Oxalic acid is a colorless, crystalline, toxic organic compound that is found in various vegetables and pulses. Its chemical formula is C2H2O4. The intake of oxalic acid affects calcium uptake inside the body. When this organic compound combines with calcium, calcium oxalate is formed. Calcium oxalate is main component of the kidney stone and has low solubility.
The excess intake of high oxalate vegetables as well as proneness of the body to form more stones in some individual can cause recurrent formation of the kidney and urinary stones. In such cases, oxalate rich foods should be used less or avoided. The highest level of oxalate is found in amaranth, aroid or arum, goosefoot, buckwheat and wood sorrel family. Here is given list of such food items that are high in oxalate contents. This is not the complete list but some of the commonly eaten vegetables.
Foods with very high oxalate content (over 50 mg per serving): Spinach, Chaulai, Swiss chard, Soy milk, Soy burger, Other Soy products, Green gooseberries, Amaranth, Buckwheat, Wheat bran, Almonds, peanuts.
Foods with high oxalate (10-50mg per serving): Beans, Celery, Dandelion, potatoes, Kiwi, grapes, Beer, Beets, Blackberries, Black and red raspberries, Blueberries, Chocolate, Eggplant, Parsley, Tea, Tofu, Tomato soup
| Vegetable | Serving size | Approximate Oxalic acid content |
| Beet greens, cooked | ½ cup | 916 mg |
| Spinach, cooked | 1/2 cup | 750 mg |
| Amaranth | 100 gm | 770 mg |
| Beets, cooked | 1/2 cup | 675 mg |
| Chard, Swiss, leaves cooked | 1/2 cup | 660 mg |
| Beets, pickled cup | 1/2 | 500 mg |
| Amla | 100 grams | 300 mg |
| Dandelion greens, cooked | 1/2 cup | 246 mg |
| Okra, cooked | 8-9 pods | 146 mg |
| Potatoes, sweet, cooked | 1/2 cup | 141 mg |
| Turnip greens, cooked | 1/2 cup | 110 mg |
| Carrots, cooked | 1/2 cup | 45 mg |
Although lots of green leafy vegetables, pulses and certain other food items contain oxalates in more or less amount, but mainly consumption of only certain oxalate-containing foods is likely to significantly increase urinary oxalate and stone formation. One such group is spinach, rhubarb, beets, nuts, chocolate, wheat bran, strawberries, peanuts, almonds, and tea. But every study has not found tea to significantly increase urinary oxalate.
If a person has history of the kidney stones formation, and genetic prone to such condition than he/she must try to avoid high oxalate foods.