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Wednesday, October 10, 2007

Investigating heatstroke: How fast do you actually have to run?

Recent marathon running news has been dominated by the Chicago Marathon and the heat which forced the closure of the race at around 11h30. There has been a great deal of reaction to this, with many people suggesting that even attempting to run in such conditions was "suicidal". Take this quote from one particularly scathing commenter:

"If you are foolhardy enough to run a marathon when the temperature outdoors is up to 88 degrees, then it is your fault, no one else's...when 35,000 people jump off a ledge, you can only catch so many in safety nets. The rest are going to fall. This is a professional competition that the public sometimes confuses with a company picnic."

But is it heatstroke?

Yet the other day, we featured a post which suggested that actually running yourself into true heatstroke is incredibly rare. We said that one should be cautious about making a knee-jerk reaction that just because it was hot, heatstroke and heat illness was the problem. And here it's really important to define the conditions very accurately - when we refer to heat stroke, we talk of a potentially fatal condition where the body temperature rises excessively because either:
a) The body is unable to lose the heat it produces, or;
b) The body is producing too much heat

The physics of heat stroke - what does it take to heat up?

Without going into massive detail on the physics of heat transfer, your body temperature at any time is dependent on heat production versus heat loss. Heat production, in the case of marathon running, comes from muscle contraction and is directly proportional to how fast you run. In otherwords, if you double your speed, you produce twice as much heat.

Heat loss is a little more complex, because it depends on evaporation, convection and radiation. Now, the specifics of each are complex and we won't go into them, but let's just say that the environment is the crucial factor that affects heat loss - high humidity prevents evaporation, high air temperatures prevent both evaporation and convection from cooling the body.

The point of this is that if we know the environmental conditions, and we know how fast a runner is going, then we can calculate what their body temperatures would be doing during the marathon. And this is what we have attempted to do in the table below, where we look at a few cases. The point of doing this is to illustrate just how difficult it is to get heat stroke, unless you are a big person running really fast.

The values calculated in this table, incidentally, are calculated based on a number of equations that have been published in the scientific literature. And while I acknowledge that they may have some small errors, they have generally been found to be very accurate and quite valid, so we use them to make a point.

Case 1 - a 60 kg elite runner going for a world record in the marathon

Take an elite Kenyan or Ethiopian runner. They tend to be very light (60kg is assumed in this case), and we're assuming the air temperature to be 15 degrees celsius. The runner goes off and runs at 2:55/km. Under these conditions, the heat production is equal to 6232 kj/hour, and heat loss is equal to 8346 kJ/hour. Therefore, this runner can lose more heat than he gains, and under normal physiological circumstances, he has NO THERMAL LIMIT. In other words, he can run without concern for over-heating. Note that there are occasions when heat stroke does occur in this kind of athlete, but that is due to something else, which we will address lower down in this post.

Case 2A - 60 kg elite going for a world record, but in hot and humid conditions

Now we have the same 60kg elite runner, but the air temperature is 28 degrees Celsius. Our runner sets off at 3:00/km, hoping for a fast time, but the problem is that because it's so hot and humid, his heat loss is vastly reduced compared to our previous example (Case 1). Therefore, our calculations predict that this runner will NOT BE ABLE TO FINISH THE MARATHON IN THIS PACE! He will gradually gain heat, because heat loss is LESS THAN heat production and his body temperature will hit 41 degrees after 1 hour 43 minutes. Therefore, this runner would be a candidate for heat stroke.

Case 2B - the 60kg elite does not get heat stroke. Why? Because he slows down!

So having just said that our 60kg elite could get heat stroke, we ask when was the last time you saw this? If it was going to happen, surely on Sunday in Chicago, it would have. Yet none of the elite runners overheated. Why? Because they slow down. Case 2B in the table above shows this clearly. If our 60kg runner is in the SAME conditions as before, but instead of running 3:00/km, he now goes 3:06/km (which is a 2:10:50 marathon), we then see that he can now finish the marathon, because his body would only reach 41 degrees after two hours and 18 minutes.

