Oxalic Acid Foods: Which Contain a Lot – the List

Oxalic acid is a natural plant compound that occurs in many types of vegetables and fruit. Particularly high amounts of oxalic acid are found in spinach, chard, rhubarb, beetroot, sorrel, cocoa and nuts. As so-called oxalates it can inhibit the absorption of minerals such as calcium and iron in the gut and, in people with a corresponding predisposition, can promote the formation of kidney stones. For healthy people, moderate consumption is generally harmless, because the body normally excretes oxalates without problems via the kidneys. This overview shows which foods contain a lot of oxalic acid, which are considered low in oxalate and how the content can be significantly reduced during preparation.

What is oxalic acid?

Oxalic acid – also called ethanedioic acid – is an organic acid that many plants form as a natural component. Among other things it serves the plants to protect against predators and to regulate the mineral balance. In the human body and in foods it is usually present as a salt, the so-called oxalate. Oxalates bind minerals such as calcium and form poorly soluble compounds. This can reduce the absorption of these nutrients. The body normally excretes oxalates via the kidneys, and a small part is also formed in the metabolism itself. In some people, particularly those with a tendency to certain kidney stones, a very oxalate-rich diet can be unfavourable. For the vast majority, however, oxalic acid from foods is not a problem, as long as the diet remains varied and is not permanently dominated by individual, very oxalate-rich foods.

How much oxalic acid is of concern?

For healthy adults, a balanced mixed diet with usual amounts of oxalate-containing foods is considered unproblematic. Only very high amounts over a longer period or an existing predisposition make the content relevant. People who have already had calcium oxalate kidney stones are often advised to limit particularly oxalate-rich foods and to ensure sufficient fluid intake. Since individual factors play a role here, those affected should coordinate their diet with a doctor. For everyone else it is usually enough not to eat very oxalate-rich foods in large amounts and not daily. Rhubarb plays a special role here: its leaves contain so much oxalic acid that they are not eaten – only the stalks are eaten, and these best cooked. This too explains why oxalic acid becomes an issue mainly with very one-sided or excessive consumption of individual foods and not with a normal, varied diet.

Foods high in oxalic acid

The following foods are considered particularly rich in oxalate (figures per 100 g vary depending on variety and origin):

• Spinach: around 600–970 mg oxalic acid per 100 g
• Chard: about 650 mg per 100 g
• Rhubarb: approx. 460–600 mg per 100 g
• Sorrel: very high, sometimes over 500 mg per 100 g
• Beetroot: around 150–180 mg per 100 g
• Cocoa and dark chocolate: several hundred mg per 100 g
• Nuts such as almonds and cashews as well as wheat bran
• Black tea in concentrated form

Foods low in oxalic acid

Anyone who wants to keep the oxalate content low can fall back on numerous low-oxalate alternatives:

• Most types of cabbage such as cauliflower, broccoli and white cabbage
• Cucumber, courgette, peppers and button mushrooms
• Potatoes in usual amounts
• Many types of fruit such as apples, bananas and melons
• Milk, yoghurt and cheese
• Rice, pasta and white bread

Oxalic acid and nutrient absorption

The reason why oxalic acid attracts attention at all lies in its ability to bind minerals. If oxalate forms a poorly soluble compound with calcium, this calcium is no longer available to the body for absorption. The same applies to iron. That is why oxalate-rich spinach, despite its considerable iron content, provides less usable iron than the pure nutrient tables suggest. For people with a balanced, varied diet this effect is of little significance, since the minerals come from many different sources. However, anyone who has to pay attention to their iron or calcium supply anyway can deliberately combine oxalate-rich foods with sources of these minerals or lower the oxalate content through preparation.

