Name: alert person
this article has been taken from IS IT SAFE TO EAT by IAN SHAW.
6
Natural Toxins in Food
6 Natural Toxins in Food
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There is much concern about horrible man-made chemicals
that find their way into our food. Pesticide residues in fruit and
vegetables, veterinary medicines in meat, nitrate in lettuces, etc,
etc (see Chapter 7). Most people think that man-made (artificial)
is bad and natural is good. This is rubbish! Some of the
most toxic chemicals that we know are natural, so perhaps we
should think again.
Just to set the scene, here are some natural and artificial
chemicals with their toxicities – (i.e. rat LD50s – remember this
is a measure of toxicity the smaller the number the more toxic).
The table of toxicities (LD50s) of natural compared with
man-made poisons shows clearly that natural is not necessarily
safe! In fact, it is abundantly clear from this very short list that
some natural toxins pose a far greater hazard than artificial
(man-made) chemicals.
The value with a single asterisk is the oral LD50 in the
mouse and the values with two asterisks are intraperitoneal
(i.p.) LD50s in the mouse – i.p is injection into the abdominal
cavity, it resembles oral dosing metabolically.
Why Do Plants Have Natural Toxins?
Plants might be eaten by animals or infected by bacteria and
fungi. The presence of natural toxins helps the plant to protect
itself from such attacks.For example North American Milkweed
(Aesclepias eriocarpa) contains a very interesting, highly potent
collection of natural toxins:
Eriocarpin – LD50 (mouse) =6.5 mg/kg body weight
Labriformidin – LD50 (mouse) =3.1 mg/kg body weight
Labriformin – LD50 (mouse) =9.2 mg/kg body weight
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Chemical (source) Rat oral LD50 (mg/kg
body weight)
For reference, a commonly consumed, accepted poisons:
Ethanol (the alcohol present in beer,wines, and spirits) 7,000
Aspirin 1,240
Artificial chemicals that might contaminate food:
Diazinon (an organophosphorus insecticide) 250
Glyphosate (Round-up – a herbicide) 4,873
Atrazine (another commonly used herbicide) 1,750*
Penicillin-G (an antibiotic) 6,900
Natural chemicals in food:
Tetrodotoxin (from Fugu Fish, a delicacy in Japan) 0.01**
Solanine (from potatoes) 42**
Psoralen (from parsnips and related plants) 791
* Oral LD50 in the mouse.
** Intraperitoneal (i.p.) LD50 in the mouse – i.p. is injection into the abdominal
cavity, it resembles oral dosing metabolically.
Just 44mg of eriocarpin will kill a sheep. This protects the Milkweed
from grazing animals. However, the Monarch Butterfly
(Danaus plexippus) is not affected by the poisons and lays its
eggs on Milkweed – its caterpillars eat the plant and accumulate
the natural toxins in their bodies so becoming toxic to any animal
that might decide to eat them.This is a clever use of natural
plant toxins in insect protection, and illustrates the importance
of these highly toxic chemicals.
Other natural plant toxins prevent insect attack – i.e.
natural insecticides, and others are natural fungicides.Many of
the toxins have unknown functions, perhaps they are accidents
of plant evolution – perhaps they just scare off grazing animals
because they taste bad.
In general, leaves, roots, and tubers are much more
likely to contain natural toxins than fruits, but this is by no
means always the case, (e.g. potato fruits contain very toxic
solanines). The seeds in fruits are the means by which plants
reproduce, and often they rely on animals eating the fruit as
part of the seed transmission mechanism. The seeds might
pass through the animal and be deposited with a dollop of
fertilizer onto the ground, or the animal might discard the
seeds which drop onto the ground beneath where they are eating.
It is therefore not in the plant’s interest to poison the vector
of its seeds. On the other hand the leaves, roots and tubers
are important to the plant in a different way. Leaves photosynthesise
(i.e. make sugars from carbon dioxide using sunlight
energy), roots take up water and nutrients, and tubers store nutrients
for the dormant months and to allow rapid growth in
the spring. Clearly the plant does not want animals to eat these
important organs, or microbes to infect them. It is for this reason
that the plant might produce natural toxins, to ward off this
attack.
There are many natural toxins that we are exposed to
everyday in the plants that we eat. The dose that we receive is
very low and therefore even though the toxins have a very high
hazard (e.g. low LD50) our exposure is low and so the risk is also
low. However, from time to time the levels of natural toxins in
plants might increase and so the consumer could get a larger
dose and become ill.