So it would be a close run thing, he would certainly be very hot and probably feel terrible at the end, from the exertion, but he would NOT be in danger of overheating. His marathon time would be around 2:10:50, and his body temperature in the mid-40's, which is hot, but safe. That is the difference made by slowing down by 6 seconds per kilometer!

The really interesting thing here is that these conditions are the same as in Chicago - 28 degrees, give or take a few. And the winning time was 2:11:11. And yes, I acknowledge that all kinds of other factors affect performance, but it's interesting to note that the prediction of the equations regarding temperature are borne out by performances.

Case 3A - an 80kg runner aiming to break 3 hours

Now we move onto a runner who perhaps more of you can relate to - weighing in at 80kg, his goal is to crack 3 hours (pace of 4:15/km). First, for Case 3A, we look at air temperatures of 20 degrees. Here, you can see that our runner is in no danger at all. His heat production is equal to 5293 kJ/hour and total heat loss is equal to 7207 kJ/hour. Therefore, running at 20 degrees celsius, an 80kg runner will have no risk of heat stroke or hyperthermia.

Case 3B - our 80kg runner aiming to break 3 hours on a hot day

Now, consider the same runner running at 30 degrees, as it was later in the day in Chicago. Our 80kg runner is still trying to run sub-3 hour pace (4:15/km). But now, because it's hotter, it's a lot more difficult to lose the heat he produces, and our equations show that he would overheat, with a body temperature of 41 degrees reached in 1 hour 58 minutes. So that means heat stroke could occur in this runner if he ran for this long at this speed.

Case 3C - 80 kg runner who slows down slightly in the heat

However, as I have no doubt all of you know, if you run at 30 degrees, and your best performance is 3 hours, you DON'T run a 3 hour marathon in 30 degree heat. So let's see what happens if our runner slows down, as for Case 2B. Assume he slows down to a 3:20 marathon time (4:44/km). Now, we see that because his HEAT PRODUCTION is much lower, it takes much longer for him to store heat and lift his temperature. In fact, we work out that it would take 5 hours 9 minutes to get up to 41 degrees. Therefore, in a marathon, there is no risk, so he's able to run his 3:19:43.

Case 4 - 100 kg runner aiming for four hours on a hot day

Now we turn to the kind of person who is most likely to feature in the medical tent. I mean this with the greatest of respect, but from experience having worked in the medical tents of Ironman triathlons and Comrades and Two Oceans Ultra-marathons, there is no doubt that MOST of the people in the medical tent are the heavier runners who run times greater than 4 hours for the marathon.

So if you take a 100kg runner, who runs at 5:41/km pace (4 hour marathon), then you will see that even if the air temperature is 30 degrees and humidity 60%, this runner would be able to run the marathon in 3:39:43 without ever getting close to overheating. In fact, it would take this person 7 hours and 53 minutes to actually reach 41 degrees celsius!

Case 5 - A typical runner in the Chicago Marathon

Lastly, let's have a look at a typical runner, weighing say 85 kg and running the Chicago marathon in about 4 hours 30. This runner would be producing heat at a much lower rate than they could lose it. And therefore, there would be no danger. It is for this reason that I am highly sceptical that most of the runners treated on Sunday had any type of heat illness.

Who is most at risk?

From the above cases, I hope it's clear that the runner most at risk during running is the slightly heavier athlete (that is, someone weighing over 75kg) who is running at a fairly fast pace and doesn't slow down to accommodate the higher temperatures. The slower runner, who finishes in times over 4 hours, is far LESS LIKELY to be in any danger. Yet on Sunday, from the comments we've received, we've heard that the biggest problems seemed to be in the guys aiming for about 3h30 to 4h30, so that they were at between 15 and 24 miles after about 2 to 3 hours. We must emphasize that unless this runner is particularly large, he's simply not running fast enough to be in REAL danger of heatstroke. So instead, we're left to suggest that maybe Chicago wasn't heatstroke - people just felt really hot. But again, that's NOT the same as BEING hot, and that's been somewhat overlooked.

