How precise does your hydration plan really need to be?

Not long ago an athlete, we'll call him Steve, sent me a spreadsheet showing his sweat rate during a number of training sessions and the results from his Advanced Sweat Test. He asked me to interpret this data and tell him exactly how much fluid and sodium he should consume during his next Ironman event.

I looked at the numbers and gave him an idea of how much fluid and sodium I thought he should aim to take in as well as some advice on how to dial this in through some further trial and error in training.

But this was not what Steve had in mind. He very politely wrote back saying that he wanted me to be more precise in my prescription. He asked me why I was ‘hedging my bets’ by giving him a window of intake to aim at rather than some hard numbers.

His (seemingly logical) argument was that if I knew his anticipated sweat rate and his sweat composition then surely I could multiply that out over the number of hours of the race, work out his net fluid and sodium losses, input the percentage of those losses that needed to be replaced and then formulate a solid plan to hit the numbers. Simple right?

The thing is, it’s very rarely ever that simple.

Around the time I was emailing Steve, I stumbled upon an excellent blog by coach, endurance athlete and author Matt Fitzgerald on why he believes training plans cannot be reduced to a simple mathematical formula. It really struck a chord with me and I referred to it when trying to persuade Steve that what he was seeking was an unnecessary - and very possibly counter-productive - level of precision.

 

The problem with breaking big problems down into lots of smaller problems

By collecting data on his sweat rate and concentration, Steve was trying to break down the complex question of how much fluid and sodium to consume during an Ironman into it’s component parts, working each element out individually and then re-combining that info to come up with ‘the solution’. 

This sort of reductionist approach has a lot of merit. It allows a large, overwhelming problem to be broken down into smaller, more manageable issues that can be solved one at a time to give a better understanding of how the bigger problem can be tackled in it’s entirety. 

In the case of sweat and sodium output, it absolutely does help to figure out the general magnitude of your losses in a given event - something that can vary a lot from person to person. We’ve even written guides on how to estimate your own sweat rate and sweat sodium losses to help with this process because it's very important in helping you understand in general terms how your body responds to temperature and intensity when racing and training.

But, in the process of breaking any complex problem down into a few simple, discreet components, inevitably some small or difficult to measure parts of the equation get lost. These missed parts can be significant when combined, or the interactions between them and other variables can cause knock on effects, and you can end up in a situation where ‘the whole is not the sum of the parts’.

This can happen with hydration planning if only an estimate of sweat rate and sweat sodium losses are taken into account. Even though these are two really important factors to consider, the body is not just a simple vessel emptied by sweating and re-filled by drinking and eating.

 

 Difficult puzzle

 

The hydration puzzle is a complex one

Here are just a few of the things that can complicate the hydration equation…

  1. There are differences in how much dehydration different athletes can tolerate before performance starts to suffer (and differences in how much any one individual can tolerate in different scenarios).
  2. An individual’s sweat rate can potentially vary from day to day. That’s because sweat rate is affected by things like your work rate, environment, clothing, type of exercise and so on.
  3. We’re all likely to have varying levels of fluid and sodium on board when starting any given session and therefore different sodium/fluid ‘stores’ that can be accessed in times of need.
  4. We all absorb fluid at differing rates and our hormones can influence the retention (or elimination) of sodium and water via our kidneys.

But because all of these things are very hard to measure, they’re largely ignored.

Dealing with that complexity

Our brains have to operate quickly and efficiently in a complex world so, when faced with a complex question, they tend to employ a practical method not guaranteed to be perfect, but sufficient for our immediate goals.

This is known as “heuristics”. Where finding a perfect solution is impossible or impractical, heuristic methods can be used to speed up the process of finding a satisfactory solution instead.    

 

Heuristics

 

In a way, what we therefore have to do with hydration planning could be described as taking a kind of heuristic approach. This is employed in order to deal with all of the hard to pin down variables that influence the hydration equation.

Whilst we do like to try to quantify sweat rates and sweat sodium concentrations as accurately as we can with testing in the lab or field rather than use this information in isolation and treat it as the only variables in play (and variables that are basically fixed once measured), we prefer to interpret them as a guide to whether an athlete is losing a low, moderate, high or very high level of fluid and sodium when they’re working out.

Once we’ve placed an athlete in the right ‘zone’ for both variables (e.g. they appear to have a high sweat rate but a low sweat sodium concentration) and we know what they’re training for (in Steve’s case, an Ironman), for roughly how long and, preferably, in what kind of conditions, we’re able to suggest a hydration strategy that takes these variables into account, draws on some actual experience and still gives enough flexibility to allow for some sensible testing and adjusting in the real world.

We usually end up suggesting an appropriate strength hydration drink (it’s the "relative concentration" of your drink that’s important) and then make suggestions on the ballpark amount of fluid the athlete is likely to need to drink per hour.

This plan is then thoroughly tested in training/race scenarios and adjusted until the athlete has a formula that actually works well for them.

This process relies on - and seeks to further hone - your own basic instincts of thirst, salt craving and other bio-feedback that are your body’s way of letting you know what it needs at a particular moment in time. This point is crucial because, if you are able to listen to it, your body is extremely good at telling you how well (or not) your pre-planned hydration strategy is bearing up and whether it needs modifying on the fly to get it just right. 

The more you practice listening to your body, the better you become at adjusting your intake and maintaining your performance. The better you also become at predicting how much you'll need to take in, in the first place and that therefore helps you get the overall balance right more frequently than not. 

Over 30,000 athletes have taken a Sweat Test with us over the last 7 years and we’re very confident that following this approach offers the best chance of success for an athlete looking to really nail in their hydration strategy in a given event. 

But, if like Steve, you are seeking an exact prescription on how much to drink during your next race, we’re sorry, but such a thing doesn’t really exist so you are probably going to end up disappointed if that's what you are looking for...


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