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Cucurbitacins in Courgettes (Zucchini)
In the early summer of 2001, several New Zealanders complained
of stomach cramps after they had eaten zucchini. Over
a few weeks more and more cases were reported, which led the
health authorities to investigate. Many of the people who became
ill remembered that the zucchini tasted bitter – I’ll return
to this important fact later.
Members of the cucumber family (Cucurbitacea) are
able to produce a group of highly potent toxins (cucurbitacins)
that have insecticidal and/or fungicidal properties. The production
of cucurbitacins is controlled by the plant so that they are
only made when they are needed. In fact the gene that codes for
cucurbitacin is only switched on if the climatic conditions are
right for insect infestation or fungal infection. The weather in
New Zealand in the early summer of 2001 was just right for insects
and fungi – wet and cool, the gene switched on the synthesis
of cucurbitacin.
The cucurbitacins are intensely toxic (cucurbitacin-B
oral LD50 [mouse]=5 mg/kg body weight – 300 mg could kill a
human) and taste very bitter indeed. The cucurbitacins have
such a terrible taste that it is very unlikely that anyone could
stand to eat a zucchini containing enough cucurbitacins to
cause them significant harm (Fig. 6-1).
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Fig. 6-1. The molecular structure of a cucurbitacin and a courgette (zucchini)
the source of this deadly poison
Rhubarb and Oxalic Acid
Most people know that rhubarb leaves are poisonous, and that
rhubarb itself is a good laxative. But why? One chemical is responsible
for both properties, oxalic acid (oral LD50 [rat]=
375mg/kg body weight – 25 g could kill a human).Rhubarb contains
about 1% oxalic acid. It takes a lot of oxalic acid to kill a human,
but its sub-lethal effects are seen at very much lower doses.
It binds calcium to form calcium oxalate which causes an
ionic imbalance in the cells of the gut and results in diarrhoea.
When you eat rhubarb your teeth sometimes take on a rough
feel if you run your tongue over them, this is because the oxalic
acid is attracted to the calcium of your teeth.
If cream, ice cream, or custard made with milk is eaten
with rhubarb, insoluble calcium oxalate is formed with the calcium
in the milk products, this stops it being absorbed so reducing
the effects of the rhubarb.
Glycoalkaloids in Potatoes
The glycoalkaloids are highly toxic components of members of
the potato/nightshade family (Solanacea). Their concentration
varies very much indeed between species. They are at highly
toxic levels in members of the nightshade genus – this is one
of the reasons that Deadly Nightshade (Atropa belladonna) is
deadly. However they also occur in plants that we eat, the most
notable being potatoes and tomatoes, but they are usually at
non-toxic concentrations in the parts of the plant that we eat.
The three main glycoalkaloids are a-solanine (Fig. 6-2),
a-chaconine, and solanidine. They are very toxic.
127
LD50 [rat] mg/kg
body weight
a-Solanine 42 [oral]
a-Chaconine 84 [i.p]
Solanidine 590 [oral]
128
Fig. 6-2. The potato glyco
129
About 2.5 g of a-solanine could kill a human. This means
that a meal of 1.5 kg of potato sprouts (see the table below showing
levels of glycoalkaloids (e.g. solanine) in different parts of
the potato plant–data from Inherent Natural Toxicants in Food
(1996), MAFF, London) would be needed to be fatal, this is
ridiculous and therefore it is extremely unlikely that anyone
would die from eating solanine-containing potatoes. However
the toxic effects of the glycoalkaloids occur at doses many times
below the lethal dose.
Levels of glycoalkaloides (e.g. solanine) in different parts of the potato plant (data
from Inherent Natural Toxicants in Food (1996), MAFF, London)
Total glycoalkaloid concentration (mg/kg)
Tubers 12–20
Leaves 30–1,000
Sprouts (“eyesâ€) 2,000–4,000
Potato skin 300–600
Glycoalkaloid in green potatoes (data from Inherent Natural Toxicants in Food
(1996), MAFF, London)
Total glycoalkaloid concentration (mg/kg)
“Normal†potato 12–20
Green tuber 250–280
Green skin 1,500–2,200
The glycoalkaloids taste very bitter indeed. Sometimes
potatoes taste bitter, especially if they have been stored in the
light; this is because in the light the potatoes synthesise glycoalkaloids.
They also turn green because they produce chlorophyll
– the green pigment found in leaves, it is responsible for capturing
sun light energy and making sugars from carbon dioxide.
Levels of glycoalkaloids in green (bitter) potatoes can be very
high indeed.
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But you would still need to eat about a kilogram of green
potato skins to kill you. This is not possible at one sitting.
