Hydration in training and racing

On the hottest day of the year with temps up to 34degrees in the UK it seems apt to put a piece out on hydration.

I’ve mentioned to my athletes doing long training sessions today to really think about hydration while on the bike, and run with a hydration vest/do loops that pass their house or leave a bottle somewhere so they can drink during their long runs. But also, I’ve made it clear to drink electrolyte and not just water. In this article I’ll go through why water isn’t the best liquid to rehydrate with. In addition I’ll look at whether you should have a drinking schedule or drink to thirst during training and racing, and how much you should be drinking.

Why is it important to stay hydrated?

During training and racing (of over 60mins) we need to consume calories to maintain performance.  We need to be hydrated in order to dilute the consumed calories so they can be easily absorbed.

Hydration is also important for controlling our blood volume.  As you exercise, sweat and get dehydrated you lose blood volume as the amount of plasma in the blood starts to reduce. When this happens less blood goes to the gut and muscles – ultimately resulting in a reduction in your performance as you can’t absorb calories so well and the muscles are lacking in oxygen to work efficiently.

At ~4% dehydration performance starts to decline. Below this there is no real effect on performance (if anything you are a bit lighter so may perform better – a fine balancing act to perfect!). You will finish races and training sessions dehydrated – the aim is not to prevent all dehydration, just to minimise it to 1-3%.

How do you know how much liquid you need to stay hydrated, & what should you drink?

  • Heat/humidity/altitude – all require more hydration
  • Level of fitness – the fitter you are the more you sweat as the body is more efficient at cooling, and so the more you need to rehydrate

“keep your calories and hydration separate – an easy rule of thumb is to eat your calories and keep bottles for hydration…”

While tempting to glug a bottle of water on hot days or during a hot training session, it isn’t the best idea. When you sweat you lose water along with salt (sodium). If you only take water on board you dilute the salt concentration in your body further and this can lead to hyponatremia. Sodium helps you retain fluid in your bloodstream, so you want to keep your sodium levels up to maintain a better hydration status, thus you want to take fluids on that contain salts – or electrolytes.

Sessions/races <60mins – drink to thirst, water is ok. Below 60mins you can drink to thirst as you don’t need to consume calories during these sessions or races so you aren’t needing to dilute those calories to ensure absorption. You are also unlikely to get too dehydrated in this timeframe so drinking to thirst is fine.

Sessions/races >60mins – a rough guide would be approx. 1 bottle/hr or for a more calculated estimate 10-12ml/kg BW/hr ie a 60kg person would need 600-720ml/hr. The longer the session or race the more important it is to keep on top of your hydration and have a schedule rather than just drinking to thirst (you are likely to feel thirsty too late!)

The easiest way to plan your hydration schedule is to keep your calories and hydration separate – an easy rule of thumb is to eat your calories and keep bottles for hydration – which will be very weak (less than 4% carb/sugar) solution with electrolyte in. When you consume calories it’s best to also consume fluid to improve absorption of those calories.

While brief I hope this has made you aware of the importance of hydration while training and racing and given you some handy tips to make staying appropriately hydrated easier. If you have any questions or want more detailed information just send me an email.

Happy training… and rehydrating!


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