How much water to drink during training
The universal rule of "eight glasses a day" is of little help to a person who trains: one person loses more than a litre of fluid in sweat during an hour of intense work in the heat, while another loses barely half a litre. The editorial team reviewed what leading sports medicine organisations say about hydration, how to determine your own water need and why drinking "in reserve" is also risky.
Why the body needs water during exertion
During physical work, muscles convert only about a quarter of energy into mechanical work; the rest is released as heat. So that body temperature does not rise dangerously, the body sheds heat mainly through the evaporation of sweat. Therefore, the more intense the workout and the warmer the surroundings, the more fluid is lost.
The position stand of the American College of Sports Medicine (ACSM, 2007) gives a range of sweat rates from roughly 0.3 to 2.4 L per hour depending on intensity, environmental conditions, acclimatisation and individual characteristics. Such a large difference between people is the main reason why universal drinking norms do not exist.
Along with sweat, electrolytes are lost, primarily sodium, as well as chloride, potassium and small amounts of magnesium and calcium. The concentration of sodium in sweat differs substantially between people: so-called "salty sweaters" leave white salt stains on their clothes after a workout. For them the question of replacing sodium is especially important.
Water also maintains blood plasma volume. When it decreases, it is harder for the heart to supply blood at the same time to the working muscles and to the skin for cooling. Heart rate at the same running speed rises, and subjectively the effort feels harder.
How dehydration affects performance
The guidelines of the ACSM and the National Athletic Trainers' Association of the USA (NATA, 2017) agree that a fluid loss of more than 2% of body weight can impair aerobic performance and cognitive function, especially in warm conditions. For a person weighing 75 kg this is only 1.5 kg — a volume easily lost in an hour or an hour and a half of intense training in summer.
The effect of dehydration on strength and short explosive efforts is less pronounced than on endurance, but at higher degrees of dehydration strength performance also declines and coordination and concentration worsen. In sports with weight categories, where rapid "weight cutting" is practised, these effects are especially noticeable.
The danger lies not only in a drop in performance. Dehydration raises body temperature and the load on the cardiovascular system, which increases the risk of heat exhaustion and heatstroke. That is exactly why, during training in the heat, fluid control is a matter of safety, not just of efficiency.
At the same time, there is no need to panic over a minor fluid loss. A small deficit within 1–2% of body weight during a workout is a normal phenomenon that the body easily compensates after the session. Current recommendations are aimed not at fully replacing every millilitre of sweat, but at preventing excessive dehydration.
Calculating your own need
The simplest and most accurate method at home is weighing before and after training. Weigh yourself without clothes before the session, note how much fluid you drank during it, and weigh yourself again afterwards, having wiped yourself dry. Each kilogram of lost weight corresponds to roughly a litre of lost fluid.
The formula looks like this: sweat rate (L/h) = (weight before − weight after + fluid drunk − urine, if any) ÷ duration of training in hours. It is worth repeating this test several times under different conditions — in cool and hot weather, on a light and an intense workout.
| Indicator | Example | Comment |
|---|---|---|
| Weight before training | 80.0 kg | Without clothes, after using the toilet |
| Weight after training | 78.9 kg | Wiped dry |
| Drunk during the session | 0.5 L | Account for all fluid |
| Duration | 1 h | — |
| Sweat rate | 1.6 L/h | (80.0 − 78.9 + 0.5) ÷ 1 |
| Weight loss | 1.4% | Below the 2% threshold |
An additional guide is urine colour: a light straw colour usually indicates adequate hydration, while dark indicates a fluid deficit. This method is imprecise: colour is changed by B-group vitamins and some foods. Nevertheless, it is useful for daily self-monitoring.
The basic daily water requirement is estimated by the European Food Safety Authority (EFSA, 2010) at roughly 2.5 L for men and 2.0 L for women, taking into account water from food. For people who train, losses through sweat are added to this figure.