Reducing the oxalic acid content when cooking

A large part of the oxalic acid is water-soluble. That is why the content can be significantly reduced through preparation. Anyone who briefly blanches oxalate-rich vegetables such as spinach or chard in plenty of water and then pours away the cooking water removes a considerable part of the soluble oxalates. Combining them with calcium-rich foods can also be favourable: if oxalate is already bound to calcium in the gut, less of it enters the body and is instead excreted. Spinach with a dash of milk or yoghurt is a classic example. Drinking enough additionally supports excretion via the kidneys. Besides blanching, soaking legumes and grains also lowers the oxalate content somewhat, since part of the soluble oxalates passes into the water. Fermenting and sprouting can also help to reduce the content. It is important not to reuse the cooking or soaking water but to pour it away, so that the dissolved oxalates do not end up back on the plate after all. Raw oxalate-rich vegetables, on the other hand, contain the full amount – anyone who is sensitive therefore prefers the cooked version.

Oxalic acid and kidney stones

The most common form of kidney stones consists of calcium oxalate. That is why oxalic acid comes to the fore with this topic. It is important to know, however, that stone formation depends on many factors – not on oxalate intake alone. Too low a drinking amount, little exercise, being overweight and a very salt-rich diet also play a role. Interestingly, with a tendency to kidney stones it is usually not sensible to reduce calcium. On the contrary: a sufficient calcium intake with meals binds oxalate already in the gut, so that less of it enters the body. Anyone affected therefore pays attention rather to a good calcium supply, plenty of fluid and a moderate approach to particularly oxalate-rich foods. The exact strategy depends on the individual situation.

Common misconceptions about oxalic acid

Some misunderstandings surround oxalic acid. A widespread error is that oxalate-rich foods are fundamentally unhealthy. In fact, spinach, chard, beetroot and nuts provide many valuable nutrients and, for most people, are part of a healthy diet. Another error concerns reheated spinach: the idea that it must not be reheated because of the oxalic acid is not correct in that form – here it is more about hygienically correct storage and the nitrate content, not about oxalates. The notion that one must generally strictly avoid oxalic acid also does not apply to healthy people. A relaxed, varied approach makes sense: anyone who does not eat large amounts of the same oxalate-rich foods every day and pays attention to a good calcium and fluid supply generally need not worry.

Practical tips for everyday life

For healthy people, variety is the best approach: anyone who enjoys oxalate-rich foods in moderation and does not eat large portions of them every day need not worry. Blanching and pouring away the cooking water noticeably lower the content. A sufficient calcium intake via dairy products or fortified alternatives helps to bind oxalates already in the gut. Anyone who drinks a lot supports the kidneys in excretion. For healthy people, that covers everything important; anyone with a tendency to kidney stones additionally follows the points mentioned above and the individual situation. Overall: oxalic acid is a normal component of many healthy foods and does not have to be avoided by most people. A conscious but relaxed approach with variety, good preparation and sufficient fluid is the best strategy and makes it possible to continue enjoying nutrient-rich vegetables such as spinach and chard regularly.

Frequently asked questions (FAQ)

Which foods contain the most oxalic acid?

Particularly rich in oxalate are spinach, chard, rhubarb, sorrel, beetroot, cocoa and dark chocolate as well as some nuts. The values vary depending on variety, ripeness and origin.

Is oxalic acid harmful?

For healthy people, oxalic acid in usual amounts is harmless. It becomes relevant mainly with a tendency to calcium oxalate kidney stones or with very large consumption amounts over a longer period.

How can I reduce the oxalic acid content?

A large part of the oxalic acid is water-soluble. By blanching and pouring away the cooking water, the content can be significantly lowered. Combining it with calcium-rich foods is also favourable.

Which foods are low in oxalate?

Low in oxalate are many types of cabbage, cucumber, courgette, peppers, button mushrooms, many types of fruit, dairy products as well as rice and pasta. They are well suited as an alternative to oxalate-rich foods.

Can I eat spinach if I have kidney stones?

With a tendency to calcium oxalate kidney stones it is often advised to limit very oxalate-rich foods such as spinach and to eat it cooked and together with calcium-rich foods. The appropriate amount depends on the individual situation.