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Nutrition and Performance in Sport

Christophe Hausswirth

Chapter 3. Nutrition and lifestyle

Topic 10. Hydration

Christophe Hausswirth, Amélie Fosse, Axel Heulin, Yann Le Meur, Véronique Rousseau, Eve Tiollier et École Lenôtre

Texte intégral

Questions and answers

Key points
Athletes must replace the fluids lost during exercise as rapidly as possible. Full fluid replacement and retention can only be achieved by ingesting 150 to 200% of the water deficiency measured.
Rehydration guided by thirst alone will always be insufficient to balance the water losses linked to the practice of exercise.
Faster water replenishment can be achieved by ingesting large volumes of liquid (rather than small amounts) rapidly over a longer period. After this initial ingestion, it is recommended that 200 ml be consumed every 15 minutes.
A carbohydrate concentration of about 2% can facilitate water and sodium absorption in the intestine.
Energy drinks are recommended when training lasts more than an hour and are necessary during competitions.

How is water distributed throughout the body?

1Water represents 40 to 70% of an individual’s body mass (depending on age, gender and body composition) and constitutes 65 to 75% of muscle mass and about 15% of fat mass. Thus, water is the main constituent of the body and athletes must optimize their water requirements. Water is necessary to several physiological functions: cellular activity, the cardiovascular system, regulation of body temperature, renal elimination, etc.

What role does water play in the body?

2Water is a remarkable and omnipresent nutrient. In the body, it acts as a mean of transport and as a reaction medium: indeed, very often gas diffusion is only possible through surfaces impregnated with water. Thanks to its incompressibility, water gives the body its structure and shape through the volume increase it creates in tissues.

3The amount of water present in the body remains relatively stable. Although significant volumes can be lost during physical activity, absorption of liquids quickly restores any liquid imbalances. An averagely active adult, in a neutral thermal environment, requires about two and a half litres per day. This water can be supplied through three different sources: drinks, food and metabolic processes.

4Thirst is not a good indicator of the rehydration requirements caused by exercise: it is thus necessary to drink before feeling thirsty, and to absorb greater amounts than those required simply to slake one’s thirst. The volume to be consumed must be determined individually and adjusted according to individual tolerance, with the objective of balancing the losses. Losses must be estimated based on the characteristics of exercise (intensity, length), environmental conditions (thermal environment, relative humidity, exposure to sun) and more individual factors (training level and state, adaptation to heat). In addition, to favour gastric emptying, it is better to ingest large volumes, in line with individual tolerance.

How can you tell if you are well hydrated?

5During exercise, dehydration is caused by sweating. If this fluid loss is moderate (equal to 0.5% of body weight), no side effects appeared. However, above this value – which is rapidly exceeded by many athletes – various complications impede good exercise. This dehydration then leads to an imbalance of fluid movements, i.e. water losses are not balanced by drinking (if the athlete was originally normo-hydrated or hyper-hydrated). To determine the level of dehydration, the simplest measure is to use double weighing, which limits the poor reliability of body scales (variations up to ± 0.5 kg). This measurement should be taken when the athlete is naked and dry, to avoid counting the water retained in clothes or remaining on the skin. The bladder should also be empty. The following formula can then be applied:

Weight before (WB) – weight after (WA) = mass of water lost (mwl) Mwl/WB x 100 = dehydration percentage

6The loss due to sweating corresponds to the difference between the two weights, minus the weight of urine collected after exercise:

Mwl – urine = losses due to sweating

7By measuring body weight before and after exercise, you can determine how much water was lost, since most of the weight loss is due to fluid loss through sweating and evaporation when breathing. Given the additional substrate losses and metabolic water losses, it is not necessary to maintain exactly the same weight to preserve water balance. It is also important to adapt water intake and the hydration plan to ambient temperature. Therefore, it is important to remember to drink more when it is hot.

8After exercise, at least 1.5 times the volume corresponding to the weight lost during exercise should be consumed. For instance, if the weight loss is estimated to be 500 g, 750 ml of water must be consumed to achieve an appropriate level of rehydration.

9Clear urine in the morning and on an empty stomach generally reflects good hydration, if the athlete does not suffer from any disease or is not taking any medication that could cause a colour change.

10Any dehydration greater than 0.5% of body weight must be balanced as quickly as possible.

Can the hydration state affect physical performance?

11Even slight decreases in the body’s water resources affect performance. As it proceeds, exercise is not well tolerated if the water losses due to sweating and respiration are not balanced. Thus, given the importance of water and the consequences of its deficiency, it is essential to compensate for its loss as quickly and completely as possible. This applies not only to the losses caused by exercise, but also to those linked to normal daily activities. It is now acknowledged that dehydration decreases athletes’ physical capacities by increasing their heart rate and internal temperature more than exercise would be expected to. For example, a water deficiency corresponding to a 2% drop in body weight reduces aerobic aptitudes by about 20%. Various studies have thus shown that dehydrated subjects cannot tolerate prolonged exercise. However, the effect of dehydration on performance is different from one physical activity to another. Because of the way dehydration appears, ambient weather conditions during exercise and the kind of exercise are also key factors.

