be a week of running-related posts, so be sure to join us for the discussion and analysis!

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
Part III: Comparison of laboratory and field studies, and implications for fluid intake

Chicago Marathon death - no evidence of dehydration

In the last few weeks, we have run a series of posts investigating the events of the 30th Chicago Marathon, where record high temperatures caused the early cancellation of the race, amid record numbers of medical cases and emergencies. We discussed the possible cause of these collapses, suggesting that blood pressure and unfamiliarity with the heat were more likely the cause of the problems than dehydration and heat stroke, which were being blamed for all the problems.

We also did a post looking at the sad death of Chad Schieber, a 35-year old policeman who collapsed at the 18 mile mark and was later pronounced dead. Initially, the death was blamed on the heat and dehydration, but the initial autopsy found that Schieber suffered from a relatively common condition known as mitral valve prolapse. This condition, which is reported to affect 2% o the US population, does not by itself cause death in athletes, but has been implicated as a contributing factor to a potentially fatal arrhythmia.

New reports from the Medical Examiner

Now, just over two weeks later, the medical examiner's office has released further reports that "tests show no evidence of dehydration." According to Nancy Jones, the Cook County Chief Medical Examiner, dehydration can now be ruled out.

In response to our initial post on this death, where we suggested that it would be prudent and wise to hold the verdict until the autopsy result was announced, a few medical doctors wrote in saying that dehydration may contribute to the death. A few things in response - firstly, there's no evidence for that, it's pure supposition, because until one has actually studied the physiological response of people with mitral valve prolapse to graded dehydration DURING exercise (very important - it must be measured during exercise, as we've tried to emphasize in other posts recently), it's supposition to say that "dehydration contributed to the death".

But in this latest news, what are referred to as "dehydration tests" (presumably measures of body water) have confirmed this. Again, there's an issue around what constitutes "dehydration" - is it 1% body weight loss, is it a change in total body water of "X" %? That's unclear. But it does seem more and more that it was not a dehydration issue.

The new blame-game - not dehydration but lack of maps

Instead, it seems that attention is now being turned to the ambulance and the length of time it took to get Schieber from the course to the nearest hospital. According to one news report,

"You can actually see the University of Illinois at Chicago Hospital from where Schieber collapsed. It takes a minute and a half to get there. But the ambulance took between 8 1/2 and 14 minutes."

Unfortunately, this type of thing does tend to happen after such tragic events. But hindsight is always 20/20, and it's a shame that blame gets assigned so easily. Not that one should gloss over potentially critical details, lest we forget to learn from (possible) mistakes

But this does make me think back to the Comrades Marathon this year, where a runner collapsed perhaps 300m from the finish line. His fellow runners, seeing him lying there without medical support, picked him up and carried him across the line. Sadly, he was pronounced dead at the scene. But then even worse for all those involved, the runners who had attempted to help him were accused of contributing to his death! There was some physiological basis for this (you can read this in a post we did on it here), but it was a shame to have to play that game at such a time, in such a public forum. The death of Chad Schieber seems the same.

Our future posts

To wrap up on a related theme, we're very much into the thick of our series on Fluid Intake and Dehydration: Exploding the myths. We've recently looked at the comparison between laboratory and field studies, having previously explained how it was the advent of "science" into marketing that drove a good deal of research in lab studies. In Post IV of that series, we'll look at the physiology of thirst and what the body is actually defending, as we ask the question: "Is thirst enough and how does it work?"

So do join us for the completion of that series.

In other news, the New York City Marathon takes place on the first weekend in November, and many runners are no doubt hoping for much cooler temperatures. Regardless of what happens, you'll be able to read all the race insights and stories right here!

Join us then!
Ross and Jonathan

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

Fluid intake, dehydration and exercise Part II

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?"

Conclusion

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!