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Fluid Replacement During Exercise

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The Basics

Our body is mostly made up of water, salty water. Water makes up a big part of out blood and also the fluid that surrounds our cells, these two things make up the extra-cellular fluid (ECF). By far most of the water in our bodies is actually within our cells, the intra-cellular fluid (ICF). If we have too much water then our blood will have too much volume (high blood pressure (BP)) and our tissues (usually legs) will become swollen (oedema), if not enough then our BP may get too low. In all of this water is a lot of dissolved electrolytes, sodium (Na), potassium (K), calcium (Ca), chloride (Cl) and so on. Our body needs these as they are used to make make our muscles work (including our heart) and our nerves (and therefore brain) function. If they get too out of line with the normal range then bad things can happen, like the heart not beating properly (arrhythmias) and the brain not working as it should (confusion, coma).

Our body's are quite good at keeping things in line with the proper range, this process is called homoeostasis. If we have too much water then our kidneys get rid of it and we wee a lot. Not enough and our brain tells us we're thirsty. Too much electrolytes and our kidneys filter extra out in our urine, not enough electrolytes and they will try and keep more in. Most of the time this works well, unless we are unwell.

It will come as no surprise that the way we get these things into our body is to eat/drink them. Our body is so good at balancing this that we don't even have to think about it too much; we drink when we need to and eat when we're hungry. Our diet will generally have enough water and electrolytes that it isn't a problem (calories and fat are a different story altogether!) The way that they get out is through urine, faeces and sweat (this is a bit oversimplified, but this isn't school, you can look up more details yourself if you want (

In our intestines the body actively takes in whatever carbohydrates, fat, protein and electrolytes it can. Water, in general, can freely move about the body, so it just diffuses into the body following the laws of osmosis (, which basically says that water will move to where things are more concentrated to try and balance things out. Our body doesn't need to actively absorb water, and water doesn't need things in it to be absorbed.

This comes to my first point:

You don't need to drink an electrolyte drink to aid water absorption. You body is more concentrated than plain water, so plain water will get into your body just fine.

Running and Fluid Balance

When we run we get hot. When we get hot we sweat. When we sweat we lose water and electrolytes (mostly Na). When our losses become significant we will suffer, ranging from a fall in performance to life threatening problems. Therefore we need to replace them. Well, nearly.

Of course we lose water when we run from sweat. In a paper by Sawaka et al on exercise and fluid replacement [1] it was found that half-marathon running in winter leads to an average fluid loss of 1.49L per hour with voluntary fluid intake of 0.15L per hour leading to an average dehydration of 2.42%. So there is no doubt that running can lead to dehydration.

The tricky part is about the loss of electrolytes. Yes, sweat is salty; but it is not as salty as blood [2]:

  • Blood: 275-299 mmol/kg [3]
  • Sweat: 117mmol/kg (male), 134 mmol/kg (female) [4]

So when we sweat we lost water faster than electrolytes so the concentration of electrolytes in our blood actually goes up! So running and sweating will not lead to us becoming hyponatraemic, or not having enough sodium in our blood (a cause of heart arrhythmias). In fact, hyponatraemia is cause by drinking too much during exercise [5]. Ross Tucker has written about sports drink on his blog, The Science Of Sport and has demonstrated that drinking sports drinks has a negligible impact on the level of blood electrolytes compared with water [6].

So basically it seems that the biggest problem is dehydration, as it has been found that dehydration of more than 2% can decrease performance [7]. Recall that we have already seen that half-marathon runners are on average 2.42% dehydrated at the end. Of course they weren't that dehydrated for the entire time, so is seems that many of the runners may have been optimally hydrated, in that they did not have degraded performance from dehydration and they did not lose more time than necessary at drink stops during the event.

So, what does all that mean? Well, my interpretation of all of that is

  • We need to make sure that we don't get too dehydrated
  • There is no absolute need to "replace lost electrolytes" when having a drink during an event

How Much to Drink

Let's assume that you start the race fully hydrated. The aim of our strategy is to optimise our performance and to avoid any medical problems. Optimising performance does not necessarily mean to be able to run the fastest at any one moment, but to finish the event in the best overall time. This might mean compromising a bit between fluid intake and hydration level.

According to Coyle [8], there is no demonstrated effect on performance for exercise lasting less that 90 minutes where dehydration is generally only 1-2%. It needs to be born in mind, however, that higher than usual levels of sweating, for example in hot and humid weather, may lead to higher levels of dehydration. He goes on to recommend that fluid should be taken at a rate that closely matches water loss (sweat), but also says that if that is not practical or if it will not help your performance (e.g. if it makes you feel bad having to drink that amount and having it slosh about in your stomach), then you may be able to tolerate dehydration of up to 2% as long as the temperature is not too high.

If we go back to something that we learned at the start, runners in half-marathons (there is no data for full marathons) lose 1.49L per hour. Lets assume that that is the same rate for full marathons as well. Let's also assume that in a marathon or half-marathon that we do not carry our own water but rely on the drink stations on the course. For the 2010 Blackmore's Sydney Marathon there were 17 drink stations, so each one would be about 2.5km apart. Let's then look at how long it would take us between each one and how much we would have to drink depending on how fast we were running. Let's also look at two different intake strategies and how they would play out for different paces.