It is still possible to get a dose of glycoalkaloids that will
cause a tummy upset. In fact skin-on potato chips (crisps) originating
from green potatoes can have enough glycoalkaloid in
two standard packets to result in toxicity in children – and it is
not outside the bounds of possibility that a kid will eat two
packets in quick succession! The table below shows the concentrations.
Glycoalkaloid in potato chips (crisps) (data from Inherent Natural Toxicants in
Food (1996), MAFF, London)
Total glycoalkaloid concentration (mg/kg)
Peeled potato chips 40–150
Skin-on potato crisps 40–720
Effects of Glycoalkaloids
The main effect is relatively mild gastrointestinal upset,
although there have been more serious cases. In London in
1979 a large number of children from a school suffered from
stomach pain, vomiting and diarrhoea after their lunch. An investigation
revealed that potatoes from the school’s kitchen had
glycoalkaloid levels of 330 mg/kg. All of the kids recovered but
several of them needed hospital treatment. Potatoes with levels
of glycoalkaloids above 200 mg/kg are now regarded as unsafe
to eat.
What Does Cooking Do to Glycoalkaloids?
The simple answer is very little. They are very heat stable. Not
even frying temperatures destroy them.
Furocoumarins and Parsnips, Celery and Parsley
The furocoumarins are a group of chemicals that occur naturally
in a wide variety of plants, but are at their highest concentrations
in members of the Umbelliferae family (having flowers
like an umbrella – parsnips, celery,parsley, etc).But they are also
found in citrus fruits and figs.
There are many different furocoumarins, but they all
have very similar molecular structures (Fig. 6.3).
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Furocoumarins in commonly eaten fresh foods (data from Inherent Natural Toxicants
in Food (1996), MAFF, London)
Plant/part Main furocoumarin Concentration (mg/kg)
Umbelliferae
Celery/stalk Bergaptan 1.3–47
Parsnip/root Bergaptan 40–1,740
Parsley/leaf Isoimperatorin 11–112
Fig. 6-3. Molecular structures of some furocoumarins found in celery, parsnips
and parsley (drawn by Barbara Thomson, ESR,New Zealand)
What Do Furocoumarins Do for the Plant?
They are produced in response to stress (e.g. bruising) and
therefore are thought to prevent fungal attack. Some might also
have insecticidal properties and could be produced in response
to insect attack.
Interestingly, organically grown vegetables often have
higher levels of furocoumarins. One explanation for this is that
conventional growing techniques use insecticides, and organic
methods prohibit the use of insecticides. The conventional
parsnips therefore do not suffer insect attack because the insects
are killed by the insecticides. On the other hand the pesticide
free organic parsnips are attacked by insects and produce
their own insecticides (or fungicides to prevent microbial infection
of the wound caused by the insect). Therefore organic produce
is likely to have higher levels of furocouarins than conventionally
grown crops (Fig. 6-4).
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Fig. 6-4. Furocoumarin levels in parsnips showing that organic parsnips are very good
at producing furocoumarins in response to damage (data from Inherent Natural Toxicants
in Food (1996), MAFF, London)
Furocoumarin Toxicity
These are nasty compounds. They are activated by light (photoactivated)
to form carcinogens. Therefore prolonged doses might cause cancer.
They can also cause skin sensitisation to UV light – i.e. if you consume
enough furocoumarins and sit out in the sun you will get
a skin rash. It is not thought that normal intakes from food will
cause photosensitisation, but high level intake (e.g. by people
who eat large quantities of organic parsnips) might get close.
This is a ‘slap in the face’ for organic food. I’d much rather eat a
low level pesticide residue than get a dose of a carcinogen with
my parsnips (Fig. 6.5).
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Fig. 6-5. Conventional vs organic parsnip production. Conventional growers
might use man-made pesticides (e.g. Phorate) to control insects, organic
farmers would shudder at the thought, so their parsnips have to defend themselves,
e.g. by producing natural insecticides such as the psoralens the problem
is that psoralens are carcinogenic
What Does Cooking Do to Furocoumarins?
They are not affected much by cooking. However the furocoumarins
are water soluble and therefore if furocoumarincontaining
vegetables are cooked in water (e.g. boiled) the levels
in the vegetables will go down, but the cooking water will
contain the leached furocoumarin – so if you use the vegetable
water to make your gravy, you will still consume the furocoumarin.
