One hour on the indoor trainer, and the floor mat has drunk more than you have. With no wind, sweat drips off instead of evaporating, and sodium goes with it. A scale, two bottles and a fan are all you need to take back control: how much to drink over one or two hours, how much sodium to make up for your losses, what to eat, on the indoor trainer, in the pool, on the treadmill and at the gym.
Why your living room makes you drip more than the road
Outdoors, at 25km/h, the air cools your body: sweat evaporates and carries the heat away with it. In your living room or garage, there is no wind. Sweat beads, rolls and drops. It leaves your body having barely cooled it.
A Japanese team measured the effect in the laboratory (Otani et al., 2018). Eight men pedalled at 70% of their maximum capacity, at 30°C, facing fans set at 30, 20, 10 and then 0km/h. With no air, they lasted 41 minutes, and 90 minutes at 30km/h. Their temperature rose faster and their sweat evaporated less. Your living room is cooler than that climate chamber, but the mechanism is the same: without wind, sweat cools you less, your body heats up faster, and the water keeps pouring out.
In terms of volume, measurements on athletes put sweat rates between 0.5 and 2 litres per hour, with rare peaks above 3 litres (Baker, 2016). The gap between two people sitting on the same indoor trainer is huge. Only weighing yourself will give you your own figure.
Weighing in: your sweat rate in one session
The protocol comes down to four steps. Weigh yourself before the session, in light clothing, with an empty bladder. Note what you drink during it. Towel off and weigh yourself again afterwards, in the same clothing. Each gram lost equals one millilitre of sweat (Baker, 2016).
An example. 74kg at the start, 73.2kg at the end, a 500ml bottle drunk during the hour. The scale shows 800g less; add the 500ml you drank and you get 1.3 litres of sweat in one hour. Even with the whole bottle down, you still finish the session at a 1.1% deficit.
Repeat the test two or three times, with and without a fan, at a similar intensity, and you get your personal range. The American College of Sports Medicine recommends this weigh-in to estimate your losses, and sets the threshold to watch: beyond 2% of body mass lost as water, performance drops (Sawka et al., 2007). For 70kg, 2% is 1.4kg.
How much to drink over one hour and over two hours
Indoors, the benchmark is the same as outdoors: 500 to 700ml per hour, in small sips, rather than half a litre in one go. Your weigh-in tells you where you sit in that range. At 0.7 litres of sweat per hour, one bottle is enough; at 1.3 litres, aim for the top of the range and make up the rest in the hour after.
Over one hour, a 500 to 650ml bottle covers the session. Over two hours, plan two bottles, at least one of them with electrolytes, and start taking carbohydrates within the first twenty minutes. To keep this rhythm without thinking about it, set a timer on your bike computer or phone: it reminds you to drink and eat at fixed times throughout the session.
Sodium, the invisible part of sweat
Sweat is water and salt. Its sodium concentration ranges from 230 to 2,070mg per litre depending on the person (Baker, 2016). With 1.3 litres of sweat per hour and moderately salty sweat, you leave around a gram of sodium on the mat.
The white rings that dry on your jersey are salt.
My benchmark, the one in the trail guide on this site: 300 to 600mg of sodium per hour when you sweat a lot. One serving of my hydration drink in a 500ml bottle provides 0.88g of salt, or about 350mg of sodium (salt is 40% sodium), with 753mg of potassium and 152mg of magnesium, for 10kcal. One serving of energy drink provides 0.75g of salt, or 300mg of sodium, and 35g of carbohydrates. One bottle per hour covers the bottom of the range and two bottles, the top.
In my bottle
Mulebar Citrus Hydration Powder
350mg of sodium, 753mg of potassium and 10kcal per 500ml bottle. Available as a 10g serving or a 250g pouch, and also in red berries flavour.
What to drink and eat depending on duration
The length of the session determines what goes in your bottle and your fuel stop. There are two cases.
The one-hour session
Over one hour, water and electrolytes are enough: a bottle of hydration drink, sipped regularly from the first few minutes. The benchmark in my gut training guide for one to two hours of effort comes down to one figure, up to 30g of carbohydrates per hour.
If the session is hard, with intervals or threshold work, switch to a bottle of energy drink. Its 35g of carbohydrates hit that benchmark, and you keep the sodium. That’s what Sybille tested on a trail run before leaving her verified review on 15 October 2026: “I enjoyed the drink’s not-too-sweet taste.” A light taste matters when you drink half a litre in an hour.
The two-hour-plus session
From the first twenty minutes, start eating, then alternate. The benchmark for two to three hours of effort is around 60g of carbohydrates per hour: one bottle of energy drink and one energy bar per hour, or one bottle and a gel taken in three goes, will get you there.
Keep the second bottle for the hydration drink: electrolytes without carbohydrates, for a light stomach. After two or three hours, sweet flavours become tiresome and savoury takes over, a trail runner’s principle that holds true in a garage too. An energy compote taken in three or four goes, with a little water after each, changes the texture and gives your taste buds a rest.
Train your stomach at home
The indoor trainer and the treadmill are the ideal lab for gut training, training your digestive system to tolerate and absorb carbohydrates during exercise. I devote a whole section to it in my complete guide to gut training: indoors, you control intensity, temperature, timing, access to products and water, and the toilet is close by. It’s the place to test a new product, a higher dose or a more concentrated drink without spoiling a ride.
The protocol in a nutshell. For the first two weeks, one intake every 20 to 30 minutes on sessions longer than an hour, 30 to 40g of carbohydrates per hour, with water after each intake. For the next two weeks, 50 to 60g per hour, alternating gel, compote, bar and drink. After each session, rate your digestive comfort from 0 to 10. Move up a level after two consecutive sessions rated 3 or lower. The heat of a closed room partly reproduces the conditions of a summer race, and in winter one fuelled indoor session a week helps you maintain the tolerance you have built.
