Hydration and Sports Performance

When you exercise, the rule fits in one sentence: drink enough to keep your weight loss to under 2% during the session, without ever finishing heavier than you started. Beyond 2%, endurance performance consistently declines, according to the National Athletic Trainers’ Association (NATA, 2017). Conversely, drinking more than you sweat exposes you to hyponatraemia, a drop in blood sodium that can be serious. Since people sweat between 0.5 and 4 litres an hour depending on the individual and conditions, there is no universal volume: you need to know your own.

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What dehydration really changes

Water is used to remove the heat produced by the muscles, through sweat, and to maintain the blood volume that carries oxygen. When body water falls, the heart’s stroke volume and the blood supply to the sweat glands decrease. Here is what studies measure, according to the level of loss.

Body weight lost as water	Observed effect	Source
More than 1%	Impaired cognitive function (reaction time, precision tasks)	NATA, 2017
2% or more	Endurance performance consistently reduced, especially in mild or hot conditions	NATA, 2017
More than 2%, often 3 to 4%	Reduced strength, power and anaerobic endurance	NATA, 2017
About 3% and above	Perceived effort clearly harder (+0.21 points on the Borg scale per 1% lost)	Deshayes et al., 2022
More than 3%	Increased risk of exertional heat illness (thermoregulation is already impaired beyond 1%)	NATA, 2017
More than 4%	Slower stomach emptying, making rehydration harder	NATA, 2017

A meta-analysis of 28 publications (Savoie et al., 2015) puts the drop in muscular endurance at 8.3% on average in dehydrated people. It also notes that the effect on muscle performance is clearly smaller in trained athletes. In practice, for 70 kg, 2% is 1.4 kg.

Two caveats. NATA notes that in cool conditions, aerobic performance can be maintained despite a loss close to 2%. And the same meta-analysis on perceived effort concludes that the effect of dehydration is probably not noticeable in practice until a loss of about 3%. The 2% threshold is therefore a safety guideline, not a cliff edge.

How much do you sweat? The scales test

This is the only reliable way to know what to drink. NATA describes it like this:

Sweat lost (L) = weight before (kg) – weight after (kg) + drink consumed during (L) – urine passed during (L)

1. Weigh yourself naked, with an empty bladder, just before the session.
2. Note what you drink during exercise.
3. Weigh yourself naked and dried, just afterwards.
4. Divide by the duration in hours to get your sweat rate.

Example: you weigh 70.0 kg before and 68.8 kg after an hour’s run, and you drank 0.5 L. Your sweat loss is 1.2 + 0.5 = 1.7 L in one hour. Without drinking, you would have lost 1.7 kg, or 2.4% of your weight. With 0.5 L drunk, your loss is 1.2 kg, or 1.7%.

Repeat the test in several conditions: heat, cool weather, short session, long session. The differences are huge. NATA cites sweat rates of 0.5 to 4.0 L per hour in adults, and sodium losses of 0.2 to 7.3 g per hour, with a sweat sodium concentration ranging from 10 to 100 mEq per litre. Our hydration calculator helps you work it out.

Before exercise: starting well hydrated

The aim is to start the session at your usual weight, give or take 1%: NATA places optimal thermoregulation within this range. The position stand of the ACSM (American College of Sports Medicine, 2007) recommends, if necessary, drinking at least several hours before exercise, so that the body absorbs the fluid and your urine returns to normal before the start.

To check your status, NATA uses three personal indicators: thirst, how often you urinate and urine colour. It specifies that first morning urine is the most reliable; during the day, colour depends too much on what you have just drunk or eaten. NATA also notes that thirst on waking is strongly linked to a water deficit measured on the scales. Dark urine on waking and a weight drop compared with previous days can therefore signal a deficit to make up before the session.

If you have coffee before training, there is no need to worry: according to NATA, caffeine can increase urine output at rest but does not cause diuresis during exercise. It does not advise against a moderate dose of about 3 mg per kilo. For daily dose guidelines, see how much coffee a day.