Phenylhydrazines and Mushrooms
It is well known that many mushrooms and toadstools contain
toxic chemicals. There are numerous examples of people picking
mushrooms and accidentally collecting a toxic species and
succumbing to its toxicity. Perhaps the best example is the
Death Cap (Amanita phalloides) mushroom. This is amongst
the most toxic plants in the world. It contains the liver toxin
phalloidin which is intensely toxic (LD50 [i.m., mouse]=
0.003mg/kg – i.m.means intra-muscular injection, i.e. injection
into the muscles. 0.2 mg could kill a human), tiny doses will
cause liver failure and death. There is likely to be enough poison
in a single Death Cap mushroom to be fatal. The problem is that
death caps look similar to field mushrooms (Psalliota
campestris), and from time to time people make mistakes.
Phalloidin is just one of many horrific mushroom/toadstool
toxins. But providing we don’t eat the nasty mushrooms
we’ll be alright. Or will we? Shop-bought mushrooms (usually
Psalliota campestris) also contain toxins, albeit far less acutely
toxic than phalloidin, but worthy of a thought or two. One of
these toxins is called agaratine (after the mushroom genus
Agaricus from which it was first isolated).Agaratine itself is not
of any great concern, however it is metabolised in the body to
the 4-hydroxymethylbenzenediazonium (HMBD) ion, and this
is a potent carcinogen.
There are no data on the toxicity of agaritine from mushrooms
in the diet of humans, it is probably just another carcinogen
in our food that plays its part in the cancer incidence
rate that humans suffer – one in four of us will get cancer, there
are a myriad chemicals that we are exposed to every day that
contribute to this risk, agaritine is likely to be a very minor risk
factor.
Herbs and Spices
Herbs and spices are used in small quantities to add flavour to
our food. Some of them are quite toxic if eaten in large quanti-
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ties, not that most people would want to eat large amounts of
them. Some of them contain interesting chemicals that have potent
pharmacological properties, indeed some of these chemicals
are components of medicines or are used as herbal remedies.
This is where food and medicines coalesce, there is much
discussion at present about when a food is a medicine because
medicines are regulated differently to foods...I’ll return to this
later.Meanwhile, back to herbs and spices.
Cloves
Cloves are a good example of a spice that contains a pharmacologically
active chemical, indeed it was once commonly used in
medicine. Cloves are the dried flower buds of a small tropical
tree, Syzygium aromaticum. They give apple pies and cakes a
wonderful flavour, and impart that very characteristic warming
smell, but if you chew on a whole clove you will find a very different
side to their character. They contain eugenol, not only
does this impart their characteristic taste and smell, but it is
also an anaesthetic. Clove oil was used by dentists as an anaesthetic
until quite recently; some people perhaps still use it. So
when you chew on a clove you will feel your mouth get numb.
Despite its pharmacological effect eugenol is of remarkably low
toxicity (oral LD50 [rat]=3,000 mg/kg body weight).
Eugenol is used commercially in the manufacture of
vanillin – the chemical that gives vanilla its characteristic
flavour and smell. It is interesting how so many of the herbs’and
spices’ flavour chemicals are related (Fig. 6-6).
Nutmeg
Nutmeg is the fruit of a large tropical tree (Myristica fragrans)
– rice pudding or egg custard without a good grating of nutmeg
on top to give that beautifully aromatic skin is a travesty!
Nutmeg has its fair share of pharmacologically active
chemicals, amongst them, pinene, camphene, dipentene and
trimyristin – they are used as perfumes, flavouring agents, and
135
136
Fig. 6-6. Flavour chemicals from nutmeg (elimicin), cloves (eugenol), thyme
(thymol) and vanilla (vanillin) showing their similarities. It is interesting to
speculate that the similarities might be because the molecules fit into tongue
flavour receptors to send a flavour message to the brain. The receptors are like
locks and the flavour chemicals act as keys to unlock the flavour signal it is just
possible that the keys have to be similar in shape to fit similar receptors
trimyristin has been used to treat rheumatism. Nutmeg also
contains the hallucinogen, elemicin.However, it would take a lot
of elemicin to get you “highâ€, a helping of mum’s rice pudding is
certainly not going to do the trick.
Thyme
Thyme (Thymus vulgaris) is evocative of roast lamb – it is wonderful
pressed with cloves of garlic into deep cuts in a leg of
lamb before roasting. It has a characteristic smell and flavour,
both due in part to one chemical, thymol. Thymol is a very low
toxicity (oral LD50 [rat]=980mg/kg body weight) antiseptic often
used in dental mouth washes, in times gone by thyme itself
was used to dress wounds because of its antiseptic properties.
Sage
Sage (Salvia officinalis) is a herb of old English gardens. It
smells lovely and has attractive blue flowers that butterflies like.
