Oxalates, what you need to know...
There is a LOT of discussion on the various parrot forums about oxalates and foods containing oxalates, and for good reason. I'm glad to see people focusing on nutrition, but what I find most frustrating are the inaccurate statements that foods that contain oxalates will deplete calcium. From a health perspective, there are two aspects of oxalates to consider: their relationship to kidney stone formation and their relationship to calcium absorption and metabolism. While many people think oxalates are an undesirable component of some foods, oxalates are actually naturally-occurring substances found in a wide variety of foods and they play a supportive role in metabolism.
Oxalic acid (oxalate) is an organic acid that binds calcium and other trace minerals and makes them unavailable to the bird. Oxalates can become problematic if they over accumulate inside the body; over accumulation will primarily affect the kidneys. If the concentration of oxalates becomes too high, along with a high concentration of calcium, the kidneys are at risk of calcium oxalate stone formation (kidney stones). High levels can also result in poor growth and bone mineralization; very high levels of oxalates can cause vomiting, diarrhea, convulsions and can also result in poor blood clotting.
Oxalates only get absorbed in the digestive tract when they are in soluble form. Sodium oxalate and potassium oxalate are the predominant soluble forms. Calcium oxalate is insoluble and magnesium oxalate is poorly soluble. So the form of the oxalate is important in the absorption process. Gut bacteria plays a critical role in the amount of oxalate available for absorption since numerous species of gut bacteria are able to break down oxalate. These species include Oxalobacter formigenes, numerous species of Lactobacillus, and several species of Bifidobacteria. Lactobacillus sp., are Gram positive rods and are part of the normal flora in parrots.
Research has shown that the overall combination of foods (including both oxalate-containing and non-oxalate-containing foods) can significantly impact the amount of soluble oxalates available for absorption from the digestive tract. The most concentrated oxalate sources include spinach (750-800 mg), beet greens (600-950 mg), almonds (380-470 mg), Swiss chard (200-640 mg), cashews (230-260 mg), and peanuts (140-184 mg).
It is important to note that you will often find very different results in plant oxalate content due to differences in varieties, planting conditions, harvesting conditions, and measurement technique. It is also worth pointing out that the leaves of plants almost always contain higher oxalate levels than the roots, stems, and stalks. Most fruits and vegetables contain measurable amounts of oxalates in the small-to-moderate range. Studies indicate grapes, for example, show 3-5 mg; pineapple 5 mg; plums 10 mg; collards 5-75 mg; celery 11-20 mg; and green beans 15 mg. Okra is a vegetable that usually shows up higher on the oxalate scale at 140-150 mg, and parsley is also worth mentioning at about 100 mg.
Do you want to control high-oxalate foods in your bird's diet? Maybe. If your bird has a history of kidney stones then yes, I might put her on a low oxalate diet. The best approach, in my opinion, is to have annual checkups with blood testing. Do your research and talk to your Vet.
Understand that there are many interrelationships between vitamins and minerals; one of the most critical in bird nutrition is the relationship between calcium and phosphorous. For proper growth, bone maintenance, and health the ratio of calcium to available phosphorous should be 1.5:1 to 2:1. In these proportions both minerals are most effectively absorbed in the GI tract and metabolized by the body. Excess levels of calcium can create deficiencies in magnesium, iron, iodine, zinc, and manganese if these are only marginally supplied. Inadequate vitamin D3 levels can cause symptoms of calcium deficiency in an otherwise calcium adequate diet. Too much vitamin D3 can produces hypercalcification even in a diet that is marginally sufficient in calcium.