The alternative to heat illness - why such high attrition in the heat?

Instead, I would suggest that the reason it was so tough on Sunday is because the vast majority of people who ran the race were simply not accustomed to the heat. The race takes place in the early winter in Chicago and for most of its participants, temperatures higher than about 20 degrees would constitute a tough day out. So standing on a start line when it is closer to 25 degrees (80F) is a near impossible task.

We know from scientific research that heat acclimitazation makes massive differences to performance. The other day, I mentioned the story of the soldiers who were sent to South-East Asia and almost all lost consciousness, felt ill and were completely incapacitated on the first day. But within 3 days, they had recovered. What was discovered there is that on arrival, it was their blood pressures that were dropping in the heat - we know this happens and it is a possible explanation for why so many people collapsed in Chicago. This is not a life-threatening condition, it simply requires some rest and then the athlete is 100% fine again.

Finally, the heat poses a huge challenge because of how it alters the perceptions of the runner. I am not for a second suggesting that these people could have continued, because clearly, some tried and failed. But to suggest it was because they overheated is not correct. They were hot, yes, and the felt like they had over-heated. But how one feels is not always how one IS, and all it takes is some experience and training in those conditions, and most of those runners would have been able to finish the race, in slower than usual times, but without any danger.

Last word on heatstroke

So having said that it's just about impossible for a runner to get heatstroke, unless they are heavy and running really fast, we still have a question - why does heatstroke occur? This is something we will look at in a future post, because it's a really fascinating question. But let me leave you with the observation that MOST of the heat stroke cases in the last ten or so years have happened in COLD conditions. Believe it or not, it's true. One case, which comes to mind, saw a runner collapse after 19km of a race where the temperature was 4.1 degree Celsius (that is 39Fahrenheit). Go figure...but we'll look at that in more detail in the future.



Anonymous said...

Hi - Very interesting stuff. I'm curious though - how did you determine that it was the slower BOP runners who were the ones being treated? Anecdotal evidence at least partially suggests that the people who were dropping in the street were those trying to stick to their fast paces in order to get their PR. I ran on Sunday, ran about a 10 min/mile pace, wound up surrounded by walkers because I had to stop a couple of times to buy water, I didn't see the mass carnage that I heard described by those who were closer to a 3-3:30 pace. Thanks! Again, fascinating information you present here.

Ross Tucker and Jonathan Dugas said...

Hi there

Thanks for the reply, it's great to hear from you!

Good question, I think that the answer is that "slower" is relative, and perhaps I should have used a wider range to illustrate my point. You are most certainly correct - the runners who "ran" into trouble are the ones who do not slow sufficiently. This was illustrated in the table above. Now, that would be the case for anyone. Had the race winner, Patrick Ivuti, attempted to run 2:08 pace, he too would have been in difficulty. But they slowed enough to avoid this.

So I guess the answer to your question is that it doesn't necessarily have to be the slower ones. But it certainly is the runners who finish outside the top 100 or in the faster times. That's been my experience from previous races where I've worked and I suspect it to be the same at Chicago. usually, the 'traffic' in the marathon gets heavier towards the middle of the event onwards.

That would mean for a marathon, from about 3h30, but on a hot day, around 3 hours. The example in the table of an 80kg man trying to run this pace illustrates it - that is the guy who will end up worst, but I still don't believe he'll actually overheat. Instead, it's just a case of feeling terrible and hot.

Also, the blood pressure theory that I put forward would be most likely to happen after someone stops having exercised. So my theory is that a lot of guys went out too hard, stopped because they simply felt dreadful, and then walked and stopped, at which point the BP dropped causing loss of consciousness and the resultant carnage. So the heat contributes no doubt, but it's because the race is run in winter and virtually no one is adapted to it, unfortunately.

