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How To Stay Hydrated When Training Indoors

An interesting infographic produced by our friends at Precision Hydration has revealed that people who train indoors believe they have a higher sweat rate than those who train outdoors. This got us thinking, if we’re all sweating buckets, how can we be sure we’re properly hydrated indoors? 

We invited Precision Hydration to explain the science behind the sweat and share their top tips for staying hydrated indoors: 

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First things first, we need ask the question do we actually sweat more indoors than outdoors? Or is that salty pool under the bike a red herring?

To answer that, it’s useful to understand what it is that makes us sweat in the first place. And the answer to that is pretty obvious. Heat.

We sweat when our core body temperature (CBT) rises beyond a certain point, as the body has to tightly control this variable to keep us alive and functioning.

What many people don’t necessarily realise is that the heat given off by your working muscles is the biggest influencer of CBT when exercising. So, how much you sweat is almost exclusively driven by how hard you’re pushing and not a lot else other than basic genetics. This was shown neatly in a recent study that also demonstrated that the contribution of things like body fat, overall size and weight actually have a minimal impact on sweat rate.

Most of us go for ‘quality over quantity’ sessions when training indoors. This makes the intensity of the average indoor workout higher than we’d normally do outdoors, leading to more sweat loss as a result.

There are other reasons why it often feels like you’re sweating more indoors than you do outside:

  1. Air flow. When outdoors you’re moving through air so, even on a still day, you get some airflow past the skin. Air movement causes heat to be drawn away from the body’s surface more effectively (via convection and sweat evaporation) and this cools you. On your Wattbike, you lose this airflow and therefore the sweat tends to drip off you, making you more aware of it. And as there's no natural cooling effect, you probably actually do sweat a little more to compensate too.
  2. Ambient temperature and humidity. Your body tries to offload heat to the environment when you’re training. The bigger the gradient between the air temperature, and the lower the humidity, the easier it is for heat to be evaporated away. As many places we indoor train are already quite warm and humid, the gradients for heat loss and evaporation are less pronounced than outside, and this further hinders thermoregulation and drives sweat rate up.

So, whilst you don’t necessarily sweat a lot more indoors than outdoors, there are reasons why total sweat loss might be higher some of the time.

So, how do I stay hydrated when training on my Wattbike? 


  • Show up well hydrated. As many Wattbike sessions are short and intense, it’s important to make sure you start well hydrated in the first place. Your ability to thermoregulate by sweating will be maximised if you start in the right shape. Aim to take on 500 - 750ml (roughly 16-25oz) water (or an electrolyte drink) a few hours before your session so that your body has time to process and absorb what it needs and eliminate any excess. 
  • That said, don’t overdo it and OD on fluid in the immediate build up. Just try to stick to good hydration practices on a day to day basis.
  • If you do find yourself low on fluid with a session looming, you can cheat a bit by adding some additional sodium to your drinks in the hours before starting to maximise absorption of the fluids you do consume.
  • During the session, drink to thirst. Don’t interfere with what you’re actually there to do by trying to take on unnecessary amounts of fluid.
  • Once the session is over, continue to drink to thirst and maybe add some sodium to the drinks (or food) you’re taking in if feel like you’re losing the battle to keep up with what is being lost. The best way to do this is through electrolyte tablets or salt capsules (both of which Precision Hydration provides to match your personalised hydration needs). By doing that you give your body the best chance of rehydrating fully before you put yourself through it all over again the next day…