12Beyond 4% of body-weight loss, the cardiovascular system becomes overloaded, with an increase in heart rate and stroke volume.

13For short and intense exercise, the effects of dehydration appear less significant. Indeed, during efforts lasting a few seconds, anaerobic performance does not seem particularly affected, provided subjects can drink freely, in line with the fluid losses observed. In addition, in a population where body mass was decreased by about 3% (water loss), a decrease of 23% in the time to exhaustion was observed when they performed isometric knee extension exercises at 25% of maximal voluntary force. This must imperatively be taken into account for strength disciplines, where static muscular contractions are frequent.

Can dehydration affect cognitive performance?

14Objective data using cortical function tests confirmed the hypothesis that even moderate dehydration affects mental performance in young adults. Short-term and working memories appear most affected, whereas long-term memory was spared. A decrease in performance was thus observed in events involving psychomotor skills, with a 10.7% increase in reaction time with 2% dehydration, and a 21.4% increase with 4% dehydration. In addition, the percentage of correct answers decreased by 10.6% with 2% dehydration, and 22.4% with 4% dehydration. Beyond a 2% loss in body mass, cognitive performance is clearly affected regardless of how dehydration came about (for instance: hot environment, prolonged exercise, etc.). This is important in sports or physical activities requiring rapid reaction time, as well as very precise gestures, but it becomes crucial in exercises with combined events – for example biathlon (rifle shooting and cross-country skiing) or modern pentathlon (during events combining pistol shooting and running) – where this kind of effect is even more deleterious.

Is it good to drink sparkling water or water enriched in bicarbonates?

15When performing sports activity, the blood acid-base imbalance perturbs the endocrine system, the availability of muscle and blood energy substrates, the cardiovascular and respiratory systems, as well as the subjective perception of fatigue. This is why, for almost 20 years, anaerobic athletes have been advised to consume alkaline drinks before and after intense exercise. However, even though this relationship appears interesting, as it enhances endurance time (between 120 and 240 seconds), consuming bicarbonates before exercise has been linked to gastrointestinal problems, especially when the bicarbonate concentration exceeds 300−1 of body weight. Therefore, it is highly recommended that alkaline drinks be consumed during the early phase of recovery. If your objective is to reduce the time until the acid-base balance is restored, you should favour bicarbonate supplementation (in the form of carbonated drinks) after exercise. As some waters contain higher levels of bicarbonates than others, it is better to choose one containing a lower concentration. This is because the amount of water ingested is more important than the bicarbonate concentration.

Is it necessary to ingest a carbohydrate drink during exercise?

16As a rule, for continuous exercise lasting less than one hour, water is enough. Beyond an hour of exercise, including during muscle-strengthening sessions, it is necessary to consume an energy drink. On the day of a competition, it is essential since carbohydrate intake during exercise increases endurance time and maximal strength capacity. Adding glucose to the drink also increases water and sodium absorption: one of the first beneficial effects of consuming carbohydrate during exercise is to restore water balance as far as possible.

What type of energy drinks should be favoured?

17Which energy drink to consume must be determined based on the aim as there is no energy drink universally adapted to all sports, all athletes and to all weather conditions. When exercise requires a lot of energy, the drink must provide a large amount of carbohydrate, at least covering the requirements linked to the practice. You must therefore choose a high-carbohydrate-concentration drink (about 80 g.l−1), mainly composed of sucrose, maltose or maltodextrins. On the contrary, if the priority is water, only about 30 to 60 g.l−1 carbohydrate is required. You can add a small amount of salt (1 to 1.2 g.l−1), which helps to maintain athletes’ hydration status and thus to prevent symptoms linked to dehydration. Importantly, it is better to test the drink several times during training rather than using it systematically during competitions. A drink that is tolerated at rest may suddenly no longer be tolerated during exercise or even become intolerable during a competition, due to stress. Energy drinks can be made more palatable by adding flavourings and maintaining it at a cool (rather than cold) temperature.


18An energy drink contains:

  • Water

  • a mixture of at least two carbohydrates (of which the concentration will be determined by the ambient temperature)

  • salt (about 1 g.l−1).

19The carbohydrate concentration of the energy drink varies between 20 and 70 g.l−1, in line with ambient temperature, because the organism cannot oxidize more than 70 g of carbohydrates per hour.

20In practice: the hotter the temperature, the more the energy drink should be diluted and vice versa.


22It is important not to mix up energy drinks for effort (such as Gatorade, Isoxan sport, etc.) and other energy drinks (such as Red Bull, Burn, etc.).

