Oxalic acid is found in many foods. It is often said to be harmful because it is said that oxalic acid interferes with the absorption of some minerals such as iron, calcium, and magnesium and contributes to the formation of kidney stones. We clarify whether oxalic acid is actually that harmful.
Oxalic acid in food – The list
Foods rich in oxalic acid are often discouraged to reduce the risk of kidney stone formation and mineral deficiencies. However, since oxalic acid is contained in almost all plant-based foods – especially vegetables and herbs – a diet low in oxalic acid is neither easy to implement nor very healthy.
Here is our list of the oxalic acid levels of some foods: Oxalic acid in foods
Oxalic acid levels can fluctuate widely
However, the oxalic acid values can vary greatly depending on the study (see the example of parsley), because the oxalic acid content depends on many factors, such as the variety, the parts of the plant examined, the cultivation conditions, the time of harvest and also the measuring technology.
Analyzes have shown that the oxalic acid content of a spinach sample varied between 506 and 981 mg per 100 g. There is also 30 percent less oxalic acid in autumn spinach than in spring spinach. And with rhubarb, it depends on the parts of the plant: there is significantly more oxalic acid in the leaves than in the stems. In the stalks, on the other hand, there is more oxalic acid in the outermost layer than in the interior.
That much oxalic acid is toxic
There is no question that pure oxalic acid is toxic in very high concentrations. In most foods, however, the substance is only present in low doses. You would have to ingest at least 600mg of oxalic acid per kg of body weight to die from it. With a body weight of 60 kg, this amount would correspond to e.g. B. around 15 kg of raw sweet potatoes, although there are no studies on this, only case studies of people who took pure oxalic acid (in the context of self-harm behavior). Pure oxalic acid is available as a bleaching agent, for example.
According to a study at Lincoln University, people consume an average of 70 to 150 mg of oxalic acid per day. For vegans, vegetarians, and other vegetable fans, the intake is of course higher because they eat more vegetables. But that is not a problem either, as the substance in food does not pose a health risk. Only people with certain pre-existing medical conditions should exercise caution.
If you have an iron deficiency, you should not eat a meal rich in oxalic acid at the same time as taking iron tablets. People with kidney stones (so-called calcium oxalate stones) should also not eat large amounts of spinach or chard every day, as oxalic acid can – under certain circumstances – promote the formation of new stones.
However, most urologists now only prescribe a strict low-oxalate diet (less than 50 mg per day) for patients with very high urinary oxalate levels, as there are many factors, as explained below, that can reduce the risk of kidney stones despite a diet high in oxalic acid.
Kidney stones could develop from oxalic acid
In healthy people, most of the oxalic acid ingested through food is bound to minerals such as calcium and simply excreted. It is problematic if this is not the case or only to an insufficient extent. Because then more oxalate salts are formed, which cannot be excreted but instead settle in the kidneys and form kidney stones (calcium oxalate stones). However, this problem seems to have less to do with the oxalic acid content of the diet than to have other causes.
Fruits and vegetables protect against kidney stones
A study at the University of California School of Medicine showed that a diet high in fruits and vegetables can prevent calcium oxalate stones. In another study, the conclusion also says A diet with u. A lot of fruit and vegetables is the best way to prevent kidney stones (together with a good supply of calcium, low salt consumption, a few animal proteins, etc.).
Fiber and phytic acid protect against kidney stones
A plant-based diet has a preventive effect on the formation of kidney stones, not only because of the high water and vital substance content. The high fiber content and even the phytic acid are also helpful in this regard. Like oxalic acid, phytic acid has a bad reputation and is often counted among the so-called anti-nutritive, i.e. anti-nutrients, because it – again like oxalic acid – can bind minerals.
However, it is now known that the benefits of phytic acid outweigh the negatives and no one will suffer from a mineral deficiency just because they eat a wholesome diet. (Phytic acid is mainly found in whole grains, legumes, nuts, and seeds.)
In a 2007 paper, researchers at Norfolk & Norwich University wrote that phytic acid strongly inhibited the formation of calcium oxalate crystals and that observational studies had therefore shown that the more phytic acid a person consumed, the less likely it was that kidney stones would occur.
It is also interesting that z. B. Green tea is considered to be a supplier of oxalic acid, but green tea drinkers do not have an increased risk of kidney stones containing oxalate. A diet containing oxalic acid alone does not lead to kidney stones – not even in the case of hyperoxaluria.
How hyperoxaluria reduces the risk of kidney stones
Hyperoxaluria is abnormally increased oxalic acid production in the liver, which results in increased urinary oxalate levels. People with hyperoxaluria are considered a risk group for kidney stones. But even here you can do a lot to avoid getting kidney stones. Because here, too, oxalic acid alone is not enough to develop kidney stones. We have presented the corresponding measures in our article on the subject of no kidney stones with vitamin C, but also in some cases below under the other measures.
How cooking and baking reduce oxalic acid
If you now – for whatever reason – want to consciously reduce the oxalic acid content of your food, then you can pay attention to the following:
After cooking vegetables high in oxalic acid, discard the cooking water. This can reduce the oxalic acid content by up to 87 percent and steaming by up to 53 percent. Of course, minerals and water-soluble vitamins are also thrown away with the cooking water.
Baking can only reduce oxalic acid levels by up to 15 percent. Blanching helps with spinach, but not with other vegetables.
Legumes are usually prepared by soaking them overnight. This measure alone reduces oxalic acid significantly. Peeling the stalks of rhubarb helps, as this is where the greatest amount of oxalic acid is found. Fermentation can also presumably reduce the oxalic acid content.
How the intestinal flora can protect against oxalic acid
Some people generally take in much more oxalic acid than usual. One speaks of hyperabsorption, the causes of which often cannot be clarified. According to the latest studies, disturbed intestinal flora can be responsible for this. According to researchers at the University of Alabama at Birmingham, affected people lack intestinal bacteria such as Oxalobacter formigenes and Lactobacillus, which feed on oxalic acid, i.e. break it down.
If the corresponding bacteria are missing in the intestine, for example, because they have been destroyed by antibiotics, there is a disproportionate intake of oxalic acid and diseases such as kidney stones. Incidentally, in 2021 a probiotic called Oxabact with Oxalobacter formigenes is to be launched, which can restore the balance in the intestine.