In the kitchen it is used with breadcrumbs and onions to make
sage and onion stuffing for poultry and pork, amongst a myriad
other uses. It contains cirisiliol, a potent inhibitor of an enzyme
(arachidonate, 5-lipoxygenase) involved in metabolising
fats – there is some suggestion that it might protect against
prostate cancer.
Chives
Chives (Allium schoenoprasum) are just very small onions.They
contain the same flavouring chemicals found in all members of
the onion family. Perhaps the most important of these is, allicin
– I’ll discuss this under garlic because the levels are much higher
in garlic. There are a number of other related chemicals, all
have a common chemical group – the sulphydryl (–SH) – this
class of chemicals are good antioxidants, and might have other
medicinal properties. Perhaps more important to the chef, they
137
smell and taste wonderful. On the negative side, a number of
them cause eye irritation and tear production (they are lachrymatory)
which is why you often cry when peeling onions. A
good example is diallyl sulphide which smells strongly of onions
or garlic and is a powerful eye irritant, but also has anti-cancer
properties. The chemical structure of allyl sulphide from onions
and garlic is shown below:
H2C=CH–CH2–S–CH2–CH=CH2
Garlic
Garlic (Allium sativum) is another member of the onion family.
As discussed above it has many of the smells and flavours of
other onions, but in different concentration ratios which is why
it tastes different to onions and chives. Garlic oil contains very
high concentrations of allyl disulphide (note this has 2 sulphur
atoms, allyl sulphide only has 1), and it is responsible for garlic’s
powerful flavour. Interestingly it has insecticidal properties
which might be why the plant produces it. The chemical structure
of allyl disulphide the chemical behind garlic’s wonderful
flavour is shown below:
H2C=CH–CH2–S–S–CH2–CH=CH2
Garlic is credited with many beneficial properties. It is said to be
antibiotic, lower blood cholesterol, reduce blood clotting, and
have anti-cancer properties. Allicin is pharmacologically active
and might explain some of these possibilities, although high
doses – higher than you would get from eating garlic, are usually
necessary for pharmacological effectiveness in animal experiments
(Fig. 6.7).
Vanilla
Vanilla is the cured unripe pod of several species of tropical
climbing orchids (Vanilla planifolia [from Central and S.Amer-
138
ica], or V. tahitensis [from Oceania]).When dried it is long and
black and imparts a delicious flavour to egg custard if the milk
is boiled with a pod before being added to the beaten egg and
sugar. Often for convenience cooks use vanilla essence. This is
an alcohol (ethanol) extract of vanilla.
The chemical responsible for vanilla’s exquisite flavour is
vanillin. It is very soluble in alcohol hence the use of vanilla
essence. Vanillin is not a natural component of the vanilla orchid,
it is produced by the curing process – therefore it has no
natural function in the plant.
Vanilla is expensive, and so often synthetic vanillin (either
made from clove extract – see above, or synthesised chemically)
is dissolved in alcohol as a cheap alternative. The main
flavour chemical is exactly the same as in natural vanillin, but
the other subtle flavours are missing.
Vanillin is of very low toxicity (LD50 [oral, rat]=
1,580 mg/kg; it would take about 100 g to kill a human).
Chilli
Chilli is a pepper (Capsicum annuum) of which there are many
varieties ranging from the mildly flavoured sweet pepper that
we use with onions, tomatoes and courgettes in ratatouille (a
Provençal [S.E.France] vegetable stew), to the pungent birds eye
chilli used in Indonesian and Thai cooking.
The burning sensation in your mouth after a curry, and
the unspeakable burning at the other end of the alimentary
canal the next morning are both due to the irritant chemicals in
chilli. The most important is capsaicin which has an incredibly
pungent taste and acrid vapour (you will know this if ever you
have breathed in over frying chillis when making Thai food),
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Fig. 6-7. The molecular structure of allicin from garlic
humans can taste 1 part in 100,000 (i.e. approx. 0.001% solution)
capsaicin.
Capsaicin is toxic (LD50 [oral, rat]=100 mg/kg; about 6 g
could kill a human), but you would have to eat a great many chillis
to achieve this dose. Interestingly, its irritant properties are used by
the police in immobilising sprays – just imagine how awful it
would be to be sprayed in the eye with this highly irritant chemical.
It is clear that herbs and spices have numerous chemical constituents,
most impart wonderful flavours and release mouth
watering aromas when they vaporise during cooking. Some are
incredibly irritant, others are toxic, but most are of relatively
low toxicity. Some even have medicinal properties. The dose
that you get eating food seasoned with herbs and spices is so
small that toxicity is unimportant. Some people claim that they
get benefit from the medicinal properties of herbs and herb extracts,
but this is questionable because often the dose is often
too low to result in a pharmacological effect.
