Magnesium salts are essential in nutrition, being required for the activity of many enzymes, especially those concerned with oxidative phosphorylation. Physiologically, it exists as an ion in the body. It is a component of both intra- and extracellular fluids and is excreted in the urine and feces. Deficiency causes irritability of the nervous system with tetany, vasodilatation, convulsions, tremors, depression, and psychotic behavior. Magnesium ion in large amounts is an ionic laxative, and magnesium sulfate (Epsom salts) is sometimes used for this purpose. So-called "milk of magnesia" is a water suspension of one of the few insoluble magnesium compounds, magnesium hydroxide; the undissolved particles give rise to its appearance and name. Milk of magnesia is a mild base, and is commonly used as an antacid. Moreover, magnesium is found to be associated with primary hypomagnesemia, which is an inborn error of metabolism.