Oxalate: Definition, Foods & Formula

Instructor: Laura Foist

Laura has a Masters of Science in Food Science and Human Nutrition and has taught college Science.

In this lesson we will learn about oxalate. We will learn why it is important in plants and the problems it can cause in humans. We will also learn about the structure of oxalate and how this structure affects calcium.

Oxalate Definitions

Popeye taught us all that eating our spinach will make us strong. Yes, spinach is very healthy and it's high in many nutrients, including iron and calcium. Yet, since it's also high in oxalate, our body can't even use all of these nutrients. Oxalate will bind iron and calcium, making it difficult for the body to absorb them.

Oxalate is found in many plants and animal tissues. When it is bound to calcium, it has been found to cause kidney stones, which are crystals that can cause problems in the urinary tract. Yet this combination of calcium and oxalate could act as a protectant for plants, which we theorize is why it's formed in the first place.

Foods with Oxalate

Some of the foods that are high in oxalate are: spinach, Swiss chard, rhubarb, beets, and nuts. Of these, spinach and Swiss chard are of the most interest because all of the calcium and iron in them is not able to be absorbed once it's bound to the oxalate. Some of the calcium and iron are not bound to oxalate and so can still be absorbed. Yet, foods high in oxalate aren't as good of a source of calcium and iron as other foods may be. Oxalate can be partially destroyed by heat (cooking). Once it is destroyed, the body can then absorb the calcium and iron. Heat will also destroy other vitamins in the vegetables, such as vitamin C and A. So, these foods can either be eaten raw and be a good source of vitamins such as C and A but not be a good source of calcium and iron, or they can be eaten cooked and be a good source of calcium and iron but not of the heat sensitive vitamins. We can also eat other vegetables such as kale, which is high in calcium and iron but does not contain oxalate.

When the oxalate binds to calcium, it forms a crystal. These crystals can grow in the kidneys as they combine with each other. The more they grow, the larger the kidney stone formed. So, for those who are prone to kidney stones, it could help to avoid foods high in oxalate.

Oxalate Formula

So what is oxalate, chemically? It's simply carbon and oxygen, containing two carbon atoms and four oxygen atoms. It is a dianion, which means that it has a negative charge of two. This is why calcium associates so strongly with oxalate; it has a positive charge of two. Just like two magnets of opposite charges are attracted to each other, calcium and oxalate are attracted to each other. Calcium has the positive charge while oxalate has the negative charge; when these two combine it's like a strong magnet holding them together. Oxalate also prevents other nutrients with a positive two charge from being absorbed, such as iron.

The oxalate formula is:

Oxalate Formula

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