ABSTRACT
Body water content, and its measurement, is important for several reasons. Body water content and hydration status affect all physiological functions, and if disturbances are sufficiently severe, both physical and cognitive functions are impaired. Equally, small fluctuations in body water content normally occur throughout the day with no perceptible effect on physical or mental performance. Assessment of hydration status—as opposed to body water content—is a topic of much controversy, and there have been many attempts over the years to assess hydration status in various different populations. Changes in blood volume and composition therefore reflect changes in hydration status. Given a constant requirement for solute excretion, the kidneys respond to changes in hydration status by increasing or decreasing the volume of urine formed in an attempt to maintain the volume and osmolality of the vascular space. Saliva flow rate, osmolality, and composition have all been identified as potential markers of hydration status.
