Water balance regulation
Water balance is maintained thanks to homeostatic controls on a small range of fluctuation.
Of the types of water losses and gains, the following are under homeostatic control:
- Fluid intake, which can be triggered by thirst
- Renal water output, which varies inversely to hydration status.1 In case of dehydration, kidneys conserve water, and in case of hyperhydration, water is eliminated in urine
The onsets of thirst and water conservation by kidneys are triggered by a rise in plasma osmolality, but at different thresholds:
- Vasopressin regulation leading to water conservation starts at an average plasma osmolality of 280 mOsm/kg H2O
- Thirst is perceived only when plasma osmolality reaches about 290 mOsm/kg H2O4
However, such a situation of plasma osmolality change is likely to happen only when a dehydration of 1 – 2% body weight loss is achieved.5 Therefore, in daily life, we may be exposed to mild dehydration without feeling thirsty. Thirst is a late signal of the potential onset of dehydration. In practice, it is often advised to drink at regular intervals throughout the day, rather than on the basis of thirst. In addition, in some people, such as the elderly, the thirst signal is less effective, which increases the risk of dehydration.
References
- Institute of Medicine (IOM). Dietary Reference Intakes for Water, Potassium, Sodium, Chloride, and Sulfate. Washington, DC: National Academies Press,2004.
- EFSA. Scientific Opinion on Dietary Reference Values for water. Available at: http://www.efsa.europa.eu/en/scdocs/scdoc/1459.htm. Accessed April 2010.
- Shirreffs SM. Markers of hydration status. J Sports Med Phys Fitness 2000;40:80-4.
- Bouby N, Fernandes S. Mild dehydration, vasopressin and the kidney: animal and human studies. Eur J ClinNutr. 2003;57:S39-46.
- Armstrong LE. Hydration assessment techniques. Nutr Rev. 2005;63:S40-54.
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