Diseases & Conditions
Overhydration is an excess of water in the body. People can have overhydration if they drink too much or if they have a disorder that decreases the body's ability to excrete water. Often, no symptoms occur, but people may become confused or have seizures. Fluid intake is restricted and diuretics may be given.
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 Minerals and Electrolytes: 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. Overhydration may also result from the inappropriate secretion of antidiuretic hormone (see Minerals and Electrolytes: 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. Premature infants may become overhydrated if they receive too large an amount of intravenous fluids.
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. With overhydration and normal blood volume, the excess water usually moves into the cells, and tissue swelling (edema) does not occur. With overhydration and excess blood volume, 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.
Did You Know...
Drinking too much fluid can be harmful, even in healthy people.
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 a drug to increase sodium and water excretion in the urine (diuretic). In general, diuretics are more useful when overhydration is accompanied by excess blood volume.
Last full review/revision August 2008 by James L. Lewis, III, MD
Source: The Merck Manual Home Edition