Drinking regimen before, during and after training
Before training.The ACSM recommends slowly drinking about 5–7 mL of fluid per kilogram of body weight roughly 4 hours before exertion. If urine remains dark or scanty, another 3–5 mL/kg can be added 2 hours before the start. The goal is to begin training in a state of normal hydration.
During training.For sessions of up to an hour in moderate conditions, water drunk according to thirst is usually enough. For longer and more intense exertion, especially in the heat, a drinking plan should be built from the results of weigh-ins so that losses do not exceed 2% of body weight.
- Drink in small portions regularly, rather than a large volume all at once.
- For sessions longer than an hour, drinks with sodium and carbohydrates are advisable.
- Cool drinks are usually consumed more willingly in the heat.
- During training, do not drink more than you lose in sweat.
After training.For rapid recovery it is recommended to drink roughly 1.25–1.5 L of fluid for each kilogram of lost weight. The volume is greater than the losses because part of the fluid is excreted in urine. Sodium from food or drink helps retain the water consumed.
If there are more than 12 hours until the next workout, ordinary meals and water according to thirst are usually enough. Aggressive fluid recovery is needed with two sessions a day or competitions in several stages.
Hyponatremia: the risk of excessive drinking
The opposite problem to dehydration is exercise-associated hyponatremia, that is, a dangerous drop in the sodium concentration in the blood. Its main cause is consuming fluid in a volume that exceeds losses, especially during prolonged exertion such as a marathon or ultramarathon.
The consensus on exercise-associated hyponatremia (Hew-Butler et al., 2015) emphasises: the greatest risk is faced by slow participants in long competitions who drink at every station "just in case" and gain weight over the distance. Weight gain over the course of the event is a warning sign.
The symptoms of mild hyponatremia — nausea, headache, swelling of the fingers, general weakness — are easily confused with dehydration. In severe cases confusion, seizures and cerebral oedema develop, and the condition becomes life-threatening. So the "the more the better" approach to drinking is a mistaken strategy.
The most practical protection is to drink according to thirst or an individual plan calculated from weigh-ins, and not to gain weight during exertion. Sports drinks with sodium can slightly reduce the risk, but do not protect if the total volume of fluid is excessive.
Editorial conclusions
The need for water during training is individual and depends on the sweat rate, which is easily determined by weighing before and after the session.
The practical goal is to avoid losing more than 2% of body weight, while at the same time not drinking more than is lost in sweat. Both extremes worsen well-being and can be dangerous.
For short workouts, water according to thirst is enough; for long and hot ones, a drinking plan with sodium and carbohydrates; and after the session, replacement of losses with a reserve.
The editorial team also recommends reading our materials about nutrition in the heat, about electrolytes and isotonic drinks and about recovery after training.
References
- American College of Sports Medicine; Sawka MN, Burke LM, Eichner ER, et al. American College of Sports Medicine position stand. Exercise and fluid replacement. Med Sci Sports Exerc. 2007;39(2):377–390.
- McDermott BP, Anderson SA, Armstrong LE, et al. National Athletic Trainers' Association position statement: fluid replacement for the physically active. J Athl Train. 2017;52(9):877–895.
- Thomas DT, Erdman KA, Burke LM. American College of Sports Medicine joint position statement. Nutrition and athletic performance. Med Sci Sports Exerc. 2016;48(3):543–568.
- Hew-Butler T, Rosner MH, Fowkes-Godek S, et al. Statement of the Third International Exercise-Associated Hyponatremia Consensus Development Conference, Carlsbad, California, 2015. Clin J Sport Med. 2015;25(4):303–320.
- Cheuvront SN, Kenefick RW. Dehydration: physiology, assessment, and performance effects. Compr Physiol. 2014;4(1):257–285.
- EFSA Panel on Dietetic Products, Nutrition, and Allergies. Scientific Opinion on Dietary Reference Values for water. EFSA J. 2010;8(3):1459.
Andriy Melnyk
A strength-sports coach and author of programs for beginner and intermediate levels. Writes about training planning.