23The latter do not present any advantage for athletes because they contain too much carbohydrate and caffeine. In addition, they contain substances which have not yet been proven to be entirely safe, like glucuronolactone.

What role does caffeine play in the athlete’s activity?

24The effects of caffeine of human physiology are complex and not easy to isolate. Ingesting caffeine significantly increases the adrenalin concentration in the blood. The effects of this are more visible at rest than during exercise. In addition, ingesting caffeine increases the plasma concentration of fatty acids and glycerol. Even though it is not unanimously approved, caffeine might, in some cases, increase the use of fats as an energy substrate during exercise, especially when circulating glucose is low. However, consuming caffeine before exercise does not appear to spare muscle and hepatic glycogen.

25In terms of performance, it has been repeatedly shown that caffeine absorption increases the aerobic endurance capacity and postpones the onset of fatigue during exercise lasting 30 to 60 minutes; for shorter exercises the results are less convincing.

Practical applications

1. Drink a lot during the day

Image 10000000000000210000001CFDD6BADD.jpg It is essential to drink a lot, because dehydration can ruin your performance

26Drink at least 1.5 litres in addition to what you drink during training. For example:

Image 100000000000040500000327404408E8.jpg

2. Drink before exercise

Before exercise, make sure you are well hydrated. The colour of your urine is a good indicator Image 10000000000000EA00000032ED6B24B6.jpg

3. Drink during exercise

27What to drink:

  • water

  • or sports drink (during a competition or intense training).

28When to drink: regularly, every 15 to 30 minutes

29How much to drink: in theory between 0.5 and 1 l/h, but it is best to adapt the volume based on your needs. To figure this out, weigh yourself before and after exercise (with dry skin and clothes). Do this 2 or 3 times in the summer and in the winter (see the table below)

Image 1000000000000400000001D8EB0A9881.jpg

4. Post-exercise hydration

30What to drink:

  • water rich in minerals

  • or sports drink (after a competition or intense training).

31When to drink: as soon as training ends and then regularly

32How much to drink: 1.5 to 2 times the volume lost (see calculation below):

Image 10000000000003680000006C52F7806A.jpg

5. Prepare your sports drink

The composition of the energy drink depends on the duration of the activity and on the climate in which it is performed

Image 10000000000004090000038E31AD70A4.jpg


Thai Broth

Image 10000000000001E7000001E73BF8561B.jpg

Photo credit: Thibaut Ruggeri.

Image 10000000000001700000017B7F304821.jpg

33Serves 2

34Preparation time: 20 minutes

35Cooking time: 30 minutes

Difficulty: Image 10000000000000290000001CC179B9F3.jpg


37 1 litre chicken stock

38 250 g skinless chicken breast

39 300 g shiitake mushrooms

40 20 g fresh ginger

41 5 g kaffir lime leaves

42 1 lemongrass stalk

43 1 lime

44 1 quarter bunch of fresh coriander

45 10 g satay spices

46 20 g Thai fish sauce

47 A pinch of salt

48Cut the shiitake into strips.

49Peel the ginger and chop it.

50Crush the lemongrass stalk and cut it into four pieces.

51Cut the lime leaves in two.

52Sweat the mushrooms, lemongrass, ginger and lime leaves without adding any fat.

53Add the chicken stock, the lime zest and juice, the fish sauce and satay spices. Bring to the boil.

54Cut the chicken fillet into thin strips and gently poach in the hot stock.

55Add the chopped coriander.


57This broth can also be made with shrimps. You can make it more spicy by adding red chilli.

58Chef’s note

59This broth is ideal for rehydration after intense exercise or if your body weight has changed rapidly (in the case of weight-category sports). It is rich in sodium (salt) and potassium, and allows the body to compensate for significant sweat loss. Since it does not contain much carbohydrate, it should be served with carbohydrate-rich side dishes (pasta salad, juice, gingerbread, etc.).

60Nutritional analysis per person

Energy (Cal)

Carbs (g)



Lipids (g)

Protein (g)



Minty cucumber and apple smoothie

61Serves 2

62Preparation time: 10 minutes

Difficulty: Image 10000000000000290000001D112908E9.jpg

Image 10000000000001E3000001DE417AA820.jpg

Photo credit: Thibaut Ruggeri.


64 Half a cucumber

65 2 Granny Smith apples

66 2 tablespoons of Greek yoghurt

67 5 fresh mint leaves

68 Half a pomegranate

69Core the apples and dice them. Blend them with the half a cucumber.

70Add the mint leaves and Greek yoghurt.

71Strain and decorate with pomengranate seeds.

72Serve chilled.

73Chef’s note

74This drink contributes to good rehydration and is a source of carbohydrates.

75Nutritional analysis per person

Energy (Cal)

Carbs (g)



Lipids (g)

Protein (g)




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