Thanks for reading!

Anonymous said...

The race officials were discussing calling off the race 3 hours in. So the medical emergencies probably occurred 1 1/2 to 2 1/2 hours into the race. It's likely these were sub-4 hour runners. I searched for news accounts for the 49 runners who were hospitalized. I found 2. Matthew McQuality collapsed after the 1/2 marathon mark averaging a 7:44 pace with heatstroke. He previously ran a marathon averaging a 8:22 pace. Dawn Dowell blacked out and was hospitalized after reaching the 30k mat. Her average pace was at the 1/2 marathon mark was 7:46. Jim Harding the 3:40 pacer, maintained a perfect 8:23 pace past the 30k mark (18.6 miles) before his DNF.

There are a lot of angry runners complaining about the lack of water at some aid stations. I am skeptical about how much effect that would have on performance and the risk of heat exhaustion. The studies that Tim Noakes has done correlates heat exhaustion to running pace rather than dehydration as % of body weight lost. In fact, I think the water shortages may have reduced the incidences of hyponatremia. Do you an opinion?

Ross Tucker and Jonathan Dugas said...

Hi there

You make an excellent point, thank you! And thanks for the really interesting information on the collapsed runners.

I can only agree that the problems will happen in the runners who are pushing faster than they are capable of, or who fail to slow down when they should in the hot conditions. There's absolutely no question that running in the heat affects performance, so the pace should be reduced. And more than this, you're quite correct - the biggest factor determining body temperature is exercise intensity (running speed, in this case). Therefore, the simple act of slowing down would prevent temperature from rising.

But I really must emphasize the point that until the body temperature is actually measured, you cannot diagnose it as heat stroke. A lot of athletes collapse and did on Sunday, and the knee jerk reaction was "Heatstroke", when in fact this may not have been true. As I wrote, I would be very surprised if there were more than a few - perhaps this Matthew McQuality was one - I'd like to know his body temperature though.

People often mistake feeling hot for being hot - not the same thing, and I think that we must be cautious about jumping to the conclusion that it was heat stroke. Heat stroke does happen, but the circumstances are some what mysterious. This is a post that we will do in the future, no doubt. But I'm very sceptical of the number of "heat strokes" in this marathon - something has to fail physiologically for this to occur, and that means either behaviourally or metabolically, there's a failure.

On the note of the dehydration, yes, the concept that drinking more would prevent the body from overheating is regrettable, because it's simply not true. Apart from the fact that I don't think many of these people had heat stroke to begin with, there's little evidence that remaining fully hydrated will prevent the temperature from rising.

Don't forget the elite runners drink the least of everyone - they finish the race MOST dehydrated. Yet, somehow, they almost never overheat (nor does anyone else, mind you). That doesn't add up - surely if you are very dehydrated you would be worst off? But the elite don't drink nearly as much as the guy at the back, even though they need it more, in theory.

That's also a post we should cover in the future! Thanks for your excellent information, I hope more comes to light so we can start understanding what happened here.


Ross Tucker and Jonathan Dugas said...

Thanks for both of you who have commented on this topic. We hope more readers chime in to continue this debate.

As Ross mentioned, the fact that so many people think being dehydrated causes an elevated body temperature is indeed regrettable. This concept is perpetuated primarily because doctors and sports physicians are taught that 1) dehydration leads to an impaired cardiovascular system, and 2) an impaired CV system cannot dissipate heat from the body. The result of these two things, they are taught, is an elevated body temp and "heat illness."

In reality, the scientific evidence shows the following three things:

1) The rectal temps of collapsed runners are not different than runners who do not collapse. Therefore the collapse must be caused by something else other than an elevated temperature, otherwise everyone with an elevated temperature would collapse, and this is not the case.