During exercise: drink what you lose, no more

NATA recommends drinking to limit the loss to under 2% of body weight, getting close to your own sweat rate without exceeding it. A few practical guidelines:

• Pace: about 200 ml every 15 to 20 minutes can be ideal, when the sport allows it.
• Volume in the stomach: keeping 400 to 600 ml of fluid in the stomach optimises its emptying.
• Intensity: above 80% of maximal oxygen uptake, stomach emptying may slow down.
• Taste and temperature: athletes drink more readily when a drink is cool and flavoured, which helps rehydration.

Water or sports drink?

For a session of under an hour, water covers your fluid needs. Carbohydrate becomes a performance lever for intense efforts of about an hour or more, and sodium for efforts of over an hour, especially in the heat. NATA points out that, normally, an active person eating a balanced diet does not need to add anything special to their drink.

• Carbohydrate: for an intense effort of about an hour, small amounts, or even simply rinsing your mouth, improve performance. For longer efforts, the target is 30 to 60 g per hour, and up to 90 g per hour beyond 2 h 30 (Burke et al., 2011).
• Concentration: NATA puts the optimal concentration at 3 to 8% carbohydrate for water absorption; above 5 to 8%, absorption may slow down.
• Sodium: for efforts of over an hour, a sodium intake can help athletes who sweat heavily and saltily. NATA cites those whose sweat exceeds 60 mEq/L of sodium with a sweat rate above 2.5 L/h. Supplementation should be individualised and tested in training.

The European Union regulates what is called a “carbohydrate-electrolyte solution”. To state that it contributes to the maintenance of endurance performance or improves water absorption during exercise, a drink must provide 80 to 350 kcal per litre from carbohydrates, of which at least 75% from carbohydrates with a high glycaemic response (glucose, glucose polymers, sucrose), 460 to 1,150 mg of sodium per litre, and have an osmolality of 200 to 330 mOsm/kg of water.

Drink (per 1 litre)	Energy	Carbohydrate	Sodium	Within EU criteria?
Tap water (average value)	0 kcal	0 g	24 mg	No (no carbohydrate or sodium)
Sports drink, sweetened	217 kcal	54 g (5.4%)	502 mg	Energy and sodium yes
Sweetened cola	401 kcal	100 g (10%)	67 mg	No: too sugary, not salty enough
Pure orange juice	454 kcal	96 g (9.6%)	48 mg	No: too sugary, not salty enough
Sweetened energy drink	409 kcal	102 g (10.2%)	462 mg	No: too sugary

Cola and fruit juice are twice as concentrated in sugars as the NATA range and almost devoid of sodium. Diluted by half with water, they come closer to the right concentration. A homemade example, calculated with Ciqual: 500 ml of orange juice, 500 ml of tap water and 1.5 g of salt give about 227 kcal, 48 g of carbohydrate (4.8%) and 620 mg of sodium per litre. That is within the energy and sodium ranges, but the sugars in orange juice do not necessarily meet the criterion of 75% high-glycaemic carbohydrate, and osmolality is not measured: it is a stopgap, not a drink that meets the European definition. Test it in training first.

Energy drinks: avoid them during exercise

In its October 2013 opinion, Anses states that so-called “energy” drinks should be reserved for adults and are not recommended for athletes. It identifies drinking them during physical activity, like combining them with alcohol, among the situations at risk of serious adverse effects. They are not sports drinks, despite their name.

After exercise: rehydrating and refuelling

When recovery is short, under 4 hours before the next session, NATA recommends drinking up to 150% of the estimated loss, to offset the urine produced by the large intake of fluid. If you lost 1 kg, that is up to 1.5 L spread over the following hours. When you have more than 12 hours ahead of you, your usual meals and drinks are enough to restore balance.

NATA also advises eating and drinking within 2 hours of the activity, to replace water, electrolytes, carbohydrate and protein. After a very intense or prolonged effort that has depleted glycogen stores, the European Union recognises that an intake of 4 g of carbohydrate per kilo, taken within the first 4 hours and no later than 6 hours afterwards, contributes to the recovery of muscle function. The detail of meals is in our guide what to eat before and after exercise.

