Radha Gopalan
No. There are several naturally occurring toxins
in plant and animal foodstuffs. Toxins
are poisonous substances of natural origin and can be harmful to human
beings. But food poisoning, in our mind,
is usually associated with microbes or environmental contaminants. A foodstuff is not always safe just because
it is natural.
Concern about the safety of food
has been reported even amongst our early ancestors. Pre-historic men were
primitive toxicologists – they only consumed food that was found safe for
animals. But they were always subject to
food hazards arising from food plant poisons, decomposition or deficiencies in
their diet. The active substances in plants can be toxic if unchanged. However,
primitive man discovered that some toxins were rendered safe in the preparation
process – cooking, soaking etc.
Poisonous plant chemicals are
called phytotoxins. These include
alkaloids, polypeptides, amines, glycosides, oxalates, resins and toxalbumins. The
natural contaminants range from toxic alkaloids in the white potato with high
carbohydrate content to oxalates in spinach.
Some edible mushrooms are also poisonous. Mycotoxins (toxicity due to fungus) on grains
and absorptions of trace elements (zinc, selenium, lead, copper, cadmium, etc)
are other toxic contaminants of natural origin.
Legumes and oil-bearing plants
that are highly proteinacious are also highly toxic. Red blood cells clump together when we eat
improperly cooked beans and some other leguminous plants. Beans also contain a
substance called tyrosine which gets metabolized in normal persons. But in heart patients, who take medicines,
the normal metabolism is hindered and eating beans can lead to a hypertensive
crisis.
Ricin, a toxalbumin, is a component of castor bean that causes burning of the mouth, throat and stomach, convulsions and respiratory distress.
Lathyrism – a disease common in
India – manifests as muscular weakness and paralysis of the lower limbs. This is due to the intake of the pulse
lathyrus (khesari) dal as a major portion of one’s diet. The chemical aminopropionitile is an enzyme
inhibitor which weakens blood vessel walls and bones. Erucic acid, a natural lipid found in
rapeseed oil and mustard seed oil, is potentially toxic and can cause heart
disease.
Honey bees visit a number of
plants to collect honey. If they visit
toxin containing flowers, the toxicity can reach us through the honey
indirectly. There has been one such
report of honey poisoning in Asia Minor.
Intolerance to lactose or milk sugar is common amongst adults in Asia and West Africa. Lactose causes diarrhea and gastric problems. This sugar is a naturally occurring component of milk. In tolerant persons, lactose is broken down to glucose or fructose which is easily absorbed but n intolerant individuals lactose remains in the gut, ferments because of the bacteria in the gut and causes gastric disorders.
Rhubarb, tea, cocoa, spinach and
beet leaves contain salts of oxalic acid in large quantities (0.02 to 1.3 per
cent). It is also present in vegetables
like lettuce, turnips, carrots, peas and beans and in certain berries. These salts are mostly found in leaves but in
the rhubarb they occur in the stalks and in beets and carrots, in the roots. Oxalic acid formed by the breakdown of the
salts in corrosive on the mouth and gastrointestinal tract. It can even cause haemorrhage.
Solanine is the glycoalkaloid
present in white potato (Solanum tuberosum), brinjal and tomato, and causes
gastrointestinal and neurological disorders.
The potato has a very high proportion of solanine and eating a fir
amount of raw potatoes can lead to death.
Linamarin and amygdalin present
in Lima beans, peaches and apricot produce hydrogen cyanide enzymatically. This chemical is highly poisonous. Dhurrin, a substance present in sorghum, denatures
the proteins. The consumption of huge
quantities of peach and apricot kernels also leads to poisoning. Cassava roots or manioc (tapioca) (Mamihot
utilissima), the basic food in many African countries, yield 15-400 mg of
cyanide per kilogram of fresh food. The
enzyme causing the release of cyanide, also contained in tapioca, is
inactivated by cooking. Low levels of
cyanide residue are observed in the cassava consumed.
Ergotis is the effect of
carcinogenic mycotoxins from the fungus ergot (Clavicepts purpurea). It contaminates flour obtained from grains
affected by the fungus and the toxin even goes up to the bread stage. The by-products of this fungus have resulted
in hallucinations and gangrene formation – ergotism. In fact ergotism affected persons were burnt
as witches in medieval times in Europe.
Other outbreaks of diseases due to mouldy foods are also well known.
Aflatoxin is a mycotoxin from
fungus Aspergillus flavus and A parasciticus.
It is carcinogenic to animals, causing injury to the liver. Ot of 400 cases of aflatoxin poisioning in
India recently, 100 succumbed to hepatic diseases. Maize, a major dietary constituent, was
contaminated with aflatoxins. Also
groundnuts grown in humid areas can cause aflatoxin poisoning because the
fungus infects the plants more easily in such climates.
Drinking herbal teas and medicines have reported to be the cause of liver injury in Jamica, Indian and
Afghanistan. Pyrrolizidine alkaloids were
found in teas that resulted in hepatic disease.
Another typical example that is carcinogenic is chewing betel leaves
with arecanut or tobacco. Forty per cent
of mouth and throat cancer prevalent in India is because of the toxicity caused
by the alkaloids arecoline and nicotine present in tobacco, betel and arecanut.Some mushrooms can be poisonous when eater, some are deadly. Gautama Buddha is said to have died of mushroom poisoning. Some mushrooms, though poisonous when fresh, are edible when cooked, dried and salted. Muscarine and amanitine are among the toxic substances present in various mushrooms that affect man.
Zootoxins are animal food toxins.
Blue fin tuna, mackerel and swordfish, when left at room temperature for long
hours, produce a chemical called saurine.
This is a histamine releasing substance causing vomiting, nausea,
itching, etc. Jellyfish have
protenacious toxins that get inactivated on heating and by gastric juices. Biotoxication is mostly caused by fish. The toxins are present in fresh fish and are
not destroyed by heat or gastric juices.
Several species of sharks can cause severe intoxication. The bones of mackerel contain an oil which is
purgative by nature. Another kind of fish, commonly known as gobi fish, causes poisoning because of the presence
of a very toxic substance called terodotoxin.
Liver poisoning by sharks, tuna and sea bass is due to the presence of
high levels of vitamin A concentration in the liver of these fish. Some turtle meat becomes extremely poisonous causing
nausea, vertigo, cramps, etc. Its pharmacological
and chemical properties are unknown.
The sea-anemone has a toxic polysaccharide
that is inactivated by heat. A popular
oriental food, sea cucumber, is a source of gastro-intestinal disorders which
are due to saponins – a group of glycosides made up of sugars. These are also present in a wide variety of
plants. The burning and itching sensations
caused by eating shell fish are due to the presence of pyrophenophorbide, a
toxic substance that is absorbed through the skin.
With the growing knowledge of
natural toxicants present in plants, and animals, our doubts about so-called “safe”
foods also increase. But they need
not. The question is not whether toxic
substances are present in some foods or not; the answer is always “yes” to
begin with. But, as Paracelsus, a Swiss
alchemist and physician, puts it, “All things are poison for there is nothing
without poisonous qualities. It is only
the dose which makes a thing a poison”.
(This essay was first published in July 1987 issue of Science Age (now defunct). Ms Radha Gopalan has an M.Phil
in mycology from Southampton University, UK)
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