It’s an age old debate and one that has seen advice swing from one extreme to another over the years. How much fluid should athletes take in during training and racing. It’s gone from drink as little as possible to drink as much as you can and everything in between. Truth is no one glove fits all. The last position stand from the American College of Sports Medicine published in 2007 suggests avoiding a body weight loss of more than 2%. So how could Haile Gebrselassie ever have run a 2:05 marathon with a body weight loss of 10%? Most published research would suggest he probably should have been on the verge of death!

Like most things to do with endurance sport the answer by working through a process and experimenting to find what is optimal for you. Now I’m not suggesting you go out and run a marathon as fast as you can and loose 10% of your body weight. Far from it! Start recording your numbers and learn from them. You’ll quite quickly get a feel for how much fluid you should be taking in, as a general rule of thumb I try not to loose more than 3% of my body weight during training sessions or races longer than 90mins. I’ve arrived at this number through a process of trial and error to build an understanding how varying levels of ‘dehydration’ effect me as an individual.

One key thing to remember is you can start races or training sessions carrying more fluid than you may do normally. In fact this was probably the case in the Gebrselassie example. Let’s say your normal day to day body weight is 70kg and you turn up to a race weighing 72kg having put on a couple of kg’s in the days leading up to the race. You actually have more fluid to loose during the race. Assuming the vast majority of your weight gain was fluid in the first place.

Here’s a couple of tools to help build an understanding of how much fluid you loose and start building your hydration strategies.

If you train on a daily basis measuring and understanding cumulative effect of fluid loss is important:

Each morning or every second morning on rising weigh yourself and record the number. If you have lost more than 2% body weight vs. prior weight give yourself 1 point.

Go for a wee, is your urine dark in colour? If it is give yourself 1 point.

Are you feeling thirsty? If you are give yourself another point.

If you scored no points happy days you’re good to go. 1 - 2 points would say you are probably somewhat dehydrated and I would suggest taking fluid. 3 points and you are dehydrated and you need to correct this by taking fluid on board until your symptoms have gone.

During Training:

This is where the rubber hits the road a bit more. Understanding how much fluid you loose during training and racing plus having a strategy to make sure you aren’t becoming too dehydrated or hyponatremic will have a positive impact on your performance. To make it easier we built this spreadsheet which you can download and use. It has been adapted from a model published by Asker Jeukendrup. The steps to follow are laid out below.

1. Before training go for a wee and record your weight, ideally with no clothes on.

2. Fill the drink bottles you intend to use and weigh them recording the weight in litres then perform your workout.

3. After your workout towel off and weigh yourself, no clothes. Record this weight. Also weigh and record the weight of your drink bottles.

4. Ideally keep a record of whether/how many times you went to the toilet during the workout. If you did add .3kg to your finish body weight for every time you went.

5. After inputing all your data you will see an hourly sweat rate has been calculated in column ‘N’ of the sheet.

Please bear in mind the model is not perfect, there are other variables that can lead to weight loss during exercise. I suggest you use it as part of your process to better understand how much fluid you loose and therefore how much you may need to replace. Like I said at the start no one glove fits all so you will need to experiment to help build your optimal hydration strategies.

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