• Meals count: according to NATA, the water and electrolytes in meals contribute to rehydration and can improve retention of the water you drink. A soup or a seasoned dish helps.
• Alcohol: according to NATA, drinks of 4% alcohol or less do not dehydrate athletes, but stronger drinks promote excessive diuresis and should be avoided for rehydration.
• Caffeine: up to 3 mg per kilo during or after exercise, it does not compromise rehydration according to NATA.

On the plate, water-rich foods help rehydration: cucumber contains 95.2 g of water per 100 g, tomato 94.1 g and watermelon 91 g according to Ciqual. A gazpacho, a carrot and ginger soup or a green spinach and pineapple smoothie combine water, minerals and carbohydrate. For recovery protein, see our high-protein recipes, including the banana-cocoa recovery shake. For the full list of the most water-rich foods, see our guide how much water to drink a day.

The opposite risk: drinking too much

Exercise-associated hyponatraemia is defined as blood sodium below 135 mmol/L during exercise or within 24 hours afterwards. It generally becomes symptomatic below 130 mmol/L and can be a medical emergency. NATA attributes it to excessive consumption of low-sodium drinks, including sports drinks, often combined with insufficient kidney excretion, during efforts of an hour or more.

The reference study covers the 2002 Boston Marathon (Almond et al., 2005). Of 488 runners sampled at the finish, 13% had hyponatraemia and 0.6% a critical form, with sodium at 120 mmol/L or less. The associated factors were: weight gain during the race, more than 3 litres drunk, a drink at every mile, a finishing time over 4 hours and a low body mass index. The composition of the drinks was not a factor.

The most common signs of mild forms are nausea, weakness, dizziness, headaches and swelling of the hands or feet (Armstrong et al., 2025). Severe forms involve confusion, vomiting, seizures or loss of consciousness. The simplest guideline: you should never weigh more at the finish than at the start. In Boston, runners who took more than 4 hours were the most exposed.

Heat, team sports, very long efforts

• Heat: NATA recommends building heat acclimatisation into the hydration plan. Repeat the scales test whenever the season or conditions change.
• Team sports: breaks are rare; make the most of every stoppage in play and keep a bottle within reach.
• Very long efforts: in ultra-endurance, a 2025 review concludes that no strategy is superior in all cases: neither a fixed schedule nor drinking to thirst. A mixed approach, based on your measured sweat rate and how you feel, seems the most practical.
• Children, older people, heart or kidney conditions: needs and risks differ; ask a sports doctor for advice.

Choosing your water: a detail that counts over time

The bottle matters more than the brand. According to Ciqual, a magnesium-rich water such as Rozana provides 160 mg of magnesium per litre, against 8 mg for Volvic. Over 3 litres drunk on a training day, the difference becomes noticeable. Conversely, very salty waters such as St-Yorre (1,710 mg of sodium per litre according to Ciqual) exceed the upper sodium range for sports drinks: they are not meant to be drunk in large quantities every day. Our bottled water comparison details 19 waters, mineral by mineral.

Myths about athletes’ hydration

“By the time you’re thirsty, it’s already too late”

This needs qualifying. At rest, thirst appears when the loss approaches 2%. When you drink to thirst during exercise, you generally replace about two thirds of the sweat lost. During exercise, these signals seem to be absent or ignored, and NATA considers that drinking only to thirst does not maximise performance. But in cool conditions, or when thirst is quenched, aerobic performance can be maintained. Thirst is a useful signal, not a systematically late alarm.

“The more you drink, the better”

False. NATA states that overhydration brings no endurance benefit and increases the risk of hyponatraemia. Drinking beyond your losses is a mistake, not a precaution.

“Coffee dehydrates you”

Not at moderate doses. In a crossover trial on 50 male coffee drinkers, 4 cups of 200 ml a day (4 mg of caffeine per kilo) gave the same hydration markers as the same amount of water (Killer et al., 2014). And during exercise, caffeine does not cause diuresis according to NATA.

