Welcome to the Science of Sport, where we bring you the second, third, and fourth level of analysis you will not find anywhere else.

Be it doping in sport, hot topics like Caster Semenya or Oscar Pistorius, or the dehydration myth, we try to translate the science behind sports and sports performance.

Consider a donation if you like what you see here!

Did you know?
We published The Runner's Body in May 2009. With an average 4.4/5 stars on Amazon.com, it has been receiving positive reviews from runners and non-runners alike.

Available for the Kindle and also in the traditional paper back. It will make a great gift for the runners you know, and helps support our work here on The Science of Sport.

Sunday, October 14, 2007

The uniqueness of exercise: When "abnormal" is normal

Over the last week, we've been running two simultaneous threads here on The Science of Sport. There's our Women vs. Men in running series, which has looked at the differences in performance between the genders and whether women will ever beat men in running events (there's one more part to come in that series, I haven't forgotten!). Then the second has been our post-Chicago Marathon analysis. In case you haven't heard yet, the Chicago Marathon last Sunday was heavily affected by high temperatures - the hottest race in the 30 year history of the event.

And we've been discussing the merits of the arguments put forward in the media after the race. Quite frankly, there has been some highly irresponsible reporting, ranging from premature assumptions around the sad death of Chad Schieber to aggressive attacks on race organizers for failing to provide water, and even for failing to cancel the race at the start. We're not particularly concerned with picking apart these arguments now, that's been partly done in recent posts. And we'll discuss it further in future posts.

But this post is about an interesting observation we've made regarding how people respond to these kinds of "physiological failures". Because what tends to happen is that people look at exercise and something like heat stroke and apply regular, text-book physiology to attempt to interpret the situation. But unfortunately, exercise is a little more complex than that...

A snap-shot from exercise - a worrying picture?

Let's take a hypothetical runner, call her Camille, aged 32. You take a physiological snapshot of Camille during the middle of an easy three hour training run. You see the following:

  • Heart rate = 160bpm
  • Breathing rate = 32 breaths/minute
  • Minute ventilation = 15L/min
  • Cardiac output = 16L/min
One can easily recognize that all these measurements are massively elevated compared to "normal". In fact, if you showed a doctor just these stats, he'd make a diagnosis that this person is hyperventilating, has a dangerously elevated heart rate and that their cardiorespiratory measurements indicate someone in severe distress. This person is likely to be admitted to ICU for observations.

But hopefully, once you tell the MD that Camille was running at 6min/km, he'd recognize that these very abnormal measurments are in fact quite normal.

The plot thickens - adding to the complexity

That's the easy part. Now let's look at a few more measurements from our "snapshot":

  • Body temperature = 38.9 degrees Celsius (102F)
  • Sweat rate = 1.5L/hour
  • Skin temperature = 29 degrees Celsius
Again, everything is elevated. The doctor is likely to see this body temperature, the fact that Camille is sweating profusely and has a high skin temperature and suggest that perhaps Camille needs a day or two of bedrest. If you are reading this at your desk and your body temperature is 38.9C, then most certainly, you would know all about it! You'd feel shivers and cold, even though you're burning up, because you'd likely have a fever.

But Camille doesn't even notice this - she feels comfortable and wouldn't know that on any other occasion, she'd be diagnosed as very ill. Again, however, MOST doctors and 'experts' will recognize that because Camille is exercising, these values are actually normal! They contradict everything you've read about "normal" in a textbook, but that's fine, because exercise changes "normal". So we make a concession that sometimes, a physiological measurement that appears dangerous is in fact quite safe. So then, why do we not have the same attitude towards the next batch of measurements....?

The acid-test: Is this measurement "abnormal"?

So Camille finishes her run and jumps on a scale.

  • Her body weight = 62 kg. Her normal body weight, by the way, is 64 kg.

Now suddenly, we have panic stations! Camille has lost 2kg! That equates to about 3% of her body weight. We immediately jump to the conclusion that she must be dehydrated! How could this happen? She had access to some water while she was running, so why didn't she drink enough to replace this? The run took her 3 hours and so assuming her sweat rate was 1.5L/hour, she sweated 4.5 L during her run! But for some reason, she only drank 2.5 L, and lost weight. "Did she not know that this would impair performance and put her at risk of overheating?" is the cry of people who hold to the theory that dehydration is dangerous during exercise.

"What might have possessed Camille to drink less than she should have? Does she not realise how dangerous dehydration is?" they say. After all, even 2% dehydration impairs performance by 20%. Or that's what the scientists from the Gatorade Sports Science Institute say!

The danger of making physiological interpretations out of context

Hang on a moment here - the first two sets of measurements showed us that when we exercise, what is "normal" is different during exercise. Yet the second we start to talk about dehydration and body weight, suddenly, there's no concession made for the possibility that perhaps during exercise, a slight body weight change is normal...Think about that for a while - we allow heart rate to rise, no problem. Cardiac output, breathing rate - no problem. Body temperature that normally suggests fever - no need to worry. But as soon as body weight is different, that's a crisis!

