Rhubarb and Other Poisons
This Delicious Food Contains a Highly Toxic Chemical
Yet its distinctive sour taste is due to a chemical that carries the same health rating, i.e. extreme poison, as cyanide.
Oxalic Acid
The name ‘oxalic’ comes, like ‘oxygen’, from the Greek ‘oxys’, meaning ‘sharp’ or ‘acidic’. Though not a very strong acid, it is much stronger than the acetic acid found in vinegar. Its toxic nature, however, owes nothing to its acidity, but is due to its ability to combine strongly with metals. In this context, it is referred to as a chelating ligand. It is known for its capacity to remove iron rust stains from clothing and is responsible for rhubarb’s dissolving of aluminium from cooking pans.
Toxic Nature
Inside the body, several metal ions, like those of sodium, potassium, magnesium, iron etc all perform important functions, which can be disrupted if they combine with oxalic acid, which can effectively remove them. The most serious consequences result from its combination with calcium ions.
While most of the 1.2 kg of calcium in the body is present in the bones, about 1% is necessary for the control of blood acidity, hormone release and electrical activity in the nerves. Bones act partly as a reservoir from which calcium ions can flow to maintain the balance in the blood, while bone calcium is continually replaced from food. Oxalic acid seriously compromises this calcium cycle.
An early symptom of oxalic acid poisoning is the presence of microscopic crystals of calcium oxalate in the urine. If these crystals grow larger, prior to excretion, they can obstruct the tubules in the kidneys, resulting in painful kidney stones and even permanent kidney damage. Deposition of calcium oxalate crystals between the joints can cause the less serious, though equally painful gout. The removal of calcium and other minerals can also lead to nutrient deficiency.
The concentration of oxalic acid in rhubarb stems is well below the danger level, though it is much higher in the leaves and roots, which should never be eaten. The well-known laxative effect of rhubarb is a safety mechanism by which the body rids itself of this toxin before much of it can be absorbed.
Other Sources of Oxalic Acid
Other foods that contain appreciable amounts of oxalic acid include cabbage, grapes, beetroot, tomatoes, sweet potatoes, chocolate, nuts, berries and tea. Plants such as sorrel, which were once used in salads also contain oxalic acid. Spinach is often thought of as a rich source of iron in the diet. Unfortunately, the high concentration of oxalic acid in the same plant is such that it effectively prevents most of this iron from being absorbed.
If large amounts of ascorbic acid (vitamin C) are eaten, the excess that the body does not use is normally excreted in the urine. However, some of the excess can be oxidised to oxalic acid. Ethylene glycol, an alcohol, is the main ingredient of anti-freeze. If this is accidentally drunk, it can also be oxidised to oxalic acid. The contamination of some alcoholic drinks with ethylene glycol has, in the past, led to fatalities.
References: John Emsley, Molecules at an Exhibition, Oxford University Press 1998
