Post-training Rehydration

Nailing hydration during training and racing is one part of the complex discipline of nutrition but possibly even more important is the post-training recovery. Not only is it important but also a little different depending on your long term goals.

No matter how well you hydrate during long or hard sessions, you will likely end up at least a little bit dehydrated and the longer and/or hotter the training the more likely this is. Simply because it’s really hard to get enough fluid in, with swimming and running being the harder of the three triathlon disciplines. But even when cycling hard, it can be difficult to get enough fluid in as the gut tends to reduce it’s functioning in order to redirect blood to the working muscles. So rehydration should always be part of your post training recovery.

In a recent sport science symposium I attended, there was a presentation looking at just that. What fluid is best for rehydration? Now we know that water is not as efficient at rehydration compared to something that has a little sugar and salt added to it (see last post for more detail). The researchers therefore focused on determining which was better, a sports drink (note this was a sports drink and so had a high carb content) or a milk based drink (Sustagen sport in this case). After getting volunteers to cycle in a hot environment for a sustained period, the change in weight from start to finish was measured in order to determine total body water loss. The volunteers were then given this volume of fluid to consume either in the form of a sports drink or Sustagen over a short period. Weight was then tracked over the next few hours to get an estimate of rehydration (note this is only an estimate because there was no measure of stomach emptying or blood plasma volume). What was noticed was that the milk based drink was able to keep the weight close to pre exercise weight better than the sports drink. Those consuming the sports drink got their weight back up but then rapidly pee’d the fluid out.

The researchers then repeated the experiment but rather than giving the volunteers the volume of fluid they had lost they were allowed to drink as much as they wanted to of either the sports drink or the milk based drink (main reason for this approach is that if you lost 2+ litres of body fluid during the cycle, it was a bit of a struggle for the volunteers to consume that much in a milk based form, plus it is more representative of real life). Those who consumed the milk based drink in this instance were able to get their body weight up closer to the pre exercise weight than those consuming the sports drink.

The explanation for this. The sports drink allows rapid rehydration, and will often overshoot the blood volume needed, at which point it passes through the kidneys to be filtered out, and so you end up in the bathroom, getting rid of that water you actually really want to hold onto. You may have also noted (what I call the leaking tap effect). Get in from training and have a drink and instantly the flood gates open and you’re suddenly sweating buckets again. This is because the body is really clever. It knows function starts to decline as you get dehydrated and so it stops cooling you down via sweating after a certain point and relies on other (less efficient cooling strategies). However as soon as you put some fluid into your stomach, sensors tell your brain don’t worry water is on the way, and so it knows that it’s ok to sacrifice a little water in order to cool you down because it can absorb it out of the gut to replace it. [Your body does the same thing with sugar, which is why holding a gel in your mouth, or swishing sports drink in your mouth can give you a quick burst of energy.] So your body actually ups it’s fluid loss when you start drinking.

Milk based drinks on the other hand stay in your stomach longer. They actually curdle when they hit the acid in your stomach (mmmm tasty) but this means it acts more like solid food rather than a liquid. FLuids move straight from the stomach into the intestines, whereas food is drip feed a little at a time so that the intestines have time to process the food. Rehydration isn’t so immediate, we don’t have a rapid increase in blood volume and a kidney response, it’s a much more controlled process which is why it is able to maintain body weight for longer, or get closer to body weight, if allowed to drink as much as you want.

Milk based drinks have all the good stuff for rehydration, a little salt and sugar and water and also has the added bonus of a quick absorbing protein (whey) which aids in immediate muscle recovery as well as a more prolonged absorbing protein (casein) for sustained recovery later in the day. This is the science behind chocolate milk being a good recovery drink.

Females please note that there is not sufficient protein in chocolate milk for your recovery, you need to add some protein to this, in order to prevent muscle breakdown. Yay hormones.

So why does it matter what your long term goals are? Dehydration can actually have a powerful physiological effect similar to altitude training. Sustained dehydration signals the body that it needs to do something to prevent this happening and it does this by increasing the resting blood volume blood volume is increased and the kidneys don’t try to get rid of it). More blood in your body when exercising means its easier to get oxygen and nutrients to the muscles, which means you can work a little bit harder. Increasing your blood volume can be a good way to help you get ready for racing in hot and humid environments. If this is your long term goal, then holding off on rehydrating can be helpful. Not only is this training your physiology but also is a great mental training exercise, for those times between aid stations when all you can think about is an ice cold drink.

But bare in mind that you do not want to do this if you have another training session the same day,  or a really hard session the following day, as you are compromising your recovery.

Addendum: Rehydration immediately following exercise can be water, sports drink or ideally a specifically formulated hydration supplement but you do want to then have a milk based recovery drink also in order to maximise the rehydration. If you can’t have milk never fear. Having some solid food with your fluid has a similar effect.

If you are training to race in the heat then dehydration training can be beneficial. This should be started weeks or months out and should be carefully planned, to not interfere with key training sessions.



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