2) More than 85% of runners who collapse do so when they stop running. Dehydration cannot explain this since if it leads to more strain on the CV system, then this would manifest more profoundly during exercise when more strain is placed on the system. This would mean that they collapse while running, and this is not the case.

3) It has been measured in the collapsed runners that they have "postural hypotension," or low blood pressure when standing. However when they lay down flat or in a head down/feet up position, their blood pressures are normal.

So therefore it cannot be solely because of an elevated temperature that runners collapse, and more importantly it seems to be from postural hypotension that occurs when they stop running.

Finally, at the New Zealand Ironman triathlon they had a relatively high incidence of hyponatremia. Their approach was to, as you suspected, reduce the number of aid stations on the cycle and run legs in the next year's event. The number of cases of hyponatremia plummeted as a result of this intervention.

It is our opinion that a simple intervention such as reducing the water stations is not feasible in America, because any race organizer that reduces the aid stations will likely be criticized harshly by the participants since the runners hold such strong beliefs that dehydration is a serious health risk.

This is a highly sensitive and widely debated topic---even within Exercise Science---and we expect much debate on this one.

Thanks again for your comments and please keep them coming!

Kind Regards,

Anonymous said...

Along with Noakes and your ideas, the idea of training in a slightly dehydrated state in order to adapt to the conditions does make sense.
However, will training in a slightly to moderate dehydrated state (10-15% loss of water as described by Noakes) be detrimental if your goal is to run in a colder environment? I understand that training for the Badwater Ultramarathon would benefit greatly from adaption to some form of dehydration, but what about colder or even sub-freezing races? Will dehydration adaption help?

Ross Tucker and Jonathan Dugas said...

Hi Anonymous, and thanks for your comments here on the Science of Sport.

First, body weight losses of 10-15% are relatively large and not seen that often during normal endurance exercise. Even during ultra-distance events we still do not see double digit weight losses especially when the athletes have access to fluids on the course.

Second, it is important to note that the body will adapt to whatever stresses you impose on it---assuming that those stresses are strong enough to create adaptations. The "strength" of a stressor is a product of its intensity, frequency, and duration. So providing that some stressor occurs in the correct intensity, frequency, and duration for the body to deem to worthy for adaptation, then the system will respond with adaptations.

Second, the adaptations will be specific to that stressor. So, if you want your body to adapt to exercising in a grossly dehydrated state, then one must exercise in a dehydrated state. However. . .if completing a very long race is your ultimate goal, then training in a dehydrated state is not likely to allow you to complete meaningful training that will help you go the distance.

The put it another way, you will likely feel like such crap as a result of not drinking any fluids that you will not feel like running 100+ miles a week to train for an ultra-run!

Rather, and I know I am going on here, humans are adapted to run long distances and still protect the fluid balance in their bodies. Therefore I would recommend concentrating on the quality of your running and worry less about the fluid aspect!

Finally, I am not sure I understand your link between the cold weather running and the dehydration. If you want to adapt to the cold, you can and that is entirely possible. However, perhaps you can elaborate on what you mean?

Thanks again for the comments and questions!

Kind Regards,

Anonymous said...

So would you tend to agree (or even theorize) that the MOP and BOP racers weren't the ones dropping, even though they might have felt parched and crappy due to lack of fluids? It sounds like you're saying it's those who started out too fast, then slowed down, and not those who slowed down much earlier. I just find this interesting, because the media (and most people!) keep talking about "all those out-of-shape people who shouldn't have even been there in the first place", assuming that the slow people were the ones needing medical attention, which isn't what I saw.

Also, separating dehydration from elevated body temp, the lack of water (and very high humidity) I assume would lead to dehydration, which would still lead to compromised performance, yes? I've read studies that show the impact that insufficient water intake can have on performance, and it seems significant.

