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
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
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