Intake 1 Intake 1
Pace Time between stations Amount to drink Actual Amount Deficit Dehydration Actual Amount Deficit Dehydration
min/km min mL mL mL % mL mL %
4:00 10:00 250 130 1382 2.0% 190 702 1.0%
4:10 10:25 260 135 1382 2.0% 198 702 1.0%
4:20 10:50 271 141 2070 3.0% 206 1050 1.5%
4:30 11:15 281 146 2070 3.0% 214 1050 1.5%
4:40 11:40 292 152 2071 3.0% 222 1051 1.5%
4:50 12:05 302 157 2071 3.0% 230 1051 1.5%
5:00 12:30 313 163 2071 3.0% 238 1051 1.5%
5:10 12:55 323 168 2071 3.0% 245 1051 1.5%
5:20 13:20 333 173 2072 3.0% 253 1052 1.5%
5:30 13:45 344 179 2072 3.0% 261 1052 1.5%
5:40 14:10 354 184 2072 3.0% 269 1052 1.5%
5:50 14:35 365 190 2760 3.9% 277 1400 2.0%
6:00 15:00 375 195 2760 3.9% 285 1400 2.0%
6:10 15:25 385 200 2761 3.9% 293 1401 2.0%
6:20 15:50 396 206 2761 3.9% 301 1401 2.0%
6:30 16:15 406 211 2761 3.9% 309 1401 2.0%

So we can see that to remain fully hydrated we need to be drinking between 250mL and 406mL per drink station depending on your pace (and therefore how long you will be running for). If that seems like a lot and you just want to stay at under 2% dehydration then you need to be drinking between 130mL and 309mL per station. Just remember to take conditions into account if you decide on this strategy!

Do Sports Drinks Make A Difference?

We have seen that we don't need sports drinks to remain hydrated, that water is well absorbed. But will drinking sports drinks give us any advantage over drinking plain water? This is where it gets a little confusing for me. In 1983 White and Ford [9] found that there was no differences between using water and an experimental sports drink. But in 2010 Bonetti and Hopkins [10] found that using a hypotonic sports drink did result in improved performance. It is unclear to me just how applicable this is to most runners, as the experiment was looking at peak power after a 2 hour warm up, hardly the sort of exercising that most people undertake.

So, what is the answer? It seems that the jury is still out on this one. My feeling is that water is fine to use to drink in exercise. you do not need the electrolytes that sports drink makers charge you for. That being said, some people may get a psychological benefit from using a sports drink, feeling that something gives you an edge (although it may not) may be enough to [actually give you an edge.

Another common comment is that the extras in a sports drink will slow gastric emptying, giving you more time to absorb it. It may be true that glucose (among other things) will slow gastric emptying. But there is nothing that I have found that supports saying that this will increase your hydration. Coyle [8] comments that the slowed gastric emptying rate is a minor factor in fluid replacement.


  • Water is absolutely fine for fluid replacement during exercise.
  • There is no independent evidence that sports drink provide a benefit for fluid replacement
  • It is important not to get more than 2% dehydrated
  • Drinking more that you lose is the main cause of hyponatraemia[/b]


1 Sawka MN, Burke LM, Eichner ER et al. American College of Sports Medicine. American College of Sports Medicine position stand. Exercise and fluid replacement. Med Sci Sports Exerc. 2007;39:377–90.
2 Wikipedia contributors. Sweat. Wikipedia, The Free Encyclopedia. 16 September 2010 at 23:24. Available at: Accessed September 17, 2010.
3 Wikipedia contributors. Plasma Osmolality. Wikipedia, The Free Encyclopedia. 31 July 2010 at 09:33. Available at: Accessed September 17, 2010.
4 Willing SK, Gamlen TR. Sweat Osmotallty Values In Normal Adults. Clin Chem. 1987;33(4):612.
5 O'Connor R. Exercise-induced hyponatremia: Causes, risks, prevention, and management. Cleav Clin J Of Med. Sept 2006;73(3):S13-18.
6 Tucker R. Sports Drinks, sweat and electrolytes. The Science Of Sport. November 27, 2007. Accessed September 17, 2010.
7 Cheuvront SN, Carter R, Sawka, MN. Fluid Balance and Endurance Exercise Performance. Current Sports Medicine Reports. 2003;2(4):202-208.
8 Coyle EF. Fluid and fuel intake during exercise. Journal of Sports Sciences. 2004;22(1):39—55
9 White J, Ford MA. The hydration and electrolyte maintenance properties of an experimental sports drink. Br J Sports Med. 1983;17:51-58
10 Bonetti DL, Hopkins WG. Effects of Hypotonic and Isotonic Sports Drinks on Endurance Performance and Physiology. Sportscience 14, 63-70, 2010 ( Accessed September 17 2010.

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