So, it's been a while since I last posted, but the hot weather has got me thinking about the subject of hydration. I would usually pop something up on Facebook, but this subject requires a little more depth than your average FB post. What's more, reading this blog post could save your life!
Considering hydration is just about the most basic requirement of human life, it's amazing that there should be so much confusion and complexity surrounding it.
I'm not going to try and tackle the oft-cited recommendations that everyone should be drinking 2 litres of water a day (water taken straight from holy wells, filtered through non-oxidative gold mesh and run over the thighs of a virgin princess - £10.99 a half-litre from your local health food store). But I will say this: your body needs approximately 3-4 litres of fluid everyday, but this fluid is ingested through all foodstuffs as well as 'drinks'. And drinks include coffee - it's not a diuretic (although you still don't want to overdo the caffeine.... note to self).
I'm going to concentrate on the hydration myths surrounding exercise. The two most common are:
1. 'Even a 2% drop in hydration can lead to a decline in performance of 10%–20%'.
2. 'Drink even if you're not thirsty; if you wait until you're thirsty it's too late, you're already dehydrated.'
These myths abound. They're repeated in running magazines, race guides, by personal trainers and 'nutritionists' – I've even heard doctors say it, reinforcing the idea that we are all capable of hearing something that sounds reasonable and accepting it without question.
The trouble is, if you stop and think about either of these statements for even a moment, neither sounds particularly reasonable.
Let's take the first. Where does this myth come from? Well, Gatorade actually. Gatorade was the first ever 'sports' drink. It was developed in the mid-Sixties through research with the Florida Gators NFL team - hence the name. The theory back then was that players were losing significant bodyweight during training/games due to fluid and electrolyte loss. They were also low on blood sugars – Gatorade replaced fluid, electrolytes and sugar, and the Gators became an incredibly successful team. Important to remember here that the drink contained sugar - that is, refined carbohydrates that your body can quickly convert to energy. And those carbs were combined in an easily digestible drink, mixed with water and salts to a level designed to optimise digestion across the gut - in other words, they were isotonic.
But let's forget the sugars for now, because that's what Gatorade seemed to do. Since then they've funded incredible amounts of research into sports drinks. One of their findings was the 2% / 10% rule. I'm not going to pick through the faults of Gatorade's research – suffice to say, much of their research looks fantastic on paper but is conducted in laboratories on running machines, which often bears little resemblance to the practise of going outside and actually running, with cooling air flow, the ability to speed up, or slow down etc etc. It's not necessary to find such faults anyway, because there's what philosophers call a 'black swan' in the room, beating it's wings and threatening to break arms.
(For more on the 'black swan problem', see here: http://en.wikipedia.org/wiki/Falsifiability#Inductive_categorical_inference)
The black swan in question is the fact that elite runners never consume the amount of fluids they're 'supposed' to. I would go so far to say that they wouldn't even if they were sponsored by Gatorade. In fact, research shows that runners who finish marathons in the quickest times are also those who have lost most body weight during the event. Elite runners often lose up to 3kg of fluids while running (around 5% of bodyweight). Yet they don't show a drop in performance as the race nears the end; in fact many will run negative splits (a faster second half of a race than the first).
So 'dehydration' clearly isn't a significant influence on performance; what about health? Most athletes finish long distance races in what would be called a 'severely dehydrated' state, but most never see the inside of a medical tent - and it's almost impossible to find documented cases of dehydration-related deaths in marathons. Research shows no difference in body-weight changes between those needing medical care at the end of races and those who get in their cars and head off for a pasta feast.
We are often warned that not drinking enough can also lead to heat stroke. Again, the evidence simply doesn't stand up. Research shows that the prime reason for dangerously high core temperatures is metabolic rate - basically, how hard we are working. Fluid has a small effect (through sweat, of course) but not as much as you'd think; certainly not enough that we need to replace all our fluid losses.
So how much do we need to drink? That leads us to myth 2.
Gatorade - and other drinks companies - would have us believe that we are physiologically impaired. Apparently our natural biological systems for alerting us to the need for water don't work. Isn't is a miracle the human race has survived so many millennia?
If we accept the drinks companies argument that overral fluid loss - represented by a change in body weight - is the most important thing for keeping us at peak health and performance, then it is a miracle. When we exercise hard, we actually 'voluntarily dehydrate'. That is, even given free and open access to water, we don't drink enough to replace all fluids lost in sweat. But evolution can't be wrong - we wouldn't be around to artificially correct it. So it can't be total fluid loss that's the important factor - and it's not, it's fluid balance. Our bodies naturally defend the concentration and balance of bodily fluids by various methods including inhibiting our urine production and, amazingly, creating a thirst reflex. This thirst reflex has kept the human race alive and evolving since before it was technically 'human'. Trying to second guess it by drinking more than we thirst for, or to some formula created by a 'nutritionist' is asking for trouble.
This is the bit of the post that could save your life... I know, it's taken a while to get here...
I mentioned that it's near-impossible to find any dehydration-related deaths in the history of endurance sports. It's actually fairly simple to find cases of both serious illness and death from 'over-hydration'. It's called hyponatremia.
When you sweat you lose a tiny amount of sodium. But not much in relation to the water loss, so the sodium levels in your blood actually increase in relative terms. This increase triggers the thirst mechanism. If you drink more than you thirst for, you actually dilute your blood's sodium levels. You will likely get a headache at first and feel nauseous; just as if you were dehydrated in fact, which often causes sufferers to drink even more.... But there will also likely be bloating - your watch may feel tight, or your rings. And if you are vomiting, you are not dehydrated, your are suffering the early stages of hyponatremia. DO NOT DRINK any more fluids. If you suspect someone you know may have over-hydrated, watch for them acting strangely - if they don't know where they are, who you are, what they are doing etc, then they are at immediate risk of suffering a seizure, falling into a coma or worse.
So, there it is... When exercising in heat, as long as you follow your own thirst mechanism, you are not at risk from dehydration. Ignore your thirst mechanism though, and you are more at risk of hyponatremia.
And if you want to know more about this, can I suggest 'The Runner's Body' by Ross Tucker and Jonathan Dugas published by Rodale. It really is one of the most clearly written and unbiased look at just about everything concerned with running - from hydration, nutrition and cramp through to stretching, running technique and training.
http://www.amazon.co.uk/Runners-Body-Exercise-Science-Stronger/dp/1605298611/ref=sr_1_1?s=books&ie=UTF8&qid=1373322860&sr=1-1&keywords=the+runner%27s+body
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