During long and/or intense exercise, the body’s thermoregulatory, metabolic and ventilatory functions gradually deplete our water reserves (50 to 70% of total body mass in adults). What attitude should we adopt then? Hydrate “at will” to offset these losses? Or hydrate methodically at the risk of not drinking enough and to brush against performance (lack of lucidity, increased exercise difficulty, fall and sprain)? Here are some useful tips.
1- Athletes who practice in warm conditions have an amplified sudoral flow due to the metabolic heat they produce AND the surrounding heat (Fig. 1). Other factors also amplify sweating: exercise intensity, level of training, degree of acclimatization to heat, level of hydration, genetic factors… For example, look at the sweat rate of the different athletes in Fig. 2.
2- When this sudoral flow becomes very important, a hydration “at will” does not make it possible to replace the water losses. Indeed, the stomach has a limited emptying capacity (~1L.h) and therefore cannot compensate for the sudoral flow (sometimes >2L.h). Consequence: the body’s water balance becomes negative.
3- A body water deficit greater than ~2% of body weight (or ~3% of total body water) is considered a “dehydrated” condition. This deficit results in a net decrease in plasma volume, which has repercussions on the filling of the heart in blood. At the end of the chain, we then have cardiovascular and muscular disturbances (especially during intense exercise).
4- In terms of impact, dehydration has no / little effect on endurance performance in cold and cool environments, it sometimes deteriorates performance in temperate environments and it systematically degrades performance in warm/hot conditions.
5- Theoretical/Practical insightful observation: for every ~1°C increase in skin temperature above 27°C, dehydration deteriorates endurance performance by ~1% (figure below). Make the count on your watts, it goes fast…
On this basis, here are some recommendations related to your hydration:
1- In a temperate environment, regulate your hydration from the last hour before exercise at a rate of 3-4 sips at intervals of 10-15′ and by combining water / exercise drinks. This helps you get started by being “well” hydrated. During the exercise, you can forget such a methodical aspect and focus on a more spontaneous hydration (i.e. to match your thirst). This is an issue that has occupied much of the scientific debate and on which there is now an emerging consensus.
2- In hot conditions (>28°C), things change. The recommendation first suggests careful control of your hydric state before exercise via your weight data (do not go down to more than 200-300g of your normal weight), urine (the darker they are, the more you should drink) and sensations of thirst. These markers allow you to adjust your intake according to how your body reacts to heat. In addition, prefer cool drinks (between 4°C and 10°C) while you are at rest. This will act directly at the center of the body to delay the internal temperature drift that occurs during exercise.
During exercise, on the other hand, prefer temperate drinks (~15°C). This is to limit the sensations of gastric discomfort while bringing a light and pleasant cold. Moreover, unlike temperate environments, you can “program” your hydration (ex: 5-6 sips every 10′). This rhythm has a strategic goal because during exercise, your thirst may be altered by heat and not encourage you to drink enough.
3- Whatever the exercise condition, estimate the amount of sweat lost at the end of the exercise to adjust your future intakes. This can be useful not only to limit dehydration, but also to prevent hyponatremia (dangerous overhydration in water). For this, a simple calculation is possible:
- Weight before exercise – weight after exercise + quantity of liquids ingested (ex: 68 kg – 66,5kg + 1,2L = 2,7 L). You can also get your sweat rate by crossing this number by the exercise time : 2.7L / 2h = 1.35L.h.
4- During recovery, keep in mind that some of the water you drink is not assimilated by the body (10 to 30%). On this basis, it is advisable to hydrate up to 150% of the calculated sweat losses, repeatedly rather than all at once. Note, milk and orange juice are the products that rehydrate best, also lean on it to recover’. They also provide carbohydrates and proteins to recover and repair your muscles.