Overhydration is an excess of water in the body.
Overhydration occurs when the body takes in more water than it loses.
Overhydration can occur, for example, when athletes drink excessive amounts of water or sports drinks to avoid dehydration, or when people drink much more water than their body needs because of a psychiatric disorder called psychogenic polydipsia. The result is too much water and not enough sodium. Thus, overhydration generally results in low sodium levels in the blood (hyponatremia—see Electrolyte Balance: Hyponatremia), which can be dangerous. However, drinking large amounts of water usually does not cause overhydration if the pituitary gland, kidneys, liver, and heart are functioning normally. To exceed the body's ability to excrete water, a young adult with normal kidney function would have to drink more than 6 gallons of water a day on a regular basis.
Overhydration is much more common among people whose kidneys do not excrete urine normally—for example, among people with a disorder of the heart, kidneys, or liver, or among premature infants, whose kidneys are immature. Overhydration may also result from the syndrome of inappropriate secretion of antidiuretic hormone (see Electrolyte Balance: Syndrome of Inappropriate Secretion of Antidiuretic Hormone). In this syndrome, the pituitary gland secretes too much antidiuretic hormone, stimulating the kidneys to conserve water when that is not needed.
Brain cells are particularly susceptible to overhydration and to low sodium levels in the blood. When overhydration occurs slowly, brain cells have time to adapt, so few symptoms occur. When overhydration occurs quickly, confusion, seizures, or coma may develop.
Doctors try to distinguish between overhydration and excess blood volume. When overhydration occurs and blood volume is normal, the excess water usually moves into the cells, and tissue swelling (edema) does not occur. When overhydration and excess blood volume occur, an excess amount of sodium prevents the excess water from moving into the cells. Instead, the excess water accumulates around the cells, resulting in edema in the chest, abdomen, and lower legs.
Regardless of the cause of overhydration, fluid intake usually must be restricted (but only as advised by doctors). Drinking less than a quart of fluids a day usually results in improvement over several days. If overhydration occurs because of heart, liver, or kidney disease, restricting the intake of sodium (sodium causes the body to retain water) is also helpful.
Sometimes, doctors prescribe drugs to increase the excretion of sodium and water in the urine. Diuretics are usually given, but other types of drugs also can increase sodium and water excretion. In general, diuretics are more useful when overhydration is accompanied by excess blood volume.
Last full review/revision February 2013 by James L. Lewis, III, MD