Science; molecule of the month: oxalic acid

Cooks in this year's Master Chef, the BBC's popular competition which ended this month, surprised judges by choosing rhubarb as an ingredient. Rarely used today, this food was once very popular and famed for its laxative properties. Rhubarb works because it contains a natural toxin, oxalic acid. Our gut is stimulated to reject this. Rhubarb became infamous in the First World War when people ate its leaves as a vegetable - and died from too much oxalic acid.

Rhubarb came to Britain in the 1770s from China, where its root has been used in traditional medicines for more than 4,000 years. It clearly met a need and, by 1860, Mrs Beeton's book Household Management reported that it was in every kitchen garden. She gave recipes for rhubarb pies, jam and wine. But the common way of eating it was to stew it with sugar and serve it with custard.

Oxalic acid occurs in lots of foods in small amounts, and a few foods in large amounts. Swiss chard has most (700mg per 100g), but spinach (600mg), rhubarb (500mg), cocoa (500mg), beetroot (300mg) and peanuts (150mg) also have a lot. A cup of tea provides 50mg and this source accounts for most of the 150mg of oxalic acid the average person consumes each day. This amount is about a hundredth of a fatal dose.

Nutritionists refer to oxalic acid as an anti-nutrient because it interferes with the essential minerals, iron, magnesium and especially calcium.

Earlier this century spinach was advocated as a rich source of iron, and indeed it has higher levels of this metal than most vegetables. However, because oxalic acid bonds to iron and renders it useless, less than 5 per cent of the iron in spinach is absorbed by our body.

The effect of oxalic acid on calcium is more worrying because it forms insoluble calcium oxalate, crystals of which can grow into painful stones in the bladder and kidneys, especially if our fluid intake is low. Such stones irritate the bladder lining and can trigger cancer, which is why oxalic acid is classed as an indirect carcinogen. Doctors put patients who are prone to develop stones on low oxalic acid diets.

Oxalic acid can kill by lowering the calcium in our blood below a critical level. (The antidote is calcium gluconate solution.) Although foods high in oxalic acid can be avoided, we cannot exclude it entirely from our body because there are other sources. For example, surplus vitamin C, which the body cannot store, is turned into oxalic acid, and a side effect of taking massive doses of this vitamin may be kidney stones.

According to John Timbrell, Professor of Toxicology at the School of Pharmacy in London and author of Introduction to Toxicology, it is possible to get a fatal dose of oxalic acid in other ways. "People who have accidentally or deliberately drunk ethylene glycol, which is used as antifreeze in cars, may die of poisoning from oxalic acid, which is formed in the body from the glycol," says Professor Timbrell.

Plant cells are known to make use of oxalic acid, but it has no role in animal cells - or so it is assumed. Yet we tolerate surprisingly high levels of oxalic acid, and research scientists in Germany recently discovered that human tissue contains more oxalic acid than previously suspected. Dr Steffen Albrecht of Dresden University challenges the view that it is merely an unwanted end-product of metabolism. He says the high levels of oxalic acid point to its having an active role.

Dr Albrecht's research team has developed a sensitive method of analysis for oxalates and can measure concentrations as low as millionths of a gram (micrograms) per litre of blood. Their work has revealed markedly different levels of oxalate in the blood, with plasma having 400 micrograms per litre while serum has 1,200 and some blood cells have 250,000.

The team also found that patients on dialysis machines were releasing higher amounts of oxalate than expected and they concluded that there is much more intracellular oxalic acid in the human body than that which is awaiting excretion. Dr Albrecht postulates that it could be used to make hydrogen peroxide, and thereby trigger a burst of phagocytes, cells that engulf and break down foreign particles such as disease-producing micro-organisms.

Oxalic acid is more correctly known as ethanedioic acid and has the formula H2C2O4. It is made commercially by treating sugar with nitric acid or cellulose with sodium hydroxide. The acid is very soluble in water - a litre will dissolve 150g - and it forms a corrosive solution. Industrially, it is used in tanning leather, dyeing cloth, cleaning metals and purifying oils and fats.

The only guise in which it might be found in the home is as stain removers for iron-based stains such as rust and fountain-pen ink. In an emergency you could try a little rhubarb juice instead - assuming you can still find some growing at the bottom of your garden.

The writer is science writer in residence at Imperial College, London.