And the reason this perception exists is because we have been told that this is the case, by a body of research that may be sound in principle, but fails to acknowledge that during exercise, the body is quite content with change...it allows the physiology to change substantially - the cardiovascular system, the blood pressure, the body temperature, everything is so tightly regulated, yet during exercise they all change. No one has ever suggested that we should aim to keep our body temperature down to 37 when we train!

Yet for some reason, we've been told that a fall in body weight is dangerous and detrimental. There are a number of studies that have 'proved' this, but they are all fraught with potential problems. These problems are not technical and nor are they related to the study design - they are good scientific studies. But they are done in laboratories, and their relevance to what really happens during exercise is so limited. They have funded much of the research of those scientists who perform the studies. Given the funding source, one can hardly be surprised that they conclude that drinking more is beneficial.

But effectively, what these scientists have done is make the same mistake a doctor would be making if he diagnosed Camille as having a fever based on her high body temperature!

Introducing a debate around the issues

So there simply is no basis for the commonly-held belief that "any level of dehydration impairs performance". The most basic evaluation of this claim is the FACT that elite marathon runners, who win the races, are THE MOST DEHYDRATED. Elite athletes almost always lose 2 or more kilograms in a marathon. Now, the claim that has been made is that "2% body weight causes a 10% impairment in performance" (this is a true claim I heard at a scientific conference once. The actual figure varies, but it's always between 5 and 20%!). So let's say you're Haile Gebrselassie - you've just run 2:04:26. But you were 2% "dehydrated" when you finished. So that means if you'd just drunk more water you'd have run 1:53:07! Anyone else think something is wrong with this picture?

But then what happens is we say "the elite are different", everyone else has to prevent dehydration. Yet for that claim, there is no proof. No proof that people who lose fluid fail to sweat enough, no proof that dehydration leads to heat stroke, no proof that it impairs performance IN FIELD EXERCISE. There's proof in the lab, allright, provided you don't use a fan. Then it seems to make a difference. But not for out of doors exercise.

Over the course of the next few weeks, we'll delve a little deeper into these concepts and look at the whole dehydration and fluid intake issue. I realise that there is a substantial part of the argument missing from THIS POST, but this is just the philosophical starting point to the discussion, which we'll go into it step by step over the coming weeks.

Join us then!

Read our post-race report from Chicago and our explanation for the high number of collapsed runners here


Lei said...

Great post, with a lot to think about.

The thing with the other stats (heart rate, sweat rate, etc) is that there are healthy ranges defined for when an individual is *resting*, and doctors obviously expect those stats to rise during exercise. In fact, it *helps* for those stats to rise during exercise. Heart rate, for example, to supply the muscles with more oxygen.

The problem with dehydration is that it impairs performance. If you're sitting at your desk and you're 2kg dehydrated, that's pretty bad. But if you're running a marathon and 2kg dehydrated, that's worse, because of all the extra stress your body's going through.

When exercising, dehydration impairs performance. But higher heart rates, breathing rates, cardiac outputs, etc... they're all benefitial.

Ross Tucker and Jonathan Dugas said...

Hi Lei

Thanks for the feedback! Much appreciated!

You're quite correct - the responses that are "abnormal" at rest are actually helpful during exercise. I did not make that point in the first post, so thank you for helping me out by making it!

I do just want to emphasize, though, that there's no evidence that 2kg of fluid loss (dehydration) is detrimental to performance in any way during normal, out-of-doors competition. It's only been found in lab situations, and there are some very basic reasons for this.

But the reason you believe that dehydration affects performance and is unhealthy is because that's what the companies who sell sports drinks have said - they fund their own research studies and conclude that more drinking is better!

So what I was trying to emphasize is that just like elevated heart rates, body temperatures and breathing rates must all be interpreted within the context of the exercise, so too, dehydration is not necessarily dangerous.

And that's what we'll do our best at looking into over the coming weeks, slowly but surely trying to explain the arguments and give the evidence.

Thanks, and keep running!

Unknown said...

Hi there - I 'ran' Chicago and was born with a mitral valve prolapse, so I very much appreciate your detailed analysis of the race happenings.
I wanted to relate my story and see if you have had this experience (although I hope no-one else does) during distance sport.
I have completed two marathons, but have not been able to absorb any fluids after I'd been on the course for 4 hours. The liquids (only water or water with electrolyte tablets, I can't stomach sugar or performance drinks) I do take in help me feel great, and then suddenly they are exiting rapidly (from the point of intake).
Have you witnessed this before, understand the root cause of this, have any pointers for me? As far as my limited understanding of this phenomenon, I'm only able to guess that I need to get my time under 4 hours and then try again (I'm at a 4:30 pace now, although I didn't come close to that in Chitown)... or can I train my body to digest Gu/Gatorade and use that to improve this? Any thoughts or insights that you have would be appreciated.
thanks much!

