What Is Oxalate (Oxalic Acid)?

Leafy greens, legumes, and most other plant foods contain a nutrient called oxalate or oxalic acid. It's a naturally occurring chemical you get through your diet. The body also produces it as waste. Foods rich in oxalates also contain other nutrients that your body needs for good health.

What is Oxalate or Oxalic Acid?

Oxalic acid occurs naturally in many plants like the following:

  • Fruits
  • Cocoa
  • Leafy green vegetables
  • Nuts
  • Seeds
  • Spinach
  • Sweet potatoes
  • Star fruit
  • Turnip greens
  • Endive
  • Swiss chard
  • Beet greens

When oxalic acid mixes with other minerals, it forms oxalate. People regularly use the two terms interchangeably to refer to the same thing.  Your body produces oxalate and also gets it from food sources. Vitamin C changes to oxalate when your body processes it.

Effects of Oxalate on Your Body

When you consume foods containing oxalate, the compound attaches itself to minerals to make other minerals. These include iron oxalate and calcium oxalate. The process happens in the colon most of the time and sometimes in the kidneys.

The body then removes the elements through stool or urine. For people with high levels of sensitivity, diets high in oxalates can cause kidney stones. They also increase their risk of other health problems.

Kidney stone formation. It's normal for calcium and small amounts of oxalate to be in the urinary tract at the same time. If they remain in their individual forms, they cause no health problems. Sometimes, they dissolve and mix to form crystals. These stones can cause problems in some people, especially if the oxalate levels are high and urine levels low. In their small form, you may not even notice that they exist. The problem is when they join together to form larger stones.

When this happens, you experience nausea, severe pain, and blood in the urine as the stones move along the urinary tract. Most kidney stones contain high levels of calcium oxalate. This explains why your doctor may advise you to avoid foods high in oxalates if you have kidney stones.

Advances in research show that restricting foods with high oxalates may no longer be practical. Most of these foods contain healthy nutrients that the body needs. Also, a large portion of the oxalate in urine comes from the body and not from food. Doctors now recommend a strict low-oxalate diet of fewer than 50 milligrams for people with a lot of oxalate in their urine.

Continued

Reduced mineral absorption. When oxalates attach themselves to other minerals in the gut, they make them hard to absorb. Spinach has both calcium and oxalate in high quantities. Oxalates block the absorption of calcium from the spinach but not from other sources like milk. Eating foods with fiber may further make it hard for the body to take in necessary nutrients.

Antibiotic interactions. When oxalates break down in the gut, they block the way for the other elements to pass through to the urinary system. When you take antibiotics, they help reduce this effect. They work by decreasing the good bacteria that absorb oxalates and making the gut activity better.

Your Gut and Oxalate Absorption

Sometimes the bacteria in your gut break down the oxalates you eat before they attach to minerals. One bacterium type, Oxalobacter formigenes, relies on oxalates as a source of energy. When it takes in the oxalate particles, it reduces the amount that your body absorbs.

For people without high levels of this bacterium in the gut, the breakdown of oxalates is sometimes not effective. Antibiotics fight these bacteria colonies in the gut.

Studies also show that you are at a higher risk of developing kidney stones if you have inflammatory bowel disease. This is because your body can't control the amount of oxalates it absorbs.

People who have had gastric bypass surgery or surgeries that affect how the gut works may also have high oxalate levels in their urine. If you have gut dysfunction or take antibiotics, you may want to eat a low-oxalate diet.

Final Thoughts

If you’re at a high risk of kidney stone formation due to high levels of oxalates in your body, low-oxalate diets can help. These include broccoli, cheese, and canned fish with bones. They also contain high levels of calcium. Boil vegetables that have high oxalate levels, get enough calcium, and drink plenty of water.

WebMD Medical Reference Reviewed by Dan Brennan, MD on April 08, 2021

Sources

SOURCES:

The American Journal of Clinical Nutrition: "Oxalate: effect on calcium absorbability.” 

Annals of Translational Medicine: "Probiotics for prevention of urinary stones." 

Asia Pacific Journal of Clinical Nutrition: “Oxalate content of foods and its effect on humans.”

Cleveland Clinic: “Kidney Stones: Oxalate-Controlled Diet.”

International Journal of Food Sciences and Nutrition: “Oxalates in some Indian green leafy vegetables.”

Journal of Endourology: “Effect of antibiotics on Oxalobacter formigenes colonization of human gastrointestinal tract.”

Journal of the American Society of Nephrology: “Oxalobacter formigenes May Reduce the Risk of Calcium Oxalate Kidney Stones.”

Journal of Urology: “Effect of ascorbic acid consumption on urinary stone risk factors.”

Kidney International: "Dietary risk factors of hyperoxaluria in calcium oxalate stone formers.”

Korean Journal of Urology: “Medical and Dietary Therapy for Kidney Stone Prevention.”

National Kidney Foundation: “Calcium Oxalate Stones.”

The New England Journal of Medicine: “Clinical Practice Calcium Kidney Stones.”

Postgraduate Medical Journal: “Treatment of enteric hyperoxaluria.”

PubMed: “Nephrolithiasis in patients with intestinal disease.”  

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