Hydration and physical activities

[Article updated on 19/09/2023]

Water is not just for quenching thirst. We can survive several weeks without food, but without water we only last a few days. Overall, the body is two-thirds water; for muscle this proportion rises to around 75%.

Before reading on

I’m not an expert in this field, but I am passionate about nutrition and health.

The articles you’ll find on my site are the result of in-depth research that I’d like to share with you. However, I would like to stress that I am not a health professional and that my advice should in no way replace that of a qualified physician. I’m here to guide you, but it’s important that you consult a professional for specific questions or medical concerns. Your well-being is important. So be sure to consult the appropriate experts and take the best possible care of yourself.

Each gram of protein retains with it 3 grams of water and glycogen, a real storehouse of glucose, does the same: one gram of glucose for 3 grams of water.

There is water everywhere: in the cells, outside the cells, between the cells.

But if there is so much water in the body, what is it for?

First, water allows the free circulation of several elements: blood is an aqueous substance in which nutrients (glucose, proteins, lipids, vitamin, minerals, etc.) are dissolved to transport them to the site of use. The body’s waste products also circulate in the blood vessels and must be eliminated from the body. For example, during muscular effort CO 2 is produced by the muscle, then released into the blood. This brings the CO 2 to the lungs which, in turn, returns it to the atmosphere.

Water also acts as a heat pump for the body. During the practice of a sport, there is a significant production of heat inside the cells: the body becomes a real furnace.

Since body temperature cannot vary considerably, efficient and well-established mechanisms are required to dissipate this heat.

Water, present in the muscle cell, captures this heat and carries it to the surface of the skin where it can be dissipated by evaporation of sweat. The more intense the effort, the greater the amount of heat produced, then you will have to sweat to restore body temperature. Finally, the hydration of tissues and cells ensures smooth sliding between the different parts of the machine. Examples of this are synovial fluid in the knee joint, saliva in the oral cavity or cerebrospinal fluid in the skull. Water is also used in many chemical reactions, including facilitating the production of energy by cells.

Mouth watering

Every day we lose fluids. On average, a sedentary person eliminates 2 to 2.5 liters of water daily.

On a hot day it’s even more and when you train the losses skyrocket. We therefore drink to replace the lost fluid since the body has no other means to compensate for these losses. Even if we swim for hours, live in a humid climate or relax in a floating bath, water does not penetrate not through the pores of the skin. Only the digestive tract allows water to enter the body.

It must therefore necessarily be obtained through the drinks and foods consumed.

hydrate during a sports session

The dehydration

When a person exercises, water loss increases for two reasons: the respiratory rate accelerates and sweating becomes more abundant.

Physical exertion generally increases the respiratory rate, that is to say the number of inhalations/exhalations per minute. However, each time we exhale we lose a little water vapor.

To convince yourself, just exhale in front of a mirror: the cloud of mist formed is water.

But this increase in losses attributable to respiration is relatively small when compared to that generated by perspiration.

Depending on the weather conditions, the intensity of the effort and the ability to sweat, water loss through sweat can easily reach 1 to 2 liters per hour, sometimes even more.

Urine production, on the other hand, is reduced during training.

To know precisely how much total water the body has lost during a sporting performance, simply weigh yourself just before and just after exercise, ideally without clothes and on the same scale.

The weight difference is attributable to dehydration, not loss of body fat.

Each kilo of weight less on the scale corresponds to one liter of water volatilized.

Since the body is not 100% efficient at reabsorbing all swallowed liquids, you should drink 1.5 liters of liquid for every kilo lost.

I’m thirsty… is it time to drink?

The thirst reflex is triggered once the body is dehydrated to a level between 1% and 2%. For optimal performance, you must follow your hydration plan, since thirst is not a good indicator

Training hydration plan

  1. Record your weight before and after training in different conditions (indoor or outdoor training, weather conditions, etc.).
  2. Evaluate the volume of fluid that was drunk during training. Write down this quantity = B
  3. Calculate the weight lost: Weight before (kg) – weight after (kg) = Weight lost (kg)
  4. Estimate the volume of water: Weight lost (kg) = Water lost (kg) = D.
  5. Evaluate the amount of fluids that should be consumed during the next workout:

B (liquids consumed during training) + D (volume of water lost) = Volume of liquids to consume.

  1. Distribute this quantity over the entire duration of the training, starting from the first minutes of the activity: Volumes of liquids + 15 minute increments.


Simon, cyclist.
2 hour training.
Weight before: 75 kg / Weight after: 73.8 / Weight lost 1.2 kg / Bus liquid: 800ml
Liquids to drink during the next training: 800ml (bus) + 1.2 liters = 2 liters.

Hydration plan:
2000 ml over 2 hours = 1000 ml/hour (1 hour = 4×15 minutes)
1000 ml/4 = 250 ml every 15 minutes over two hours