Ross Tucker and Jonathan Dugas said...

In response to Jennergy's comment:

Thanks for sharing this with us and participating here on the Science of Sport.

First we must clarify some of the concepts and details here. As you explain it, the problem sounds like not one of absorption but rather one of emptying. In other words you fail to empty the contents of your stomach.

At first glance we would hypothesize that you are ingesting too much fluid. However, to help get to the bottom of this, please can you tell us approximately how much fluid you are drinking, or how frequently you are ingesting fluid?

Second, with regards to your mitral valve propapse, there is nothing to suggest that your problems with gastric emptying are related to your MVP. Rather it is likely related to the volume of fluid you are consuming. However, we will leave for now until you write back to let us know what kind of volumes you are ingesting during the marathon.

Thanks again for commenting, and we look forward to hearing back from you so we can continue this discussion!

Kind Regards,

Unknown said...

Thanks so much for your quick reply, Jonathan!

I wasn't worried about the MVP, I haven't thought about it for decades until the news of Chad Scheiber's death in Chicago. The only thing that made me think to ask help on this is that my fluid absorption could cause other issues.

I carried with me a 20 oz bottle (in order to use the electrolyte tabs I was also carrying) and drank two bottles between the start and mile 12.

After mile 12, I drank about 4 oz of plain water, and then drank another 6 oz or so of electrolytes between mile 14 and 17, at mile 18 i had 4oz water and at mile 19 it jumped ship. I started sipping water (maybe 4oz total, just to get my mouth refreshed)and released that at mile 23 (in front of some medics who checked my pulse and bp and found them to be good: 120bpm and 120/70) and then again I took in about 6 oz water and released it and more at the finish. each evacuation was more forceful than the previous, but I always felt good afterward and didn't experience any sloshing feelings in my stomach.
Overall I feel that my intake was pretty low given the day and also because I have a pretty high sweat rate, which slowed dramatically around mile 19.

Interestingly enough, I don't have a sensitive stomach and, prior to my marathon experience, I don't recall any vomiting since I was about 6 years old.

Now that I re-read this, this is pretty gross, and I really appreciate your willingness to dive into this issue as it's really confusing me and I hope there's a way to train my stomach as well as my legs/heart/lungs.

If you ever want to study rowers, let me know as I'm probably going back to sculling as a primary sport for a while and work on my speed in the 10K and half.

Lei said...

Thanks for your reply, Ross. Is your argument that the 2kg dehydration is good because it makes the body lighter? I would disagree because, if that were the case, elite runners would aim to start the race 2kg-dehydrated, and then maintain that throughout the race. No, I still believe that elite runners may lose 2kg out of negligence, or simply the impracticality of drinking that much on the run.

Ross Tucker and Jonathan Dugas said...

Hi Lei

Not quite, I am saying that the 2kg that the elite runners lose (and the other 90% of runners, by the way, they're just as important - it's only about 5% who don't lose weight, and they are the ones who end up very ill and often die, by the way) is NORMAL

So it's not a case of it being good or bad, but it is certainly a case of the weight loss of a few kg being normal. For example, if you go out and run, and you do an easy 40 minute training jog, and you DON'T drink, I can pretty much guarantee you that you will LOSE about 1kg, maybe 2kg. Now, is that dangerous? Do you suffer because of it? Of course not! Your body is quite capable of losing some weight during exercise, because as soon as you finish, it can replace that weight/fluid and you'll be fine.

The only time fluid becomes a problem is when you become really, really thirsty. Then it's the sensation of thirst that is bad for you. But the approach that you are advocating (along with most other people, incidentally), is wrong - it assumes that the 90% of people who drink to thirst and lose weight are somehow at risk. It's just untrue. I can almost guarantee you that if you run for 30minutes and just sip water once, you'll lose weight and feel 100% fine afterwards. Weight loss and fluid intake - not vital to health.

The concept of dehydration being bad is a myth. We're attacking that one in the new series we're running, with discussions of the evidence. I know you're a medical student, so I'm sure you'll relate to much of the science. Will be good to hear your views once we've dissected the whole argument.


Ross Tucker and Jonathan Dugas said...

Hi again Jennergy,

It would seem that your fluid ingestion was indeed relatively low. By my calculations I have you ingesting 1.5 L (54 oz) before you vomited. That fluid was ingested over 19 miles. I suspect you were not running faster than 6 min per mile, and therefore your hourly volume was some amount less than 750 mL per hour, and perhaps even less than 500 mL per hour.

In addition you say that you did not experience any bloating or feelings of fullness, and that would suggest that some of the fluid was emptying from your stomach.

Were you accustomed to these electroylte tablets you were taking? For example, had you used them in training or during another marathon?

Also, what was your running speed, and at what intensity were you running (i.e. hard, medium, easy effort)?

It sounds like an isolated incident not related to exercise or the heat and instead perhaps it resulted from a stomach bug, or from the tablets you used.

Kind Regards,