Emsley Oxalate
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This is also from the NY Times article ... see phosphoric acid .. Molecules at an
exhibition.
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Rhubarb pie--oxalic acid
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Chocolate also harbours oxalic acid, a dangerous chemical that can kill--but rarely does.
We take in oxalic acid every day from a variety of sources. It occurs in lots of foods in
small amounts, and in a few foods in large amounts, cocoa having one of the largest with
500 mg per 100 g. Green leaf vegetables have the most, such as Swiss chard with 700 mg per
100 g, spinach with 600 mg, and rhubarb with 500 mg. Rhubarb, also known as pieplant in
the USA, is popularly thought to have dangerously high levels because this food has killed
people. Perhaps less well appreciated is that beetroot (300 mg) and peanuts (150 mg) also
have a lot of oxalic acid.
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The average person consumes about 150 mg of oxalic acid a day, and in countries where tea
is popular the level is generally higher because a cup of tea provides 50 mg. A fatal dose
of oxalic acid is around 1500 mg. Could we reach a deadly dose during the course of a
normal day? And what effect do even the lower levels of oxalic acid have on us?
<br/>
Rhubarb is less popular than it used to be, but it was once widely eaten stewed with
sugar. It was famed for its laxative properties, and it works because it stimulates our
gut to reject the natural toxin, oxalic acid. A bowl of stewed rhubarb could provide us
with a sizeable fraction of the toxic dose. To poison yourself by eating bars of milk
chocolate would be virtually impossible, no matter how chocoholic you felt, because these
contain much less oxalic acid and you would be satiated with them before you reached even
the laxative level.
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Rhubarb became infamous in World War I when people ate its leaves as a vegetable, and some
died through oxalic acid poisoning. The level of oxalic acid in rhubarb leaves is much
higher than in the plant stalks, but you are not at risk from eating those.
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Rhubarb has long been known as a medicament. In AD 70 the Greek physician and botanist
Dioscorides recommended it for treating a variety of conditions. This was European
rhubarb, and was used until the twelfth century, when a superior rhubarb appeared from the
East. There was much speculation as to where it was grown. Most came from China and it
continued to be imported on a large scale as powdered root for hundreds of years.
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Rhubarb root has been used in traditional Chinese medicine for more than four thousand
years. The Royal Society of Arts, Manufacturers and Commerce decided to promote the
cultivation of the new rhubarb in the British Empire, and during the next 30 years it
awarded several gold medals to those who grew the best varieties. In 1784 the Swedish
apothecary Carl Wilhelm Scheele detected oxalic acid (which he knew as acid of sorrel) in
rhubarb roots, and showed that the amounts in the plant's leaves were too large for these
to be edible. The oxalic acid is thought to be the plant's protection against cattle. In
1860 the Victorian best-seller,
<i>Mrs Beeton's Book of Household Management</i>
, reported that rhubarb was in every kitchen garden, and she gave recipes for rhubarb
pies, rhubarb jam and even rhubarb wine. The easiest way of cooking it was to stew the
chopped stalks with sugar. When aluminium pans became popular, stewing rhubarb was
discovered to have another bonus: it cleaned the pans beautifully. It did this because the
oxalic acid dissolved off the top layer of metal, although the amount that was removed
this way was so tiny it was no threat to health.
<br/>
This affinity of oxalic acid for metals also explains another curious anomaly, and is the
reason why nutritionists refer to it as an antinutrient. Oxalic acid 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. For example spinach has 4 mg of iron per 100 g, compared to peas which
have 2 mg, brussels sprouts 1 mg, and cabbage only 0.5 mg. Despite having more iron, the
oxalic acid in spinach renders 95% of this metal useless as a nutrient, and only 5% can be
absorbed by our body. The cartoon character Popeye attributed his strength to spinach, but
he was sadly misinformed. By all means eat spinach as a vegetable, enjoy it, but expect
very little from it except a modest amount of vegetable protein and a little vitamin C.
<br/>
Oxalic acid kills by lowering the calcium in our blood below a critical level. (The
antidote is calcium gluconate.) Calcium is essential for keeping the blood at a constant
level of acidity and viscosity, as well as for clotting and transporting phosphate around
the body. But even in non-lethal doses the effect of oxalic acid on calcium is worrying,
because it forms insoluble calcium oxalate, crystals of which can grow into painful stones
in the bladder and kidneys. The development of these is more likely if our fluid intake is
low. Doctors whose patients are prone to develop such stones put them on a low-oxalic acid
diet, which excludes the foods we have been talking about. Although such foods can be
avoided, we cannot exclude oxalic acid entirely from the body because there are other
sources. For example, surplus vitamin C, which the body cannot store, may be turned into
oxalic acid, and a side-effect of taking massive doses of this vitamin may also be kidney
stones.
<br/>
According to John Timbrell, a toxicologist at the London School of Pharmacy and author of
<i>Introduction to Toxicology</i>
, 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 oxalic acid poisoning because the body converts ethylene glycol to oxalic acid.
<br/>
Plant cells are known to make use of oxalic acid, but it has no role in animal cells--or
so it was once assumed. Recent research suggests otherwise. Despite the apparent toxicity
of oxalic acid, the body will tolerate surprisingly high levels of it, and research
scientists in Germany discovered that human tissue contains more oxalic acid than was
previously suspected. Dr Steffen Albrecht and co-workers at Dresden University have
challenged the view that oxalic acid is merely an unwanted end-product of metabolism. They
have developed a sensitive method of analysis for oxalic acid, and can measure
concentrations as low as millionths of a gram (mcgs) per litre of blood. Their work has
revealed markedly different levels of oxalic acid in the blood: plasma, the fluid part of
the blood, has 400 mcg per litre while serum, the clear solution which separates from
blood after the plasma has coagulated, has 1200 mcg. Some blood cells have 250 000 mcg,
which sounds a lot but when converted to milligrams is only 25 mg per 100 g, which is low
compared to the levels in some foods. Albrecht says the high levels of oxalic acid point
to its having an active role in human metabolism, although what that is remains unknown.
<br/>
Oxalic acid is made commercially by treating either sugar with nitric acid or cellulose
with sodium hydroxide. The acid is very soluble in water--a litre will dissolve 150 g--and
it forms a corrosive solution. Industrially it is used in tanning leather, dyeing cloth,
cleaning metals and for purifying oils and fats. The only guise in which it might be found
in the home is in stain-removers for iron-based stains, such as rust and ink from
fountain-pens.
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