The chemistry of spinach

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“Spinach is a good source of iron” – a myth, but a surprisingly persistent one. The story behind the myth and the chemistry that debunks it are fascinating. Here we look at both, as well as the chemical explanation behind the ‘spinach teeth’ phenomenon.

Let’s start with the ‘good source of iron’ myth. If the myth itself wasn’t enough, its origins also have their own sub-myth. The story goes that the iron content of spinach was overestimated by a factor of ten in the early 1900s due to a misplaced decimal point in a scientific paper. It’s a fantastic tale, but unfortunately, one that’s a complete falsehood. The full story of the investigation of this myth can be found here.
Another victim of the myth was the cartoon character Popeye, who famously ate spinach. The perception that the original comic strips promoted his consumption of spinach for its iron content is an incorrect one. His creator, Elzie Segar, actually chose spinach for its high vitamin A content.
Even without any decimal point errors, it’s easy to understand how the myth of spinach being a good source of iron originated. Spinach’s iron content actually trumps that of many other vegetables. It clocks in at approximately 2.6 milligrams of iron per 100 grams (according to the USDA food composition database). That exceeds the amount of iron found in some types of meat, with the amount of iron found in beef sirloin steak coming in at around 2.5 milligrams per 100 grams.
However, the full story isn’t quite so simple. To start with, iron can be found in different forms in food. In meat, iron is commonly found in the form of haem iron, which is easily absorbed in the body. In vegetables, on the other hand, the iron content is non-haem iron. This isn’t absorbed as easily by the body, and other factors can impact on its absorption.
When taking this into account, we can refine the figures for comparing spinach and meat. Only about 1.7% of the non-haem iron in spinach is absorbed when we eat it. That means that the 2.6 milligrams of iron per 100 grams only translates into a paltry 0.044 milligrams of iron absorbed. Compare this to our sirloin steak, from which approximately 20% of the available iron is absorbed. That’s 0.50 milligrams from the original 2.5 milligrams per 100 grams.
Part of the reason so little of the iron in spinach is absorbed when we eat it is the other compounds present. For a time, it was thought that the high oxalic acid content of spinach played a part. More recent studies have suggested that this isn’t the case. Instead, it’s now thought that the polyphenolic compounds found in spinach are responsible. They reduce the amount of iron available for absorption by binding to it and forming insoluble compounds.
There are still good reasons to eat spinach, even if it’s not for the iron content. It’s particularly high in vitamin A precursor compounds called carotenoids. Vitamin A is important for our eyesight and immune system, amongst other things. Beta-carotene is an example carotenoid, well known as the chemical that gives carrots their orange colour. It doesn’t give spinach the same colouration as the green of chlorophyll masks it.
Oxalic acid might not have much to do with the absorption of spinach’s iron content, but it does play a part in one of the consequences of eating spinach. You might have experienced the so-called ‘spinach teeth’ effect. This is a kind of ‘chalky’ feeling on the surface of your teeth after eating spinach.
The effect is due to the oxalic acid in spinach forming calcium oxalate by reacting with calcium ions in saliva and in the spinach. The calcium oxalate is insoluble and coats your teeth to give the ‘chalky’ effect. Rhubarb is another oxalic acid-containing plant which can cause a similar effect.





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