When is a Food a Medicine?
[Reproduced in full, by permission of the Editor from Shaw, I.C.
(2001) International Journal of Pharmaceutical Medicine 17:69]
140
There is an upsurge of interest in functional foods (“nutraceuticalsâ€)
[foods that have medicinal properties], but is their use to
replace real medicines justified? Before we can address this issue
we must question why people appear more interested in
these potential remedies than they did a decade or two ago. An
idealist’s answer might be that they cure ills without the adverse
effects of conventional medicines and the need to visit the doctor.
This hypothesis is almost certainly wrong! My cynical viewpoint
is far more likely to explain their popularity, namely that
advertising is making people think that functional foods work
and that they are worth buying. All of this is just about to change
as legislation around the world places anything with a medicinal
claim firmly into the medicines camp. So if preparations of
Echinacea purpurea (Purple Cone Flower from the American
prairies) have the merest sniff of a claim to treat colds and influenza
they will be considered medicines and not foods. At the
moment in New Zealand there is a successful industry producing
these, and other plants and their extracts. They are used to enhance
the functionality of foods, or for export to other countries
as components of herbal remedies which might be classified as
either foods or medicines according to the regulations of the particular
country.
Echinacea is a good example because it contains numerous
phenolics that might well have pharmacological activity. Indeed,
the purified phenolics have been shown to have effects in isolated
cells and in in vitro systems. However, simple randomised
placebo-controlled clinical trials have shown equivocal efficacy
with respect to an enhanced phagocytosis endpoint; three of the
five studies showed no efficacy (Melchart D, Linde K, Worku F
et al (1995) Results of five randomized studies on the immunomodulatory
activity of preparations of Echinacea. J Alt
Comp Med 1:145–159). This might simply be a dose effect, but
it points to the need for dose relationship efficacy studies with
standardised test material.
Preparations of the herb are said to stimulate the immune system
(possibly by enhancing phagocytosis) and so prevent cold,
flu and minor infections. Yoghurt containing Echinacea is available
in supermarkets in New Zealand (and elsewhere), it is
claimed to “enhance the body’s ability to resist infection†and to
be “prized for its antibiotic propertiesâ€. But are these claims
true? The honest answer is that we don’t know because they have
not been tested in proper clinical trials, because Echinacea is regarded
as a food and foods do not need to undergo clinical trials
before they are marketed. But surely these are medicinal claims.
The New Zealand Medicines Act 1981 defines a medicine as
something having a “therapeutic purpose†and for use in “treat-
141
ing or preventing diseaseâ€. In my opinion, the Echinacea
yoghurt pot labels fall within the Medicines Act definition.
The world is waking up to this conundrum and Regulatory
Authorities are grappling with the problems of medicinal claims
and unconventional medicines. This will have a major impact on
the nutraceutical and functional food industries. I suspect that either
the foods (or are they medicines?) will disappear, or the medicinal
claims will be removed from their labels because of the
enormous cost of generating toxicological and efficacy data.
New Zealand has introduced an Advertisement Pre-Vetting
Service run by the Advertising Standards Authority and Medsafe,
through which all advertisements for “fringe†medicines
must pass before the media will publish them. This is an excellent
quality assurance system that helps to protect the consumer
of these products. Despite this, advertisement vetting cannot replace
toxicological and efficacy assessment. Surely the time is
right for the manufacturers of ALL medicines, even if they prefer
to call them foods, to have to support their claims with real
science. And if they are not prepared to test their claims they
should withdraw them.
142
Red Kidney Beans
Back in the early 1970s (when I was a student) there were several
unexpected cases of severe illness amongst university students
who created their version of the student all time favourite,
chilli con carne, in crock pots (slow cookers) in their university
lodgings.
These severe health effects were due to a group of potent
protein toxins found in red kidney beans (Phaseolus vulgaris) –
a crucial ingredient of “chilliâ€. Red kidney beans contain a
group of toxins called lectins. The two most important lectins in
red kidney beans are arcelin and phasin. Phasin is a phytohemagglutinin
– a plant chemical that causes blood to clot, and
while present in other types of beans (broad beans have 5–10%
of the amount) its concentration is particularly high in the red
kidney variety that we use for chilli con carne.
