Water, a Vital Molecule for the Human Body
Water is the most abundant component in the human body, playing a crucial role in its physiological functioning, from the cellular level to the entire organism. Within cells, water acts as a solvent, creating a favorable environment for the chemical reactions necessary for cellular function¹. Additionally, the movement of water between intra- and extracellular spaces is responsible for transporting essential molecules to the cells, such as nutrients and hormones¹. At the whole-body level, water is the main component of blood, facilitating the transport of vital molecules between organs and is predominantly present in protective tissues like mucus and joint fluid.
Therefore, the total amount of water and its distribution between intra- and extracellular spaces must be controlled and maintained over time for optimal functioning of the body. Specifically, the total water volume should be maintained at a volume corresponding to 73% of fat-free mass, with a distribution of about 60% intracellular water and 40% extracellular water². Physiologically, the regulation of these two parameters is a mix of mechanisms at the molecular level and the whole organism level.
From a cellular perspective, the main parameter used by the body to assess hydration is osmolarity, which depends on the concentration of electrolytes, particularly sodium and potassium. More precisely, if the electrolyte concentration changes, osmolarity will also change, activating regulatory mechanisms controlled by the kidneys. In the case of overhydration, osmolarity decreases, stimulating the kidneys to filter excess water from the blood and turn it into urine³. Conversely, during dehydration, osmolarity increases, reducing the filtration activity of the kidneys and thus urine production to minimize water loss. In addition to this water-saving mechanism, the body also needs to stimulate fluid intake. To achieve this, the brain secretes an antidiuretic hormone that activates the sensation of thirst, signaling the need to drink, in addition to its effect on kidney filtration. However, it seems that this hormone is released when osmolarity reaches a level corresponding to a 2% loss of body weight, at which point the physiological effects of dehydration begin to appear, as we will see later in this article.
Summary of Water Regulation in the Body
Dehydration
Throughout the day, the human body loses between 2 and 2.5 liters of water and is not able to produce an equivalent volume endogenously to compensate for these losses, although 300 ml can be obtained as by-products of body metabolism¹. These losses can increase if we are in a hot environment, perform physical exercise (through sweating), or ingest diuretic beverages (e.g., coffee, tea, alcohol)⁴.
Daily hydration variations represent about 1% of body weight, and it is currently considered that dehydration appears after a water volume loss equivalent to 2% of body weight³. Many studies have first analyzed the effects of dehydration on sports performance due to its high prevalence during exercise, and the data obtained show that it decreases performance only in endurance efforts and little or none in resistance sports, like weightlifting³. More specifically, dehydration exceeding 2% of body weight reduces the ability to maintain high exercise intensity and reduces exercise duration, two key factors in performance during this type of effort. Mechanistically, this is caused by an alteration in muscle energy metabolism associated with decreased blood flow to the muscles⁵. However, it is possible to extrapolate these results to daily life, where this dehydration limits everyday tasks such as shopping, walking, or cleaning.
Beyond the effects on physical performance, dehydration also affects cognition, notably by lowering attention span, short-term memory, and increasing fatigue⁷.
Summary of the Physiological and Cognitive Effects of Dehydration
Given these results, it is necessary to maintain an adequate hydration level and sufficient fluid intake throughout the day.
Fluid Intake
As mentioned in the previous paragraph, dehydration has a significant impact on the physiology of the human body and, therefore, it is necessary to have adequate fluid intake throughout the day. It is generally recommended to drink between 1 and 1.5 liters of water per day to compensate for fluid losses, with the rest being provided by food and metabolism. When temperatures are warmer or physical exercise is performed, it is recommended to drink at least 2.5 liters per day. Considering that the sensation of thirst appears when dehydration is already present, it is also recommended not to wait until you are thirsty to drink, especially in the elderly or children, as they do not always have a sense of thirst.
During exercise, water loss is caused by sweating, which serves to lower body temperature, but this is accompanied by a loss of sodium that limits performance, in addition to dehydration. Therefore, it is also necessary to limit sodium losses during exercise, especially during endurance exercises where these losses can be significant. For this, the best strategy is to have fluid intake during and after exercise, containing sodium, with a total volume greater than the volume lost⁴⁸. However, recent studies also suggest that this intake should be gradual to prevent part of the absorbed water from being converted into urine and not participating in rehydration⁸.
Conclusion
Water is a crucial component of the body as it participates in the functioning of the human body, both at the cellular and systemic levels, and dehydration is responsible for a decrease in physical and cognitive performance. Naturally, the human body loses between 2 and 2.5 liters of water every day, so it is necessary to have adequate and sufficient fluid intake to compensate for these losses, especially during warm temperatures and during exercise.
References
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- Wang Z, Deurenberg P, Wang W, Pietrobelli A, Baumgartner RN, Heymsfield SB. Hydration of fat-free body mass: review and critique of a classic body-composition constant. Am J Clin Nutr. mai 1999;69(5):833‑41.
- Cheuvront SN, Kenefick RW. Dehydration: physiology, assessment, and performance effects. Compr Physiol. janv 2014;4(1):257‑85.
- \Armstrong LE. Rehydration during Endurance Exercise: Challenges, Research, Options, Methods. Nutrients [Internet]. mars 2021 [cité 12 juin 2024];13(3). Disponible sur: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8001428/
- Febbraio MA. Does muscle function and metabolism affect exercise performance in the heat? Exerc Sport Sci Rev. oct 2000;28(4):171‑6.
- González-Alonso J, Calbet JAL, Nielsen B. Muscle blood flow is reduced with dehydration during prolonged exercise in humans. J Physiol. 15 déc 1998;513(Pt 3):895‑905.
- Adan A. Cognitive performance and dehydration. J Am Coll Nutr. avr 2012;31(2):71‑8.
- Evans GH, James LJ, Shirreffs SM, Maughan RJ. Optimizing the restoration and maintenance of fluid balance after exercise-induced dehydration. Journal of Applied Physiology. avr 2017;122(4):945‑51.
