Hypohydration

Hypohydration is a physiological state in which the body contains less water than its euhydrated state. It can develop when water losses exceed water intake, such as through sweating, increased urine production, gastrointestinal losses, or restricted access to fluid. The term describes the resulting state of reduced body water rather than the process by which water is being lost .^1,2

Loss of body water can alter both the volume and concentration of body-fluid compartments. Depending on the relative losses of water and electrolytes, hypohydration may reduce plasma volume, increase plasma osmolality, or produce combinations of volume and osmotic changes. These changes can activate water-conserving responses, including thirst and increased secretion of arginine vasopressin, as well as mechanisms that promote sodium and water retention .^2,3

The degree of acute hypohydration is often estimated from changes in body mass when a reliable euhydrated baseline is available, because short-term body-mass losses during exercise or heat exposure largely reflect fluid loss. A body-water deficit corresponding to about 2% or more of body mass is commonly used in exercise physiology as a threshold for physiologically meaningful hypohydration, although hydration status cannot always be characterized by a single numerical measure .^1,2

Hypohydration and dehydration are closely related but are sometimes distinguished in physiological terminology. Dehydration can describe the process of losing body water, whereas hypohydration describes the state produced by that loss. In clinical literature, however, terminology is not fully standardized, and “dehydration” is also commonly used to describe a state of water deficit .^1,3