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Hydration for Indoor Cycling – How Much Do You Sweat on Zwift?

Sports hydration and electrolytes are vital to effective training, even more so in the indoor environment. Whilst Zwift is amazing. Its indoors. As a result you are are going to SWEAT BUCKETS. But serious sweating, when not accompanied by the right electrolyte replacement can leave you underperforming at best, and in danger at the worst. Read on for some of the science of hydration for indoor cycling!

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Hydration for indoor cycling

When I do physical exertion to any major degree indoors, I sweat. Actually that’s not fair – I sweat like I’m a leaking hose pipe! Lovely picture I know, but it does mean that I have to be aware of replacing my electrolytes after rides/work outs. I have seen people collapse after doing an hour race on rowing machines during my uni days as a result of not doing this properly – and that includes just drinking water.

Warning Science sports post here! 

So based on this experience, the sweat pools around my Wahoo KICKR when on Zwift, and in conjunction with some work I’m doing at the medical school I’m going to look at staying hydrated. When I say staying hydrated, I don’t just mean pushing water into your face, but also maintaining the correct levels of electrolytes/salts to keep you and your muscles functioning at maximum performance.

Hot Turbo

Its only when you you have actually started to do frequent sessions of cycling indoors that you realise the true value of a turbo trainer mat. Its got, in my opinion, nothing to do with sounds deadening, and everything to do with making it easier to clean up the mess afterwards! It was seeing the pools of sweat from me, coupled with the recent increase in century rides being posted on the Zwift groups, that made me think it might be worth while finishing this article – I started the attempt about a month ago, but there is a lot of background reading that was needed

Some riders have gone it ingenious lengths to help cool down – none more so than Marc Atthecorner on the Zwift riders group, has appears to have a pain cave set up inside a wind tunnel!

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Fans = evaporation = cooling. But dont forget the fluids!


To start off, sweat has one purpose and that is to cool you down. The process is very elegant, you make sweat, and it takes heat any from your body in order to evaporate it. Simples. But if you go indoors, on a static trainer, without a fan, the sweat doesn’t evaporate well, but your body keeps on pumping it out. The result is a big pool – so big in fact that an intense ONE HOUR session a rider can lose up to 3.5 LITRES of sweat. Yes litres and between 1.5 and 5 GRAMS of sodium.

Even if you have the fan/jet engines pointed straight at you, and hour can still pull away over 2 LITRES of sweat. If you don’t believe me, or want to see how much you actually use, rather than collecting the sweat in buckets – both difficult and messy – weigh yourself before doing an event,  record your total fluid intake during the exercise and re-weigh yourself afterward. Essentially 1litre of water is 1kg. The difference might surprise you!

Before a 55.5km Zwift ride with the TNW Early Birds I weighed 73.0kg, according to my Withings scales. After the ride, in spite of consuming 1 litre of fluid I weighed in at 72.0kg after the ride, supporting the fact that I’d left 2 litres of sweat around the bike and in the air – nice!

The crucial point though is that these are NOT small volumes of and must to be replaced. If doing longer rides, then they need to be carefully and continually across the ride, and this is where things can get tricky.

Even with the amount of sweat I produce, its unlikely that I’m going to be at any major risk of hyponaturaemia – a lack of sodium in the blood. BUT with the numbers of people doing 3-5 hour and more turbo trainer sessions, multiple centurys and vEveresting, the concept of exercise-associated hyponaturaemia does need to be considered.

it’s important to note that this is not a purely theoretical issue either – to put the issue into context, 10-15% of marathon runners upon crossing the finish line are found to have hyponatraemia – and each year there are many sports people hospitalised from their exertions and an inability to replenish the salts they lose whilst competing.

Balance is key

But someone is always going to turn round and say – “I drink plenty of fluids when I’m on the bike/running/training, I just use regular juices etc I’m fine.” For a short hot ride, that’s ok, but when you are doing serious distances/durations, and producing a lot of sweat, all drinks are not made equally. One of the reasons why this is important is due to how your guts work and are affected when trying to replace fluids during an event.

Its possible that you have actually taken in plenty of fluid and electrolytes, but because of the form, and concentrations, they are just sitting there in your stomach. Fruit juice might seem a natural choice for on a long ride in terms of calories, but the amount of sugar and electrolytes can actually be too high to be absorbed effectively during the ride/long turbo sessions. That’s not good and is one of the reasons sports drinks manufacturers talk so much about isotonic drinks, which I’ll come to in a moment.

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Electrolytes need to be balanced

The main reason for this post is to look at when you are not able to replace enough of the electrolytes you are loosing in the sweat One of the signs of low sodium, hyponaturaemia is a loss of appetite. But that could also just be a sensation you are feeling as a result Of everything sitting in your stomach like a brick

All about the sodium

After an hour or two the turbo, I dont really feel like eating a meal for about an hour after. One of the reason for this may be post exercise, put there is also the possibility of a temporary dip in sodium levels. Even though I’m drinking on the bike, the electrolytes might not be getting into my system in time with the loss, hence the feeling off for a little after the ride.

