What Are Oxalates and Are They an Issue for CKD Patients?

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Stephanie Legin RD, LDN,
Registered Dietitian |

What Are Oxalates?

An antinutrient is defined as “a naturally-occurring substance found in plant-derived foods that interfere with absorption or proper functioning of nutrients in the body” (i.e. iron, calcium, protein).  Oxalic acid is an antinutrient. Some other antinutrients are phytates, tannins, lectins, and protease inhibitors.

Where Do You Find Oxalates?

Oxalic acid is an organic compound found in plants.  When other minerals found in plants bind with oxalic acid, they form oxalates.  After consuming plant based foods, they start to breakdown in the digestive tract.  During this process, oxalates bind with other minerals like calcium, forming calcium oxalate crystals.  Healthy kidneys filter the excess calcium oxalate and excrete it in the urine. If the kidneys are unable to effectively filter out excess calcium oxalate crystals in the urine, a build up of these crystals can occur.  They can deposit themselves in the kidneys which can irritate the kidneys and cause inflammation. This can lead to the formation of calcium oxalate kidney stones, or worse, kidney damage.

Your body can also make oxalates from vitamin C.  Oxalates are a by-product of vitamin C metabolism.  Vitamin C, when taken in recommended amounts, is necessary for good health.  Consuming high doses of vitamin C per day (2000mg or greater) from foods and/or supplements has not been found to provide health benefits.  In fact, it increases the amount of oxalate in your urine. If excess vitamin C cannot be excreted by your kidneys, it too can increase the risk of kidney stone formation and can potentially damage your kidneys. 


If you have CKD, you also have a high risk for developing oxalosis from excess vitamin C intake. Oxalosis is the accumulation of oxalate crystals in other parts of your body, such as your skin, bones, eyes, heart, and joints.

Diet Recommendations for CKD

If you have Chronic Kidney Disease (CKD), this is concerning, due to already diminished kidney function.  Excessive oxalates in the diet or vitamin C supplementation can contribute to the progression of kidney disease.  Depending on your level of kidney function, dietary restriction of foods high in oxalates may be recommended by your nephrologist and renal dietitian.  If you have CKD, supplementation of vitamin C should be limited to 60-100mg/day.  

A diet rich in a variety of plant foods is encouraged because of all the potential health benefits.  Foods from plant sources are usually low in calories, high in fiber, rich in vitamins and minerals, plentiful in phytochemicals and antioxidants, and some are a good source of heart healthy fats such as monounsaturated and polyunsaturated fats.  However, some plant foods are higher in oxalates than others and should be avoided or limited in the diet if you have CKD or are at increased risk for developing calcium oxalate kidney stones.  

Your nephrologist and renal RD may advise you to avoid or limit the following plant based foods.  While this list of high oxalate foods may seem very restrictive, your renal RD can help you learn about the long list of other plant foods that you can and should include in your diet for optimal health.

Foods High in Oxalates

Spinach, Swiss chard, Turnip greens, Collards, Endive, Kale, Sweet potatoes,  Beets, Beet greens, Starfruit, Raspberries, Currants, Figs, Rhubarb, Beans, Wheat bran, Quinoa, Soy,  Nuts and nut butters, (especially almonds), Dark chocolate, cocoa powder, Tea (hot or iced)

Kidney Stones

It’s important to understand not all kidney stones are calcium oxalate stones.  If you have a history of kidney stones, your doctor can test them to decipher what kinds of stones you develop.  Based on the type of stone, your nephrologist and renal dietitian can best meet your health needs (i.e. with meds, diet, or both, etc.) for optimal kidney health.


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