Oxalic acid is a naturally- occurring chemical compound found in many plants or vegetables, such as spinach, Swiss chard, beet greens and rhubarb. Vegetables like rhubarb leaves, which contain high quantities of oxalic acid, can be poisonous, states Dr. Andrew Weil. However, the oxalic acid amount found in most other plants is low and can be safe for consumption.
An issue with foods or vegetables that contain even low levels of oxalic acid is that this compound interferes with calcium absorption. To avoid this problem, it is important to eat other foods that are high in calcium, such as cheese and milk. Because spinach and other plant foods that contain oxalic acid are also good source of many vitamins and minerals, they should still be included in a diet, notes Dr. Weil.
Besides oxalic acid, it is important to note that that rhubarb leaves should not be consumed because they contain another poisonous ingredient called anthraquinone glycoside, states the National Institutes of Health's MedlinePlus.