Physiology of Dehydration and Overhydration

When the body is in a state of dehydration, many substrates and neurotransmitters are influenced by circulating vasopressin (antidiuretic hormone) and angiotensin II (1,2).

Dehydration can increase levels of cortisol (3). Interestingly, even a decrease in cell volume caused by hypohydration promotes insulin resistance (4,5,6).
Conditions dehydrating insulin target tissues such as hyperosmolarity or amino acid deprivation are associated with insulin resistance; blockage of the cell volume response to insulin may be the common denominator in dehydration-induced insulin resistance (4).

As a consequence of dehydration, the blood–brain barrier permeability is altered by serotonergic and dopaminergic systems, potentially causing central nervous system dysfunction if dehydration is prolonged (7).  

Chronic dehydration influence inhibitory and excitatory activities of the brain by increasing aminobutyric acid and glutamate levels (8), by stimulating γ-aminobutyric acid and N-methyl-D-aspartate receptors, to synthesize and release antidiuretic hormone (9).

Even mild dehydration produces significant changes at the neural level: total brain volume shrinkage and over-recruitment of specific brain areas during cognitively demanding tasks (10).


Hyperhydrating means drinking more water than you could possibly need, so much fluid that you dilute the amount of sodium in your blood. The low sodium level can cause a cerebral edema, swelling of the brain, leading to death. Sodium and potassium are essential electrolytes. An inadequate amount of sodium in the blood is defined as "hyponatremia” (11).
Women’s menstrual cycle phase influences body water, during the luteal phase total body water can increase up to 2 kg (12,13).

Water intoxication

Water intoxication can lead to life-threatening hyponatremia, which can result in central nervous system edema, lung congestion, and muscle weakness (14). Hyponatremia can also occur from excessive fluid intake, underreplacement of sodium, or both during or after prolonged endurance athletic events. This is common during marathon events (15). For example 13% of the athletes who finished the 2002 Boston Marathon were in a hyponatremic state (16).

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