Oxalic acid is a naturally occurring compound in many fruits, vegetables, nuts, and whole grains. While small amounts of oxalic acid are not harmful, this compound can inhibit the absorption of other important nutrients.

For this reason, some people refer to oxalic acid, or oxalate, as an anti-nutrient. In some people, it can also increase the risk of kidney stones.

Certain gut bacteria can metabolize, or break down, oxalic acid. This prevents it from binding to minerals and affecting nutrient absorption.

This article looks at oxalic acid and its associated risks in more detail. It also lists dietary sources of the compound and explains how people can decrease their intake.

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Oxalic acid is a common organic compound. A range of living organisms — including fungi, bacteria, plants, animals, and humans — produce it.

Technically, oxalate occurs when the oxalic acid in plants binds to minerals. However, many people use the terms interchangeably.

The body can either produce oxalate as a waste product or obtain it from the diet.

Oxalate can combine with other minerals in the body to form compounds such as calcium oxalate and iron oxalate. People can then eliminate these oxalate compounds in the urine or stool.

However, some individuals with high oxalate levels may develop kidney stones.

Although oxalic acid naturally occurs in plants and humans, it also has a variety of uses in industry. These uses include:

  • removing rust
  • removing stains
  • stripping and cleaning
  • removing wax
  • cleaning wood
  • dyeing textiles

Laboratories may also use oxalic acid and oxalate salts as anticoagulants in blood specimens.

In small amounts, oxalate is harmless. However, higher levels may reduce the body’s mineral absorption and contribute to kidney stone formation.

Nutrient absorption

People refer to oxalates as anti-nutrients. This is because they bind to certain minerals and prevent the body from absorbing and utilizing them.

One good example is spinach. Although it is rich in the important nutrients calcium and magnesium, it is also high in oxalate. The oxalate forms a complex with these minerals and can inhibit absorption.

Kidney stones

The other concern with oxalate is that it can contribute to kidney stones.

Most people have a small amount of oxalate and calcium in the urinary tract at some point. Usually, they remain dissolved, and there are no associated problems.

However, in some people, the compounds form crystals and then kidney stones. Research has shown that this is a particular problem if oxalate levels are high and urinary volume is low.

There are several types of kidney stones, which comprise different minerals, but calcium oxalate is the most common. Therefore, if a person is prone to developing kidney stones, their doctor may advise them to reduce their intake of oxalate-rich foods. They should also consume enough calcium and avoid vitamin C supplements.

Learn more about kidney stones.

Certain bacteria in the digestive system can metabolize some dietary oxalate before it binds to minerals. This process can prevent oxalate’s potentially harmful effects.

A type of gut bacteria called Oxalobacter formigenes breaks down oxalate and uses it as energy. Having this bacteria in the gut microbiome significantly reduces the amount of oxalate in a person’s body.

Many people have O. formigenes in their gut. According to research, it is present in the feces of about 60–80% of adults.

Some people who experience recurrent calcium oxalate kidney stones have less of this bacteria.

Certain factors, such as taking antibiotics, can reduce the number of good bacteria in the gut. Additionally, people with altered gut function, such as those with inflammatory bowel disease (IBD), have an increased risk of developing kidney stones.

People typically consume oxalate through dietary sources. Almost all plants, including fruits and vegetables, contain oxalates, having the highest concentration in the leaves and seeds. However, the amount they contain varies considerably.

Vegetables that are particularly high in oxalate include:

Other dietary sources rich in oxalate include:

The amount the body can absorb from foods depends on the quantity of soluble oxalates the foods contain and the bioavailability of these oxalates. For example, tea has a significantly higher rate of oxalate absorption than spinach or rhubarb.

The method of food preparation and cooking can also affect the oxalate content. Soaking certain vegetables and legumes can reduce the amount of oxalate they contain.

The best way to avoid oxalate is to reduce the intake of oxalate-rich foods.

It is important to eat a variety of fruits and vegetables. However, if a person is concerned about the number of oxalates in certain foods, they can replace them with low oxalate alternatives. Some examples include:

High oxalate foodLow oxalate alternative
rhubarb apples
black teafruit tea
dark chocolatewhite chocolate

Boiling vegetables also reduces their oxalate content. Research shows that boiling could remove up to 76% of oxalates, depending on the vegetable.

It is also important to eat enough dietary calcium. This mineral can bind to oxalate in the digestive system and reduce how much the body absorbs. Therefore, eating milk, yogurt, or cheese alongside foods containing oxalates can reduce the likelihood of developing issues such as kidney stones.

Oxalate is a compound present in many plant-based foods, including spinach, chard, and kale.

In the digestive system, oxalate can form complexes with minerals such as calcium and interfere with their absorption. Additionally, a high oxalate diet may lead to some people developing kidney stones.

Doctors may advise some individuals to consume a low oxalate diet. They can do this by replacing high oxalate foods with low oxalate alternatives, boiling vegetables, and eating foods rich in calcium alongside high oxalate foods.