Finally (sorry for all the questions) - about hyponatremia, since this is in part an electrolyte imbalance, could it be caused by perhaps not an intake of too much water, but perhaps water in the absence of other fluids? Another issue has been the lack of Gatorade on the course, so thirsty runners only had water to drink, unless they brought along their own electrolyte supplements. I generally try to take in more gatorade or the like during longer races, especially when it's hot, because otherwise I start to exude the smell of ammonia, which I guess is another point altogether.

In any case, thanks for the earlier reply!

Ross Tucker and Jonathan Dugas said...

Hi Anonymous

Thank you for the reply and follow up questions. I'll do my best to answer them.

Firstly, I'm not convinced that only MOP and BOP runners were not the ones dropping. My experience in race medical tents is that most of the runners who require medical attention are those who run the marathon in betwen 3h30 and 4h30. I guess one would call that MOP, but in SA, the cut off time is 5 hours in a marathon, and so I guess my perception is that they are BOP. It seems to me, from your comments and what I have read, that most of the people who had problems on the course were those in this group? If that is the case, then perhaps I can 'unconfuse' myself by saying the following:

1. The runner most likely to actually develop heat stroke is the elite runner (running sub 2h15), or one who is pushing for a good time and also happens to be a larger runner. So an 80+ kg runner aiming for 3h15 is a marginal case. The much slower runners are almost in ZERO danger of heatstroke, which is what I was emphasizing in the table of this post.

2. HOWEVER, the runner who is most likely to end up in the medical tent at the finish line is the slower runner. And by slower, I mean one who will finish in the second half of the race. By this, I would say anyone who runs over 4 hours. Now, there is a very obvious reason for this - they are exposed to the heat for almost twice as long as the elite, so it stands to reason that if you were going to feel lousy, it would be later on!

So I think we agree that the elite guys and heavy fast ones are in danger of real, true heatstroke, but the guy most likely to be unable to finish is the one finishing slower than 4 hours. This will hopefully resolve the current paradox?

Now, the second question, on dehydration. This is something we will definitely post on in the future. But, to give a summary now, there is VERY LITTLE evidence that dehydration (as we define it) leads to impaired performance. But I'll tell you what kills performance more than anything else - thirst. If you are thirsty, and you don't drink, then you suffer hugely. However, that is not the same as being dehydrated.

Many experiments have been done where athletes are allowed to drink voluntarily - when they do this, and have access to fluids when they want them, then what we find is that they usually drink about 60% o f their fluid losses. This means they end up losing about 2% of their body weight. So clinically speaking, these people are "dehydrated". But in reality, they are fine!

You see, during exercise, "normal" is very different from rest. In fact, the elite runners are always the MOST DEHYDRATED. They tend to lose about 3 to 4% of their body weight. Yet they are fine, and no one would dare suggest to Haile Gebrselassie that if he drank more, he'd be even faster! So we can learn a great lesson from the elite, that you DON'T HAVE TO DRINK TO REPLACE 100% OF WHAT YOU LOSE. There is simply no evidence for that approach. The body is quite happy to lose 3 to 5%, and there is zero evidence that performance is impaired when you do.

Now, final question, the issue of hyponatremia. Hyponatremia is without doubt a problem of excessive water intake. The problem is that Gatorade have told people that it's due to salt intake being too low as well. And that's why they say that if you drink Gatorade, which contains sodium, then it will help prevent hyponatremia. This is untrue, because the sodium content of Gatorade is so low that it just doesn't have the effect it needs to.

Basically, if you are going to replace all the sodium you lose in sweat, you'd have to drink something with the same sodium content as sweat - you may as well be bottling sweat and selling it! Now, they do have a new "high sodium" drink out, which I've seen advertised. But the reality is that you get hyponatremia because you drink too much. There are actually very many cases reported where people have ended up in hospital and even died because they drank too much GATORADE! Not water, but Gatorade - it doesn't protect you, but they say drink it as much as you can, because every time you do, their bottom line is boosted. It's a cynical view, but unfortunately, the true on.

