Oxalic acid: Where it is and when it is a concern

Oxalic acid is a toxin that can attack the kidneys in high concentrations. It is contained in many foods, but is rarely dangerous. We’ll show you what you need to pay attention to.

Basically, oxalic acid is found in every vegetable and fruit. For example, it causes the dull feeling on your teeth when you eat rhubarb.

Consumed in high concentrations, oxalic acid can damage the kidneys, among other things. But the good news: Only a few foods contain such quantities that their massive consumption would have consequences for health.

You can also reduce the oxalic acid content in foods through proper preparation.

Oxalic acid: The concentration is particularly high in these foods

Plants produce oxalic acid to protect themselves against predators. It is therefore found in plant foods.

These foods contain high concentrations of oxalic acid:

• Chard
• sorrel
• rhubarb
• spinach
• sweet potatoes

• soybeans and tofu

• Beetroot
• Nuts such as almonds, cashews or hazelnuts
• black tea
• cocoa

Why oxalic acid concentration can fluctuate

In general, a concentration of oxalic acid of 50 milligrams per 100 grams or more is considered harmful to health. But the calculation is not that simple, because the oxalic acid value of plants is not constant.

• Seasonal fluctuations – For example, in spinach, the amount of oxalic acid increases in the summer and then decreases again.

• Environmental fluctuations – Weather conditions and soil quality influence oxalic acid levels. It also plays a role how much carbon the plant has absorbed.

• Variety-related variations – For example, rhubarb varieties with red stems have less oxalic acid than varieties where the stems are green. By the way: The rhubarb leaves always contain more oxalic acid than the stalks and are therefore not edible.

Studies on spinach varieties showed oxalic acid levels between 647.2 and 1,286.9 milligrams per 100 grams of fresh spinach leaves. However, that does not mean that a portion of spinach could be dangerous to your health – because the bioavailability of oxalic acid for humans is far lower than the content in the plants themselves. The body excretes a large part of it undigested or intestinal bacteria break down the oxalic acid. According to a specialist article in Nutrition Umschau, the average absorption rate is only between two and five percent.

The article further reports that with a normal mixed diet we consume around 50 to 200 milligrams a day. If you follow a vegan or vegetarian diet, the values ​​can also be slightly higher. Nevertheless, for a healthy person there is no cause for concern.

Not all oxalic acid is toxic

In its pure form, oxalic acid is toxic and can damage the kidneys to the point that they fail. It can cause bleeding in the intestines or circulatory collapse.

In plants, however, oxalic acid is usually present in combination with other elements. They are the so-called oxalates or salts of oxalic acid. They are considered less critical for health.

According to the Nutrition Survey, they can be divided into two groups according to the connection:

• The body can absorb oxalate salts dissolved in water – the kidneys excrete these substances through the urine. If you eat large amounts of these oxalate salts over a longer period of time, they can settle in the kidneys and form kidney stones. Anyone who has eaten rhubarb or spinach, for example, has felt the oxalate salts. They are responsible for the dull feeling on your teeth.

• Water-insoluble oxalate salts, on the other hand, are largely harmless to the kidneys because the body excretes them through the intestines. These salts are often associated with iron, calcium or magnesium. For example, amaranth contains a lot of oxalic acid at over 1,500 milligrams, but the calcium content in the grain reduces the amount of oxalic acid that the body can absorb to around 500 milligrams.

Cooking reduces the oxalic acid content in food

The best way to combat oxalic acid is to cook the food – this reduces the oxalic acid. In general, prepare vegetables as gently and healthily as possible. So if you cook rhubarb or blanch spinach, your meal already contains less oxalic acid. The cooking water also contains dissolved oxalate salts – so don’t use it.

Oxalic acid combines well with calcium. Cook spinach or chard in cream sauce or prepare rhubarb compote with vanilla pudding.

You can also do this:

• Drink plenty of water or unsweetened herbal tea with meals. The fluid flushes the kidneys thoroughly and reduces the chance that the oxalic acid salts will build up.
• A diet high in fiber also reduces the possibility that the body can absorb oxalic acid through the intestines.
• It’s better to prepare a spinach salad or green smoothie in spring, when oxalic acid levels are still low.
• Make sure you have a varied diet – including vegetables.