By sport: indoor trainer, pool, treadmill and gym
The common core applies to everyone. Here is what changes depending on where you sweat.
Cycling and triathlon: the indoor trainer
The fan is your first piece of nutrition kit. Place it facing you, as powerful as possible: above 10km/h of airflow, the study mentioned above already measures a clear gain in exercise tolerance. Two bottles within reach, one of energy drink and one of hydration drink, a towel, and the scale before and after.
Triathletes who combine indoor trainer and treadmill follow the rule on the Triathlon page: the bike is the ideal time to eat and drink both bottles before starting the run. Cyclists will find the full table by ride duration on the Cycling page, and a 650ml cycling bottle for the bottle cage.
Triathlon: the pool
You sweat in the water too. An Australian study of elite male and female swimmers measured an average of 138ml of sweat per kilometre swum in men, 107ml in women, with large individual differences (Cox et al., 2002). For a 3km session, that works out at 300 to 400ml. The water washes the sweat away before you feel it, and thirst stays quiet.
The same study looked at what the swimmers drank of their own accord: 155ml per kilometre for the men, who roughly made up their losses, and 95ml for the women, who stayed in deficit.
My advice: a bottle of hydration drink at the end of your lane, a few sips after each set, and you leave the pool ready for the bike session that often follows.
Road and trail: the treadmill and the gym
On the treadmill, same story as on the indoor trainer: the Japanese study mentioned above also measures warmer skin every time the airflow slows. You run at your long-run pace in a heated room, with about as much wind as a shoebox. The weigh-in works for you too, and the result often comes as a surprise.
For a 45 to 60 minute easy run, a bottle of hydration drink in the treadmill’s bottle holder does the job. For a long run on the treadmill or a trail runner’s incline hill session, switch to the energy drink and a gel taken in three goes. Road runners preparing for a spring marathon will find the full plan in my article Marathon hydration.
What the puddle teaches you
Winter is the season to get to know your needs: your litres of sweat per hour, your sodium, the carbohydrates your stomach accepts. Three weigh-ins, a notebook, and you arrive in spring knowing your body better than most of your rivals on the start line. For the carbohydrate side, the “diagnostic” app gives you your plan in two minutes, based on your sport, the length of your effort, your digestion and your tastes.
Discipline makes sense when you know why you are pedalling in your basement on a Tuesday evening. And next time you wring out the floor mat, look at the puddle before it dries: it’s your most honest training log, and it reads in grams.
Sources
- Otani H. et al., “Air velocity influences thermoregulation and endurance exercise capacity in the heat”, Applied Physiology, Nutrition, and Metabolism, vol. 43, no. 2, pp. 131-138, 2018. Record consulted on 23 October 2026: https://vufind.lboro.ac.uk/PrimoRecord/cdi_proquest_miscellaneous_1948760709
- Baker L. B., “Sweat testing methodology in the field: challenges and best practices”, Sports Science Exchange no. 161, vol. 29, 2016. Consulted on 23 October 2026: https://gssiweb.org/docs/default-source/sse-docs/baker_sse_161.pdf?sfvrsn=2
- Sawka M. N. et al., “American College of Sports Medicine position stand. Exercise and fluid replacement”, Medicine & Science in Sports & Exercise, 2007. Consulted on 23 October 2026: https://www.sportgeneeskunde.com/files/bestanden/VSG/VSG6667.pdf
- Cox G. R., Broad E. M., Riley M. D., Burke L. M., “Body mass changes and voluntary fluid intakes of elite level water polo players and swimmers”, Journal of Science and Medicine in Sport, vol. 5, no. 3, pp. 183-193, 2002. Record consulted on 23 October 2026: https://research.bond.edu.au/en/publications/body-mass-changes-and-voluntary-fluid-intakes-of-elite-level-wate/
- Regulation (EU) No 1169/2011 of 25 October 2011, Annex I, point 11: salt = sodium × 2.5. Consulted on 23 October 2026: https://eur-lex.europa.eu/eli/reg/2011/1169/oj/fra
- Mulebar product pages reviewed on 23 October 2026: citrus hydration powder (10g in 500ml, 0.88g of salt, 753mg of potassium, 152mg of magnesium, 10kcal per serving), citrus energy drink (40g in 500ml, 0.75g of salt, 35.4g of carbohydrates per serving); 650ml cycling bottle, page reviewed on 29 October 2026.
- Mulebar, trail nutrition and eating guide (8 October 2025) and Running page: 500 to 700ml of water and 300 to 600mg of sodium per hour. https://mulebir.com/blogs/news/nutrition-trail and https://mulebir.com/pages/nutrition-running-performance-bio-mulebar
- Mulebar, “What is gut training? The complete guide”, published 12 February 2026, updated 6 October 2026: benchmarks of 30g/h (1 to 2h) and 60g/h (2 to 3h), six-week protocol, section “At home: indoor trainer and treadmill, the ideal lab”. https://mulebir.com/blogs/news/quest-ce-que-le-gut-training
- Mulebar, Cycling and Triathlon pages, reviewed on 23 October 2026: https://mulebir.com/pages/nutrition-cyclisme-sans-chimie-mulebar and https://mulebir.com/pages/nutrition-triathlon-multi-distance-mulebar
- Verified review by Sybille, 15 October 2026, Société des Avis Garantis, collected on 23 October 2026: https://www.societe-des-avis-garantis.fr/www-mulebir.com/