So when you look at research, you need to ask "who funded this research?". You will find that ALL the studies that show that dehydration is dangerous and that Gatorade is good for you are FUNDED BY GATORADE, and CONDUCTED BY THE GATORADE SPORTS SCIENCE INSTITUTE.

So there's a massive conflict of interest there. I'm reminded of the tobacco industry in the 60's, 70's and 80's. Tobacco was a glamour product, and the people telling you this were the people who sold it in cigarettes! But over time, the truth came out...

Anyway, this is such a vast topic and one that we will certainly address in more detail in the future in posts here.

Thanks for your comments!

Ross Tucker and Jonathan Dugas said...

Not sure which "Anonymous" poster is which (!), but in reply to the comment on 12 October starting with, "So would you tend to agree. . "

First and foremost, hyponatremia is NOT an electrolyte imbalance. Rather it is a disorder of fluid balance in which there is too much water in the fluid outside the cells, otherwise called the extra-cellular fluid, or the plasma (the water component of the blood).

The sports drink companies will tell you that if you ingest sodium during exercise you will be fine, but this is false. They will also tell you to replace all the weight you lose during exercise, which is, again, incorrect.

In fact, the main and most important concept in this debate on fluid replacement is that the body does NOT regulate the body weight. Instead it regulates the concentration of the blood.

In other words, when you run a marathon and sweat off some of your fluid volume, the body does not say to itself, "Ok, we lost one kg of fluid, let's therefore drink one Liter of fluid to replace that." Instead the body weight has nothing to do with this physiological mechanism and with fluid balance. Rather, your body controls the concentration of the blood, and it does this by telling you to drink just enough to lower the concentration. Then, when it rises again as you sweat, it tells you to drink again. Of course, the way it tells you to drink is by stimulating thirst.

The second important point in this debate is that thirst is a very sensitive and accurate fluid balance mechanism, and therefore is an entirely accurate gauge of how much fluid your body needs.

The thirst mechanism will optimize the amount of fluid you ingest, always preventing you from drinking too much OR too little fluid.

There is most certainly such thing as drinking too little, although only when performance is a desired outcome. If only finishing the race is the goal, then not drinking anything, although not advisable, does not pose any major health risk as your body will always protect you and make you slow down before you can suffer from any detrimental effects of not ingesting sufficient fluid.

If you abstain from drinking, will you feel like crap? Most certainly. Will you die from heat stroke? No, firstly because dehydration does not cause heat stroke but more importantly because your body will prohibit you from running sufficiently fast to get to that point.

In case it is not evident to all the readers, this is a much more complex issue and the interaction between all these concepts is not as clear cut as it seems on the surface. . .but we promise to try to clarify the science behind these issues, so be sure to tune in for those posts.

And thanks so much again for the great comments and questions, please keep them coming!

Kind Regards,

Tasha the Triathlon Goddess said...

LOL, sorry for the secrecy. I'm that anonymous poster with the questions about slower vs. faster runners and hyponatremia. I'll try to sign in under my blogger name. Anyway, what you said makes sense - that the faster people are more likely to have heatstroke, slower more likely to feel like crap and not finish.

I'm curious about the dehydration/thirst issue and how those are different. If you're thirsty and you don't drink, does that mean that your body is signaling a slide towards dehydration and if you don't heed that, then you hit dehydration at a level that can be impairing and/or dangerous? I know you said you'd discuss this later, so you don't have to get into it now.

I'm also curious about the electrolyte/Gatorade/hyponatremia issue. I do triathlons and have asked people with a medical background if there's a danger to taking in too many electrolytes in the form of salt tablets and the like. I was told that for the level of exercise we do (training for Ironman, for example), that it would be basically impossible to take in too much. We were talking about salt tabs and liquids (endurolytes, Nuun, etc.), but I took it to include Gatorade as well - and here I mean the gatorade endurance formula, which has 200mg sodium and 90mg potassium per serving. I don't use this in my everyday training (I use Infinit), but I train with it leading up to races when that's what'll be on the course. I also tend to err on the side of drinking too little when cycling, and have been told that that's impacting my performance. Ah, it's all so confusing. Thanks for trying to clear things up!

