Oxalates are natural substances that occur in plants and perform various functions, such as repelling herbivores as an internal pesticide, regulating calcium levels and chelating heavy metal ions.

There are two sources of oxalates - the main ones - endogenous 90% (they are produced in our body as a result of metabolic changes in the liver) and exogenous approx. 10% (coming from food).

Oxalates come in the form of soluble salts - sodium and potassium oxalates and insoluble salts - calcium oxalates. Oxalic acid creates insoluble salts with di- and trivalent metals, reducing the use of these metals from food.

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For this reason, oxalates are referred to as antinutritional substances in relation to minerals (divalent and trivalent metals).

The term oxalates refers to chemical compounds - salts or esters, oxalic acid. The most common in food are calcium and magnesium oxalates, although potassium, sodium, anion and iron oxalates can also be found. Salts that are insoluble in water are salts of oxalic acid with magnesium, calcium and iron.

They are excreted by the human body in urine in unchanged form. Water-soluble salts, i.e. potassium, sodium and anion oxalates, are decomposed in the body into metal ions and oxalic acid, and then it can combine with, among others, magnesium or calcium and prevent the absorption of these elements into the body.

Precursors of oxalic acid include, among others: glyoxylate, gioxalic acid, glycine, proline, hydroxyproline and L-ascorbic acid. Oxalic acid is also the end product of the metabolism of serine, alanine and glycine.

The purpose of the formation of this acid in the plant has not been fully explained, but it is suspected that it can protect plants against bacteria, viruses, fungi, parasites and herbivores by giving them a bitter taste, texture and changing their acidity.

However, other sources report that it may be a pH regulator and influence osmoregulation.

In the human body, oxalates may appear through ingestion with food - exogenously, or be produced during metabolic processes.

Calcium, magnesium and iron oxalates have been considered anti-nutritional ingredients due to their impact on reducing the bioavailability of minerals.

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Sources of oxalates in the daily diet - exogenous oxalates

Oxalic acid is found in plant tissue. When its concentration is too high, as in the case of, e.g., hyacinths or king Solomon's seal, the plant is considered inedible.

Consuming too much oxalic acid causes chemical burns and damage to the epithelium of the oral cavity and other parts of the digestive tract.

Additionally, it may lead to swelling of the larynx, hemorrhage in the stomach, diarrhea and fever. The lethal single oral dose of oxalic acid is 15-30 g, depending on the person's weight.

The main sources of oxalic acid in the daily diet is coffee (on average 100mg/100g - it does not contain an aggravating amount of oxalic acid, but due to its very frequent consumption in large quantities it is listed as one of the main sources):

  • spinach (on average 970 mg/100 g)
  • sorrel (on average 500 mg/100 g)
  • rhubarb (on average 805 mg/100 g)
  • beetroot (in roots and leaves, for chard on average 610 mg/100 g)
  • quinoa (on average 1100 mg/100 g)
  • tea (on average 1150 mg/100 g, the infusion contains much smaller amounts of acid)
  • cocoa (average 700 mg/100 g)

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The content of oxalates in teas varies, depending on, among others: on the region of origin, harvest time (leaves harvested in autumn have a higher oxalate content due to their ripeness), leaf size and method of processing, and in tea infusions also on water temperature and brewing time.

Products that contribute to increasing the concentration of oxalic acid in the diet also include products rich in glyoxylic acid, i.e. beer, coffee, tea, yogurts, bread, rice, soy paste and sauce, and oils. The reason for this is the very easy conversion of this acid to oxalic acid.

The absorption of oxalic acid from food products ranges from 2-10% depending on the type of product. However, when pure oxalic acid is administered, absorption can reach a level of approximately 17%.

The acceptable daily intake (ADI) of oxalates for an adult should be approximately 250 mg per day. Of which it is estimated that the average oxalate intake in Western Europe is from 100 to 150 mg per day.

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Endogenous sources of oxalates

Oxalic acid can also be produced in our body, it is a product of liver metabolism, and its main source is hydroxyproline (an amino acid typical of animal proteins - collagen, elastin).

Hydroxyproline is the main source of glyoxylic acid in the human body, and oxalic acid can be produced from it (food sources of glyoxylic acid include: beer, wine, coffee, tea, yogurt, bread, rice, oils, soy sauce, baked, grilled and fried foods).

The condition for the conversion of glyoxylic acid to oxalic acid (instead of glycolic acid or glycine) is oxidative stress, which consumes glutathione (antioxidant) resources. That is why it is so important to eat a diet rich in antioxidants (which protect the body against free radicals and oxidative stress).

Consuming excessive amounts of meat, tendons or gelatin provides large amounts of hydroxyproline (which can only be catabolized and contribute to the production of oxalic acid). Even though plant products are an exogenous source of oxalic acid, people who eat large amounts of meat are mainly at risk of oxalate urolithiasis.

Oxalate stone disease

The most common type of kidney stones (70-80%) is oxalate stones (calcium oxalate deposits). Oxalic acid is excreted mainly by the kidneys, to a small extent by the bile ducts.

Precipitation of calcium oxalates in the kidneys may occur, for example, in the case of high concentration of tubular oxalic acid, high calcium concentration, urinary tract infections, damage to the renal tubular epithelium and oxidative stress. The basic factors for the formation of oxalate deposits include inflammation.

In the prevention and treatment of oxalate urolithiasis, it is mainly recommended to reduce the consumption of oxalic acid along with the diet, but it should be remembered that exogenous oxalates constitute only 10% of the total number in the body.

The excretion of oxalic acid depends primarily on its endogenous production, which is influenced by dietary and metabolic factors. In order to prevent the development of oxalate urolithiasis, we should pay attention to where 90% of oxalates present in the blood and filtered by the kidneys come from.


Oxalates from plant products do not pose a serious threat to our health. The causes of oxalate urolithiasis can be seen mainly in too much eating of meat (a source of hydroxyproline), sodium, fructose syrup, too little supply of vegetables and fruits, i.e. antioxidants, and insufficient fluid intake.

Limiting the consumption of plant products that are a source of oxalates may be important in people with advanced urolithiasis or kidney dysfunction, or in the case of a genetic predisposition to the development of oxalate urolithiasis.


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