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Spinach (Spinacia oleracea)

A flask containing spinach leaves, overlaid with the structure of retinol.

In the 1930s Popeye the Sailor Man, one of the world’s most famous consumers of spinach, was held responsible for a 33% increase in the consumption of it in the USA.  Spinach was portrayed as the source of his muscles and gave him strength. The reason behind this was thought to be the iron found in spinach, but is it true? Does spinach really contain as much goodness as people originally thought?

Figure 1: Popeye eating some spinach.

1. Everybody knows that Popeye is strong because he eats lots of spinach (Figure 1), and that his creator chose this vegetable because of its high iron content. Almost as many people know the interesting factoid associated with this idea: that the iron content of spinach as originally reported was actually incorrect, because a misplaced decimal point in an early report of its iron content led to a value ten times higher than it should have been. What fewer people know is that everything written in this paragraph so far is a myth – it originates in this article in the BMJ, and has been thoroughly debunked. Spinach does contain lots of iron: according to the US Department of Agriculture, 100 g of raw spinach contains 2.71 mg of iron, as well as lots of vitamin A, vitamin C, vitamin K, magnesium, manganese, and folate. It is also a good source of B vitamins (riboflavin and vitamin B6), vitamin E, calcium, potassium, and dietary fibre.


Figure 2: Structure of the oxalate anion.

2. As well as all these things, spinach also contains high levels of oxalate (Figure 2). Oxalate, the conjugate base of oxalic acid, is a fantastic ligand for metal ions, and will react with the calcium in your saliva to form minute crystals of calcium oxalate, which stick to your teeth and give them a chalky feeling (known to the internet as Spinach Teeth). Most oxalate that is ingested is simply excreted in the urine as small calcium oxalate crystals (Figure 3), and one study (International Urology and Nephrology, 2008, 40, 589–594) compared the absorption of calcium from spinach and from milk. The results showed that 5.1% of the calcium in spinach is available compared to 27.6 % in milk. Diets which are low in calcium do not remove the oxalate from the body, which can lead to the formation of kidney stones, one form of which is just large crystals of calcium oxalate (BMJ 2004, 328, 1420). Equally, oxalic acid is poisonous because the crystals it produces when it reacts with the calcium in the blood plasma both lower blood calcium levels and lead to kidney failure (J. Occupational and Medical Toxicology, 2012, 7, 17).

Figure 3: Calcium oxalate crystals (indicated by arrows) in urine. The round things are blood cells.

The internet will tell you that the bioavailability of iron from spinach is also low due to its high oxalate levels (see, for example, this site selling dietary supplements); the theory goes that the formation of iron oxalate removes the iron in the same way as for calcium. However, this ignores the very different solubilities of calcium and iron oxalates, and it has been shown that this is not the case (European Journal of Clinical Nutrition, 2008, 62, 336–341) – in fact, it appears to be other compounds called phenolics in the spinach that limit iron uptake (European Journal of Clinical Nutrition,1989, 43, 547-557). Phenolics are also found in tea, and drinking tea with your meals can also reduce the amount of iron your body absorbs.  (Human Nutrition Applied Nutrition, 1982, 36, 116-23.)


Scheme 1: The change in structure of cis-retinal to trans-retinal

3. Spinach is also rich in vitamin A (a Molecule of the Month in January 2017), which is also known as retinol – its structure is shown in the main image at the top – and this is probably why Popeye’s creator chose spinach as his food source.

Vitamin A is important when it comes to vision. Retinol is converted to cis-retinal by the action of the enzyme retinol dehydrogenase. If cis-retinal absorbs light in the visible range, it can isomerize to form trans-retinal. It does this by the excitation of an electron in a double bond to a higher energy orbital, weakening the double bond and allowing it to rotate to form trans-retinal. This is illustrated in Scheme 1. It was the American biochemist George Wald who first discovered this isomerisation, with experiments using fish oil. When retinal is bonded to the protein opsin, the resulting complex is called rhodopsin. The change in shape of retinal on absorbing light causes a change in the shape of the opsin, triggering a signal to be sent down the optic nerve, and this is how vision occurs. The trans-retinal is ejected by the opsin and then is converted back to cis-retinal via the action of enzymes.

Not only is retinol important for vision, but it can also be used as an acne treatment, as well as to remedy other skin conditions like psoriasis and ichthyosis. However, there are many issues with using it as an active ingredient in medicines. It is hydrophobic, at low concentrations is ineffectual, at high concentrations is poisonous, and both light and oxygen exposure can lead to its degradation. A research group at Duke university in the US have addressed these issues using encapsulation of retinol into silicone particles. Sol-gel polymerisation was involved in the preparation of the silicone particles. The sol-gel process involves using small molecules (silane monomers in this case) to produce solid materials (silicone particles in this case). The encapsulation of the retinol allows for controlled release of the active ingredient reducing skin irritation and preventing degradation.

Contributors: Chris Adams, Natalie Fey, Lucy Bird (editing, images), Bobby Walker (initial research, photo & writing)

Picture credits

Figure 1: Taken from Wikimedia commons (Thomas Quine [CC BY 2.0])

Figure 3: Taken from Wikimedia commons (Doruk Salancı [CC BY-SA 3.0 ])

Categories: Cookery, Biochemistry, Metals, Metallurgy, Food

Tags: chemistry, iron, vitamin A, oxalic acid, science