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An oxalate is any chemical compound that contains the oxalate ion, which is the anionic or negatively-charged form of oxalic acid. Oxalates are therefore salts or esters of oxalic acid. Oxalic acid is the simplest dicarboxylic acid and has the formula HO2CCO2H.
Oxalic acid itself is unusually strong for an organic acid and is a good reducing agent. Commercially, it finds wide use as a bleaching agent, rust remover, and wood restorer. As an acid, it is toxic, extremely irritating, and must be used with caution to avoid ingestion or skin contact.
Sodium and potassium oxalates are quite water-soluble, while calcium and magnesium salts are notably less so, with the solubility of calcium oxalate being only 0.005 grams/Liter (g/L) at physiological pH. Oxalic acid and oxalates occur widely in nature, usually as calcium, sodium, or potassium salts. They are common in fruits and vegetables, in which they often contribute an astringent or bitter taste. Foods high in oxalates include spinach, rhubarb, strawberries, and parsley, though almost all plant foods contain some amount. Normally they are not especially harmful, although the leaves and roots of the rhubarb plant are especially rich in oxalates, and eating them can be toxic.
It is the insolubility of calcium oxalate that gives oxalates their medical significance. Formed when soluble oxalate encounters naturally-occurring calcium ions in the body, calcium oxalate precipitates as a solid and can cause considerable harm, especially in the kidneys. Calcium oxalate is the most common component of kidney stones, and the oxalate content of urine is the most significant factor in causing kidney stone formation. Patients prone to kidney stone formation may be put on low-oxalate diets.
Oxalate is also a good chelating agent, a substance that can bind to various metal ions through electrostatic attraction and thus block the ion from being used by the body, either for good or ill. Chelated iron oxalate appears to be a major factor in gout. Due to its low solubility, it precipitates, from the blood, in joints where the crystals can cause excruciating pain.
Disturbances in oxalate metabolism have recently been noticed in individuals with autism, and several theories have been put forth to account for this. There are reports that a controlled-oxalate diet can be beneficial in autistic children, but so far this information is only anecdotal.
The average daily oxalate intake is usually in the range of 80-120 milligrams per day (mg/day), though it can range from 44-350 mg/day. It may be even higher in individuals who eat a typical Western diet. Oxalate may also be produced through the metabolism of vitamin C, and large doses of this vitamin — in excess of 2 grams a day — are discouraged for this reason. Most soluble oxalate is eliminated in the urine, while the insoluble salts are eliminated in stools. An excess of oxalate in the urine is known as hyperoxaluria, and an excess in the body is called hyperoxalosis.
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