Oxalic acid is a naturally-occurring substance found in several foods. It is usually excreted in urine as a harmless metabolism byproduct. It can also combine with calcium, interfering with its absorption, or accumulate and lead to the formation of kidney and bladder stones. According to Litholink, a metabolic testing and disease management service for kidney stone patients, a low oxalate diet of no more than 50 mg per 100 g food serving is necessary to avoid an excess concentration.
Rhubarb
The richest natural source of oxalic acid is rhubarb. Oxalates are found in all parts of the plant, but levels are so high in the green leaves that they can be toxic. Canned rhubarb contains 600 mg of oxalic acid per 100 g serving, while unsweetened stewed rhubarb can have up to 860 mg.
Spinach
Spinach is another excellent source of oxalate. Frozen spinach has 600 mg per 100 g serving. Because cooking increases the oxalic acid content, boiled spinach will give you about 750 mg of oxalic acid.
Other Fruits and Vegetables
Because oxalic acid is a natural substance in many plants, various fruits and vegetables contain significant amounts. Beet roots offer anywhere from 500 to 675 mg, and Swiss chard can have up to 645 mg. Pokeweed, an herb, has about 475 mg, and okra offers 145 mg. Lemon and lime peels have between 83 and 110 mg of oxalic acid per 100 g. Strawberries are another source of oxalic acid.
Tea
Certain varieties of tea contain considerable amounts of oxalic acid. You will get between 12 and 30 mg per cup of black regular and decaffeinated teas. Green and white teas have less oxalic acid -- about 6 to 18 mg per cup. Herbal teas contain very little oxalic acid.
Other Foods
Nuts, chocolate and some grains are other good sources of oxalic acid. You will obtain 187 mg from roasted peanuts, while pecans have slightly more at 202 mg. Plain chocolate has about 120 mg. Dry cocoa powder, however, has a whopping 625 mg per 100 g serving. Wheat germ provides about 270 mg per 100 g. Amaranth also contains moderate amounts of oxalic acid.
