Green, leafy vegetables such as spinach, swiss chard, beet tops, parsley and rhubarb contain high amounts of oxalic acid. Other foods with significant levels of the acid include blackberries, blueberries, raspberries and strawberries.
Oxalic acid, a colorless, organic compound, occurs naturally. The human body also makes it from substances such as vitamin C. When ingested from food, it sometimes imparts a sharp taste felt in the throat. The amounts of oxalic acid in food increases as food matures, producing more bitter vegetables. For example, baby spinach has lower levels of oxalic acid than fully grown spinach. Other items with high levels of oxalic acid include currants, kiwifruit, almonds, cashews, buckwheat, unhulled sesame seeds, tea, coffee, chocolate and beets.
Cooking slightly lowers the amount of oxalic acid in a food. Consequently, to consume all of the oxalic acid present in a food, it needs to be eaten raw. Foods high in oxalic acid should be consumed alongside foods low in oxalic acid for a balance of nutrients.
Even if consuming foods high in oxalic acid, the body does not always readily absorb their oxalate content. In fact, the body absorbs significant amounts of the acid from only a handful of foods, such as peanuts, spinach, beets, pecans and wheat bran.