Phytohemagglutinins are large complex proteins that
work by sticking red blood cells (erythrocytes) together by
binding to one cell via its surface proteins and attaching another
cell to another part of the protein molecule. This process
leads to many cells being held together by the phytohaemagglutinin
molecule (i.e. haemagglutination or clotting). This rapidly
results in sever harm or death, because the clots block important
small blood vessels (e.g. the brain’s blood supply), and diminish
the function of crucial organs.
Phasin is highly toxic, it takes only 5 μg/kg body weight
to kill a human. This could be present in only one or two beans.
So beware!
Does Cooking Beans Make Them Safe?
As is the case for most proteins, phasin is de-activated by heat.
Therefore thorough cooking (i.e. at 100°C) significantly reduces
the level of toxic protein and so makes the beans safer.
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Levels of phasin in cooked and uncooked red kidney beans (data from Foodborne
Pathogenic Microorganisms and Natural Toxins Handbook, US Food & Drug
Admisistration at http://vm.cfsan.fda.gov/~mow/chap43.htlm)
Bean Phasin level (phytohaemagglutinating units)
Uncooked 20,000–70,000
Thoroughly cooked 200–400
If cooked beans are safer,why did people get ill after eating
crock pot-cooked chilli con carne? The answer is simple,
food cooked in crock pots doesn’t get to much above 60°C, and
this is not hot enough to destroy the toxin. The solution is equally
simple;make sure that red kidney beans are boiled for at least
10 minutes as part of their cooking process and all will be well.
The alternative is to use canned beans that have been heated to
a high temperature (121°) as part of the canning process.
Why Do Beans Contain Lectins?
It is often difficult to explain the purpose of toxins in plants.
However, as discussed previously, pesticidal effects are often associated
with natural plant toxins. Phasin and arcelin (particularly
the latter) have insecticidal properties. This might explain
why the plant expends so much energy to synthesise a complex
protein molecule.But, perhaps the plant is simply trying to stop
animals eating its seed,which after all is the plant’s future.
Mycotoxins
Mycotoxins (from the Greek Mukes for mushroom [fungus]) are
fungal toxins sometimes found in food that has been infected
with a fungus. They are horrifically toxic and many are thought
to cause cancer. The most important food mycotoxins are produced
by fungi of the Genera Fusarium,Aspergilus and Penicillium.
These fungi grow on carbohydrate-rich substrates like
grains and nuts. They produce a range of mycotoxins, all of
which are important food contaminants;most have specific legislation
covering their levels in food.The most important mycotoxins
and the fungi that produce themAflatoxins are shown in
the table.
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Fungus Mycotoxin
Fusarium graminearum Deoxynivalenol, zearalenone
F. culmorum Deoxynivalenol, zearalenone
Aspergillus flavus Aflatoxin
F. verticilloides Fumosin
Penicillium verrucosum Ochratoxin
Aflatoxins
The aflatoxins (Aflatoxin-B1, -B2, -B2a, -M1, -M2, -G1, -G2 and
-G2a) are perhaps the most notorious group of mycotoxin.They
are a particular problem in peanuts which can become infected
with A. flavus post-harvest. During storage and transport of the
peanuts from the tropics where they are grown to temperate
countries (e.g. USA) where they are very popular (e.g. as peanut
butter), the fungus continues to grow and contaminates the nuts
with highly toxic aflatoxins.
Very low doses of aflatoxin can have significant effects
on consumers – Aflatoxin-B1 LD50[oral, rat]=5 mg/kg body
weight, which means that 300 mg could kill a human. Peanuts
can contain 500 ug/kg if they are grown, stored and transported
under conditions ideal for A. flavus’s growth. If in the exceptionally
unlikely event that you ate only highly contaminated
nuts, you would need about 600 kg of nuts to kill you. This, of
course, is ridiculous, therefore the acute toxicity of aflatoxin is
of little concern to consumers. Of very much greater concern is
the long-term effect, namely cancer. Low doses over a long period
of time might cause cancer. This is why regulators test
peanuts and make sure that consumers are only exposed to low
levels of aflatoxins that are unlikely to result in cancer. Codex
Alimentarius (the international committee that sets standards
for food) has set an MRL (maximum residues level) of 15 μg/kg
for total aflatoxin in peanuts. This is a very low value that reflects
concerns about these cancer-causing contaminants. It is
illegal to sell peanuts with a level of total aflatoxins above the
MRL.
Natural Toxins from Animals
Plants are not the only living things that produce toxic chemicals
to ward off would be attackers. Animals also produce a
broad array of ingenious natural toxins.Most of these (e.g. the
snake venoms) are of no importance in the context of food (except
perhaps to the pickers of tropical fruit who might encounter
a poisonous snake). But there are just a few that could
be included in food and present a significant risk to the consumer.