“Creatine dehydrates you”

NATA concludes that at recommended doses, it does not compromise hydration. The detail is in our creatine guide.

“A sports drink is essential”

Not for a short session. Carbohydrate and sodium become useful when the effort exceeds an hour, when it is hot, or when you sweat heavily and saltily. Otherwise, water does the job. And a sugary drink consumed outside exercise mainly provides sugars: Manger Bouger recommends not exceeding one glass of sugary drink a day.

Frequently asked questions

How much should you drink during an hour of exercise?

It depends on your sweat rate, which varies from 0.5 to 4 L an hour. Measure it with the scales test, then drink enough to limit the loss to under 2% of your weight, without gaining weight. About 200 ml every 15 to 20 minutes is a starting point suggested by NATA.

Should you drink before you feel thirsty?

During a long effort or in the heat, yes: thirst signals are unreliable during exercise. For a short session in cool conditions, drinking to thirst is generally enough.

How can you tell if you are well hydrated in the morning?

Look at the colour of your first morning urine, which NATA considers the most reliable, and compare your weight with previous days. Dark urine and a dropping weight signal a deficit.

Are salt tablets useful?

Only for certain profiles: efforts of over an hour, with heavy and very salty sweat. NATA notes that few data support salt tablets for retaining water during exercise, and recommends individualising sodium and testing it in training.

How much should you drink after exercise?

If the next session is less than 4 hours away, up to 150% of the weight lost: 1.5 L for 1 kg lost, spread over time. Otherwise, your usual meals and drinks are enough.

This article is for information only and does not replace the advice of your doctor or a dietitian.

Our sources

• McDermott BP, Anderson SA, Armstrong LE et al. National Athletic Trainers’ Association Position Statement: Fluid Replacement for the Physically Active. Journal of Athletic Training, 2017. PMID 28985128.
• American College of Sports Medicine, Sawka MN, Burke LM et al. American College of Sports Medicine position stand. Exercise and fluid replacement. Medicine and Science in Sports and Exercise, 2007. PMID 17277604.
• Savoie FA, Kenefick RW, Ely BR et al. Effect of Hypohydration on Muscle Endurance, Strength, Anaerobic Power and Capacity and Vertical Jumping Ability: A Meta-Analysis. Sports Medicine, 2015. PMID 26178327.
• Deshayes TA, Pancrate T, Goulet EDB. Impact of dehydration on perceived exertion during endurance exercise: A systematic review with meta-analysis. Journal of Exercise Science and Fitness, 2022. PMID 35601980.
• Burke LM, Hawley JA, Wong SH, Jeukendrup AE. Carbohydrates for training and competition. Journal of Sports Sciences, 2011. PMID 21660838.
• Almond CS, Shin AY, Fortescue EB et al. Hyponatremia among runners in the Boston Marathon. New England Journal of Medicine, 2005. PMID 15829535.
• Armstrong LE, McDermott BP, Young SL, Casa DJ. Exercise-Associated Hyponatremia: Serum Sodium, Symptomatology, Severity, and Sport Specificity. Open Access Journal of Sports Medicine, 2025. PMID 41280657.
• Wierick SC, Perez RI, Zhao X, McDermott BP. Hydration Strategies in Ultra-Endurance Running: A Narrative Review of Programmed Versus Thirst-Driven Approaches. Nutrients, 2025. PMID 41305577.
• Killer SC, Blannin AK, Jeukendrup AE. No evidence of dehydration with moderate daily coffee intake: a counterbalanced cross-over study in a free-living population. PLoS One, 2014. PMID 24416202.
• European Commission. Regulation (EU) No 432/2012, annex: carbohydrate-electrolyte solutions; carbohydrates and recovery (EFSA opinion 2011;9(6):2211).
• Anses. Opinion on the risk assessment of so-called energy drinks (in French), 1 October 2013.
• Anses. Ciqual food composition table (in French), 2025.