The main reason for this is going to be just because the food has gone in your mouth, doesn’t mean its actually entered you body. Remember that whilst we think eating is putting something into our body, the physiology doesn’t quite work like that.

You see, we, and all mammals are actually a tube! A tube with a mouth at one end, and a bum on the other. Nutrients/electrolytes need to be absorbed from the inside of that tube, into the material that makes up the tube and into our blood. Just smearing it around the inside on the tube won’t really cut it!

Many sports drinks have higher concentrations of salts/solutes than the blood does. If you consume a sports drinks with more salt in it than your blood (hypertonic drinks like Gatorade), it slows the rate that the stomach empties, so again, you take in food and drink, but it just sits there. Remember there is little to no absorption done in the stomach, that all happens later in the intestines.

So spending time on a turbo, drinking/eating sports drinks with too much (or that you haven’t diluted enough – both are bad) electrolytes will give you the special joy of stomach cramps mid-ride, but not really get into your blood fast enough to even out the losses you are sweating out over your turbo. Sport companies try to get round this problem by making isotonic sports drinks which have the same level of salts as your blood

SIS market their energy gels as being isotonic, so allowing easier absorption and not slowing the gut – as ever, this isn’t advertising, merely information I’m aware of. Personally, I’m not a fan for SiS due to the inclusion of sweeteners, but that is just a taste thing as much as anything else.

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SIS = isotonic gels

Does sodium really matter than much?

So far I’ve basically said that if you drink too dilute/excessively concentrated drinks, the sodium doesn’t get replaced. But why does this really matter?
Yes people have died from this in the past, but those are rare cases. There are however a raft of other symptoms which can suggest you are not replacing enough salt – and these will matter as they will affect your performance on the bike/run.

Fatigue – we’ve all been in the situation of training heavily over several days, and just not had the get up and go, but pair this is muscle weakness, and it might indicate you need to up your intake of salts off the bike. Personally I’ve found a power meter very effective for this, there has been at least one occasion in recent memory where I’d over trained, under replaced, and I simply couldn’t get the watts up (- yes make your own jokes with it). Now its probable that there was a lot of general fatigue as well, but at the end of a hot, sweaty day already, its possible that I wasn’t in the best biochemical state before I got on the bike either

Other symptoms that can be seen in hyponatraemia are listed below. The further down the list, the greater the sodium deficiency

– Lethargy – from a medical perspective, if I see a sports person in clinic who is “tired all the time” one of the first things I’ll be looking for in the bloods is their sodium levels.

– Headache

– Nausea

– Vomiting

– Confusion

– Seizure – this is the sort of thing which kills marathon runners/iron man contenders 

skim and you’ll pay

It is possible to think that you are looking after yourself, yet may actually be putting yourself at risk, with the “we all need to drink more water adage”. What I mean by that is putting too little electrolyte solution in your ride bottle, and taking on extra water, compared to the salts during the day before a race

Normally if you are not replacing enough salts, yet you are still taking in the fluid, your body will detect this and act accordingly. In this situation your kidneys will keep the electrolytes your body needs and pass the rest of the fluid into the urine. Your body controls this though checking bloods osmolality. But if you are doing major hours on the turbo, and lots of sweat, drinking too much water, or not enough salts whilst exercising, your body can go a bit…odd and you get something call SIADH. Syndrome of Inappropriate Anti Duretic Hormone release.

Dont worry about the medical-sounding name. It essentially means that the training stress your body is under, combined with excess unbalanced water you are taking on gets absorbed, and not filtered out into the urine. Your body was too busy chasing that KOM to focus on which hormones it should be producing. This then further lowers the sodium in your blood, in addition to that you are already pushing out in sweat, which can lead to some very nasty consequences – essentially water intoxication.

Previously marathon runners have died as a result drinking too much – even in 2012, in spite of guidelines being changed to advise approximately 500ml of fluid to be consumed over each hour of a race, (previously, when trying to take on a litre of water an hour.

Slow and steady

The main point about this is to drink about 500ml of isotonic solution for hydration for every hour of indoor cycling or any endurance activity. This should be relatively easily absorbed. But dont aim to just be able to come off the bike thinking that everything you needed has already been put back in whilst on the ride. For your best results, that loss should be replaced steadily over the next 24hrs post-race.

Ok that’s all – if people like the idea of some of the science behind the sports, I’ll do further/longer articles

Other TitaniumGeek Nutrition posts

James Gill

Author of TitaniumGeek, which started after smashing off my RIGHT elbow. Feel free to drop me a line about sports tech, medicine, or frankly anything that you want to chat about!!