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Tetrodotoxin and Sashimi
This intensely poisonous fish toxin has already been discussed
in Chapter 2. It is produced by the Fugu Fish (a puffer fish) used
for sashimi (raw fish).Tiny amounts can be fatal (see Chapter 2),
but still Japanese people spend significant amounts of money
on this prestigious delicacy.
Marine Toxins
These are not strictly speaking animal toxins, they are present
in marine food animals (e.g. fish), but are derived from other
creatures that have either been eaten by the animal,or have contaminated
the animal after its death.
Histamine and Scromboid Fish Poisoning
An enzyme (histidine decarboxylase) present in the bacteria
that colonise some warm water fish (e.g. Tuna – a scromboid
fish) makes histamine from the amino acid histidine (present
naturally in the fish). Histamine is intimately involved in allergic
reactions (hence the use of anti-histamine drugs for allergy
treatment), high doses of histamine cause symptoms similar to
allergy.Histamine is present naturally in fish at levels of the order
of 1 mg/kg, but in “toxic†fish concentrations can reach
100 mg/kg; the toxic threshold is 20–50 mg/kg.
Scromboid fish poisoning (or pseudo-allergic fish poisoning
as it is sometimes called) is characterised by very rapid
(as soon as 2 or 3 minutes after eating contaminated fish) onset
of symptoms, including burning or swelling of the mouth, rash,
diarrhoea, flushing, sweating, headache and vomiting. As you
might expect treatment of severe cases is with anti-histamines.
The disease is not as rare as you might expect. In the USA
there were 1,400 cases between 1973 and 1997, all were in states
that have ready access to fresh fish.
146
Ciguatera Poisoning
This is associated with tropical reef fish (e.g.Red Snapper) consumption,
and is caused by a toxin produced by a microscopic
plant (a dinoflagellate – Gambierdiscus toxicus) that the fish eat.
The toxin concentrates up the food chain, so large predatory
fish (e.g. Barracuda) are more likely to harbour toxic levels of
ciguatera toxin than their small prey.
Ciguatera toxin is an incredibly complex, large molecule
that is horrifically toxic. It is amongst the most toxic chemicals
known – LD50[i.p., mouse]=0.45 μg/kg body weight; 27 μg
could kill a person and 0.1 μg can cause illness. The toxin works
by interfering with the passage (transmission) of nerve impulses.
Ciguatera toxin’s effects occur very rapidly after consumption
of contaminated fish (as soon as a few minutes), and
include a plethora of symptoms such as vomiting, diarrhoea,
cramps, sweating,“pins and needlesâ€, and the strange sensation
of reversed temperature sensitivity in the mouth – hot feels
cold, and cold feels hot.
Ciguatera fish poisoning is common in parts of the world
where reef fish are eaten. For example, in the US Virgin Islands
and French West Indies it is estimated that 3% of the population
are affected each year.
Paralytic Shellfish Poisoning (PSP)
This is caused by a different dinoflagellate (e.g. Gonyaulax
catenella) to the species that causes ciguatera fish poisoning,
and involves a different group of toxins. The PSP dinoflagellates
are red and cause red tides when their numbers are so great (in
excess of 50,000,000/ml) that they colour the water red. The
Red Sea is so called because of its regular blooms of red algae.
The PSP dinoflagellates are filtered from sea water by bivalve
molluscs (e.g. mussels) as a component of their microscopic
diet. If you eat one of these shellfish you might suffer PSP.
The symptoms include, tingling of the face, numbness,
headache, weakness, partial paralysis, and rarely death. Symp-
147
toms occur as soon as 10 minutes (usually within 2 hours) after
eating contaminated shellfish.
PSP toxins are a complex array of closely related molecules.
There are likely to be many hundreds, all having similar
toxic effects. They are often named after the shellfish from
which they were originally isolated. For example, saxitoxin
came from the Alaska butter clam (Saxidomus giganteus). PSP
toxins are intensely poisonous; saxitoxin is the most toxic –
LD50 [mouse, oral]=263 μg/kg body weight. It would take only
15 mg of saxitoxin to kill a person. It works by interfering with
nerve impulses, hence the tingling and numbness that it causes.
The Last Word
This has been just a brief foray into the world of natural toxins
in food. I have covered a tiny fraction of the chemicals we eat
each day with our food. I hope that it has persuaded you never
to believe anyone who tries to convince you that natural is always
safe! But also remember that even though you are eating
these ingenious natural toxins every day, you are alive, and
rarely succumb to their evil aspirations.
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