December 9, 2020

Hydrating for Indoor Rides

With the northern hemisphere firmly in the grim grip of winter, many cyclists turn to indoor sessions for their training.

There are many differences between riding indoor and outdoor, and one of them is how we need to deal with hydration.

Indoor riding provides a unique challenge because we’re not physically moving, and there is no cooling effect of air flowing around us. You’re standing still while furiously pedalling. In this setting, it is much more difficult for the body to get rid of the heat generated by your muscles.

Why we sweat

Why we’re sweating is actually pretty simple. Physical activity creates heat, and this heat must be dissipated from the body in order for it to perform properly.

The main strategy of the body is to produce sweat on the skin. The evaporation of the sweat leads to a cooling of the skin, effectively cooling the whole body.

Hopefully, the more we sweat, the more heat gets dissipated. At least, that’s the basic strategy of our body. (But sadly, that’s not really true, because it depends on environmental factors like humidity)

How we lose water

Many people use one or multiple fans to cool them when training indoors. This is of course a great strategy against overheating and increased sweat. But is it enough to stay hydrated?

Say - if I don’t sweat at all during my session, do I need to drink something at all?

It’s tempting to think that as long as you’re not sweating, you’re not losing any fluids. But sweating is not the only way water moves out of our body. We also lose water through breathing:

"The typical water content of air is .5 percent, while the water content of expired air is approximately 6 percent, which clearly explains why rapid respiration is a major route of water loss in athletes."1

So, even if you cool yourself with multiple fans during your indoor rides, you will still lose significant amounts of water.

The problem with dehydration

Why is that even a problem? The body consists of roughly 55% to 70% of water, what difference can it make if you lose 1 liter or 3?2

Given that such a vast part of our bodies consist of this resource, it's astounding how sensitive we react to fluctuations of its availability. Even a reduction of water volume of 2% of our body weight leads to a significant reduction in athletic performance.3

For a 75 kg athlete, 2% of body weight are 1,5kg, which is a water loss that is easily reachable even with modest intensity during an hour or so.

It’s clear from the available data that you want to avoid dehydration as much as possible.

The problem with thirst

Ideally, there would be a a bodily response of dehydration that tells us that we should definitely drink something. Something like being hungry, but for drinking. Something like … thirst. Oh.

The problem with thirst is that it is a lousy indicator of our hydration status. Athletes typically lose 1,5 to 2 liters before the onset of thirst. 4

And given that this is well within the ballpark of fluid loss that results in significant performance drops, that’s actually bad news.

Rule of thumb: If you start drinking when you feel thirsty, it’s already too late, and given the rate you sweat when exercising, and the ability of your intestines to absorb water, you don’t have a chance getting to a well hydrated state again.

A hydration strategy for indoor trainer sessions - timing and volume

As we’ve seen, two obvious signals for the need for hydration (sweat and thirst) are bad indicators of dehydration: Your body loses water even if you’re sweating lightly or not at all, and when you feel thirst, you are already in a dehydrated state. Instead of relying on these indicators, consider the following strategy:

"To achieve a hydrated state before competition or practice, 14 to 22 ounces (420 to 660 milliliters) of fluid should be consumed"5

From then on, you should try to maintain hydration levels by continously drinking something, at least every 10 to 15 minutes. Drinking is difficult during high intensity phases of your workout, so make sure you use rest intervals to catch your breath and hydrate.

It’s hard to come up with a general recommendation about how much to drink because people are so different. But here’s a quick way to find out how much liquid you lose during an indoor session:

Take your weight (nude if possible) before your session. Drink as you want during session, and make a note how much you drink. After the session pee, then take your weight again.

The difference in the before and after body weight plus the amount of liquid you consumed is the amount of liquid that left your body.

So, for example, if you weight 75 kg before a training session, had one bottle of 500ml of water during your training, and weight 74kg after the session, you lost 1,5kg of water (75 - 74 is 1kg difference, plus the 500ml water is 1,5kg).

During the session, you lost 1,5kg of water, and replenished 500ml, leaving you in a deficit of 1 liter.

(It’s useful to calculate your sweat rate per hour so you can extrapolate on workouts with other durations)

You can use our sweat rate calculator for that.

Sports drink or plain water?

So, now we have established how much you should drink, and when.

Another question is: What should I drink? Many cyclists rely on plain water when training indoors. When considering wether to consume a sports drink or plain water, most athletes focus on the energy aspect of sports drinks: Indoor sessions are typically shorter than outside rides, so there is less need for feeding during the session. While this is debatable, it is missing one big point: Adding carbs to a fluid greatly improves the absorption rate of the fluid in the intestine. Meaning: If you add carbohydrates to your liquids, the fluid will go into your system much faster than plain water.

This is isn’t much of an issue if you’re training for less than 1 hour. But on longer sessions, consider using a sports drink or a home-made drink to improve your hydration efforts.


Dehydration is a serious problem for all athletes. The special challenges with indoor riding compound this problem and make it necessary to develop strategies to avoid dehydration. Common and obvious signs like sweating or the feeling of thirst are bad indicators of hydration status because they either set on too late, or show only a part of the processes that lead to dehydration.

Instead, follow a hydration strategy that is independent from these unreliable indicators:

  • Start your hydration strategy by drinking a bottle of fluid before your session
  • Drink regularly during the workout in smaller sips
  • Check your sweat rate to get a better understanding of your individual intake needs
  • Consider using a sports drink for any sessions longer than 1 hour
  • Re-hydrate immediately after your session by consuming another 0.5 liters, and then drink plenty of fluid in the next couple of hours

1 Benardot, Dan. Advanced Sports Nutrition. Human Kinetics, Inc.

2 Sawka, Michael; Cheuvront, Samuel; Carter, Robert (2005). Human Water Needs.

3Goulet, Eric (2012). Dehydration and endurance performance in competitive athletes.

4 Benardot, Dan. Advanced Sports Nutrition. Human Kinetics, Inc.

5 Benardot, Dan. Advanced Sports Nutrition. Human Kinetics, Inc.

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