Oxalic acid and oxalates occur naturally in varying degrees in plants and animals. Oxalic acid is made by the human body from other substances such as Vitamin C. It can also be ingested by the human body from the foods consumed.
This substance is found in fairly high concentrations in such foods as, berries, spinach, beet greens, and parsley. Okra, leeks, and collard greens are also known to be high in oxalic acid. Although tea leaves have an extremely high concentration, not much of the oxalic acid is passed on to the brewed tea.
For most people on a balanced diet, oxalic acid will have no appreciable effect on health or longevity. However, for people with kidney stones, gall stones or severe calcium deficiency problems oxalic acid intake should be severely restricted. Oxalic acid seems to assist in the formation of crystals in the body by combining with calcium and various metals. (The precipitate is referred to as an oxalate.) Consequently oxalic acid also inhibits calcium absorption by the body. Even so, most of the foods that are high in oxalates have many beneficial effects that more than offset the downsides of the organic acid. Also, it is not currently known if oxalic acid has some other beneficial effect on the system.
Since the body can produce oxalic acid from excess Vitamin C, it is not wise to take extremely large doses of the vitamin for extended periods of time.
It is known that cooking has little effect on the oxalic acid content of food. About 10 to 15 percent reduction is common at normal temperatures and cooking times. Cooking foods longer or hotter will have an overall detrimental effect by destroying the good nutrients.
As noted above, oxalic acid consumption, on the whole, should not be a concern or factor in considering the diet of the average person. Those with special concerns should consult a physician or a nutritionist about restricted intake.