Other edible plants that contain significant concentrations of oxalate include—in decreasing order—star fruit (carambola), black pepper, parsley, poppy seed, amaranth, spinach, chard, beets, cocoa, chocolate, most nuts, most berries, fishtail palms, New Zealand spinach (Tetragonia tetragonioides) and beans. The gritty “mouth feel” one experiences when drinking milk with a rhubarb dessert is caused by precipitation of calcium oxalate. The calcium is abstracted from the casein in dairy products.
Leaves of the tea plant (''Camellia sinensis'') contain among the greatest measured concentrations of oxalic acid relative to other plants. However the infusion beverage typically contains only low to moderate amounts of oxalic acid per serving, due to the small mass of leaves used for brewing.
Physiological effects
The affinity of divalent metal ions is sometimes reflected in their tendency to form insoluble precipitates. Thus in the body, oxalic acid also combines with metallic ions such as Ca2+ and Fe2+ to form crystals of the corresponding oxalates which are then excreted in urine. Those with kidney disorders, gout, rheumatoid arthritis, or certain forms of chronic vulvar pain (vulvodynia) are typically advised to avoid foods high in oxalic acid. The calcium oxalate crystals or precipitate (better known as kidney stones) can obstruct the kidney tubules. An estimated 80% of kidney stones are formed from calcium oxalate.Mg oxalate is 567 times more soluble than Ca oxalate, so that Ca oxalate crystals are more likely to precipitate out when Mg levels are low and Ca and oxalate levels are high. Mg oxalate is a million times more soluble than Hg oxalate. Oxalate solubility: Mg>>Ca>Cd>Zn>Mn, Ni, Fe, Cu>As, Sb, Pb> Hg. The highly insoluble Fe oxalate appears to play a major role in the nucleation and growth of the otherwise extremely soluble Na urate, to form the Na urate crystals in gout, which is how gout usually appears after 40 years of age, when ferritin levels (which reflect Fe reserves) exceed 100 ng/dl. Beer is rich in oxalate and Fe and ethanol increases Fe absorption and Mg elimination (both Mg urate and oxalate are very soluble and prevent the precipitation of Ca oxalate and Na urate crystals), so beer intake greatly increases the risk of a gout attack. Cadmiun catalyzed the transformation of vitamin C into oxalic acid and high Cd levels are often high in smokers, people who eat produce tainted with Cd or who are exposed industrially to Cd.
In studies with rats, calcium supplements given along with foods high in oxalic acid can cause calcium oxalate to precipitate out in the gut and reduce the levels of oxalate absorbed by the body (by 97% in some cases.)
Oxalic acid can also be produced by the metabolism of ethylene glycol ("antifreeze"), glyoxylic acid or ascorbic acid (vitamin C).
Powdered oxalate is used to eliminate the bee mite that can destroy complete hives.
Some ''Aspergillus'' species produce oxalic acid, which reacts with blood or tissue calcium to precipitate calcium oxalate. There is some preliminary evidence that the administration of probiotics can affect oxalic acid excretion rates (and presumably oxalic acid levels as well.)
Methods to reduce the oxalate content in food are of current interest.
Adapted from the Wikipedia article Oxalate, under the G. N. U. Free Documentation License. Please also see http://en.wikipedia.org/wiki