Hung-Kwong Ng said...

I found one more runner who was hospitalized. Dave Schwantes - 7:56 pace at halfway. He has averaged a 5:43 pace in a 5k. He collapsed at mile 19 with "heatstroke" and received 4 liters of saline.

Have you tried contacting Dr. Martin Lucenti at Northwestern Memorial Hospital. He is quoted as saying "One runner's body temperature had reached 107 degrees Fahrenheit (42 degrees Celsius) when he arrived at Northwestern". (FYI the London marathon in April experienced record heat and water shortages; one person died from hyponatremia)

While I agree with you there were probably very few cases of heatstroke and that low blood pressure caused quite a few runners to collapse - would you agree that runners with nausea, vomiting and cramping had heat exhaustion? Although, the symptoms of hyponatremia are simliar: nausea, vomiting, mental confusion and odd behavior.

Could gels and Gatorade create a hypertonic solution that is not being absorbed and cause nausea?

As the temperature rises, does the optimum osmolarity of ingested fluids go down? If so, it seems that simply diluting Gatorade would further dilute the blood of sodium. It would seem that in hot weather, it would be better to reduce the carbohyrdrates and increase the electroytes subject to an optimum osmolarity.

I plan to apply for Badwater if I have a good 48 hour run in November. Are oral thermometers or ear thermometers useful at all compared to rectal thermometers for monitoring core temperature? There are ingestible thermometers but they are not affordable.

Ross Tucker and Jonathan Dugas said...

This is turning into a great discussion, and the reason is that you are all asking such good and relevant questions.

In response to Tasha's comments:

"If you're thirsty and you don't drink, does that mean that your body is signaling a slide towards dehydration and if you don't heed that, then you hit dehydration at a level that can be impairing and/or dangerous?"

Your body will "defend" the concentration, or osmolality, of the blood plasma. When that concentration rises, it stimulates thirst. Therefore, if you do not ingest fluid when you are thirsty, the osmolality will continue to rise, and you will become more thirsty. There will be some physiological consequences to that, but most important is that your brain will slow you down before you suffer from any of these consequences.

So even it a loss of fluid results in a lesser ability to transfer heat from the core to the skin, you do not get too hot. Rather, your brain slows you down and keeps your temperature in a safe range.

"I was told that for the level of exercise we do (training for Ironman, for example), that it would be basically impossible to take in too much."

As with most things in life, moderation is the key!

First, it is entirely unnecessary to ingest sodium during exercise. Humans conserve sodium extremely well---as we have discussed here on the blog, these things are very tightly regulated in the body.

Second, hyponatremia is a disorder of fluid balance and not sodium balance.

So yes, there is such thing as taking in too much sodium. While it will reduce slightly the fall in sodium concentration that accompanies ingesting large volumes of fluid, it does not prevent the fall in sodium concentration.

In addition, such relatively large quantities of sodium will promote the retention of fluid. Why? Back to the osmolality. The body will defend the osmolality by hanging on to the fluid so that the osmilality does not go too high---which is the result of adding so much sodium to the plasma volume.

Finally, restricting fluid during exercise will likely affect your performance, although this effect has not been studied in cool or thermo-neutral environments.

However, and we just cannot stress this enough, drinking to thirst will optimize your fluid replacement, and will always keep you from drinking too much OR too little. You do not even need to factor in fluid replacement and do not need to think about it. Just follow your thirst and you will be fine.

Thanks again for the great questions, we are really enjoying this and please keep them coming!

Kind Regards,

Dave said...

@ Hung-Kwong Ng Hey! That was me. I'm the Dave Schwantes who passed out at mile 19. I am curious where you got my pace and treatment information from.