An oxalate is a naturally occurring compound found in many plants, and in smaller amounts, produced by your own body. Chemically, it’s the salt or ester of oxalic acid, a simple molecule made of just two carbon atoms bonded to oxygen and hydrogen (C₂H₂O₄). Oxalates matter to most people for one reason: in high enough concentrations, they bind with calcium in the kidneys and form the most common type of kidney stone.
Why Plants Make Oxalates
Plants produce oxalates for practical reasons. Calcium oxalate crystals help regulate calcium levels within plant tissues and act as a defense against animals that would otherwise eat them. The sharp, needle-like crystals can irritate the mouth and digestive tract of herbivores, discouraging grazing. This is why biting into raw taro or rhubarb leaves can cause an immediate stinging sensation on your tongue.
Foods With the Highest Oxalate Levels
Oxalate content varies enormously across foods. The numbers below are total oxalates per 100 grams of fresh weight:
- Spinach (raw): 330–2,350 mg
- Swiss chard (raw): 874–1,458 mg
- Rhubarb (raw): 1,235 mg
- Sorrel (raw): 1,079 mg
- Sweet potato (raw): 496 mg
- Soybeans (raw): 124–497 mg
- Okra (raw): 317 mg
- Star fruit (raw): 160–295 mg
- Parsley (raw): 271–782 mg
Spinach stands out. A single 100-gram serving of raw spinach can contain more oxalate than most people should excrete in an entire day. Rhubarb, Swiss chard, and beets are also consistently high. Lower-oxalate vegetables like broccoli, kale, and lettuce are generally safe to eat in large quantities without concern.
How Your Body Handles Oxalates
Your body gets oxalates from two sources: the food you eat and your own internal metabolism (your liver produces a small amount as a byproduct of processing certain amino acids and vitamin C). Once oxalate enters the bloodstream, your kidneys filter it out and excrete it in urine. Normal urinary oxalate excretion is less than 50 mg per day, but the risk of kidney stones begins to climb at levels above just 25 mg per day.
One of your body’s built-in defenses happens in the gut. When calcium is present alongside oxalate in the intestines, the two bind together into an insoluble compound that passes through you without being absorbed. This is why eating calcium with oxalate-rich meals is protective. Only the soluble, unbound form of oxalate can cross the intestinal wall into your bloodstream. The main factors controlling how much oxalate stays soluble are the pH of your gut contents and the concentrations of calcium and magnesium available to bind it.
Your gut bacteria also play a role. A specialized bacterium called Oxalobacter formigenes lives in the intestinal tract and feeds exclusively on oxalate. People colonized with this bacterium tend to have lower urinary oxalate levels. One study found that colonization with O. formigenes was associated with a 70% decreased risk of recurrent kidney stones. The bacterium doesn’t just break down oxalate in the gut; it also appears to signal the intestinal lining to pull oxalate out of the bloodstream and back into the intestines for elimination. Antibiotic use can wipe out these bacteria, which may partly explain why some people develop higher oxalate levels after courses of antibiotics.
Oxalates and Kidney Stones
About 80% of kidney stones are calcium oxalate stones. The process starts when oxalate levels in the urine rise high enough that calcium and oxalate can no longer stay dissolved. This state, called supersaturation, is the driving force behind stone formation. Once the urine is supersaturated, tiny crystals begin to form. Those crystals can grow, clump together, and attach to the inner surfaces of the kidney. Over time, layers of crystalline material and organic matter accumulate into a stone large enough to cause symptoms.
Oxalate is especially potent in this process. Small increases in urinary oxalate have a more pronounced effect on calcium oxalate supersaturation than equivalent increases in urinary calcium. High oxalate levels can also directly injure the cells lining the kidney’s tiny tubules, making it easier for crystals to stick and accumulate rather than being flushed out.
When Oxalate Levels Get Too High
Abnormally high oxalate levels in the body, called hyperoxaluria, can happen for different reasons. Primary hyperoxaluria is a rare genetic condition where the liver lacks specific enzymes needed to process oxalate precursors. People with this condition produce far more oxalate than normal, and it can lead to severe kidney damage starting in childhood.
Enteric hyperoxaluria is more common and develops when the gut absorbs too much oxalate. Normally, calcium in your intestines binds to oxalate and prevents absorption. But conditions that cause fat malabsorption, like Crohn’s disease, chronic pancreatic disorders, short bowel syndrome, or bariatric surgery, disrupt this system. Unabsorbed fatty acids and bile salts bind to the calcium instead, leaving oxalate free to be absorbed in much larger quantities. People with inflammatory bowel disease often show elevated intestinal oxalate levels along with reduced populations of oxalate-degrading gut bacteria, a double hit that raises kidney stone risk significantly.
Reducing Oxalate Absorption From Food
If you’re prone to kidney stones or have been told your urinary oxalate is elevated, the most effective dietary strategies target both what you eat and how you eat it.
The simplest step is avoiding the highest-oxalate foods: spinach, Swiss chard, rhubarb, nuts, star fruit, beets, and bran. Potatoes should be kept to modest portions (under 100 grams), and chocolate should be limited. These are the foods with enough oxalate to meaningfully shift your urinary levels.
Equally important is eating calcium with each meal, roughly 300 to 400 mg per serving. This doesn’t mean taking a supplement on an empty stomach; it means having dairy, calcium-fortified foods, or other calcium sources alongside the meal so the calcium is present in your gut at the same time as the oxalate. The two bind in the intestines, and the bound complex passes harmlessly through.
Cooking methods offer some help, though the picture is more nuanced than often claimed. Boiling can leach soluble oxalate into the cooking water (which you then discard), reducing the amount you actually consume. Steaming for extended periods can break down oxalate crystals, with one study showing roughly 80% fewer crystals after two hours of steam heating. However, the total oxalate measured by chemical analysis didn’t always decrease significantly, suggesting that cooking changes the physical form of oxalate more than it eliminates it entirely. Boiling in water you discard is likely more effective at removing soluble oxalate than steaming.
Staying well hydrated dilutes oxalate in the urine and reduces the chance of supersaturation. For most people who eat a varied diet and drink enough fluids, oxalates in food are not a health concern. The people who benefit most from tracking oxalate intake are those with a history of calcium oxalate kidney stones, genetic hyperoxaluria, or gut conditions that increase oxalate absorption.