Food Allergy - General Facts
Most people think that allergies are
commonplace, with around one in five of the population
suffering from some type of food allergy. However, food
allergy is actually much rarer with about 1-2 in a hundred of
the adult population actually having adverse reactions to
foods.
Whilst uncommon, allergic reactions to
foods are real and can take a number of forms. The commonly
experienced symptoms are described below and when experienced
in association with the consumption of particular foods it is
likely that an individual is suffering from a food allergy.
Anyone who
suspects they have a food allergy should see a medical
doctor, and seek a referral to an allergy clinic.
These pages have information, together with links to other websites with
useful information, which relate to
Symptoms of Food
Allergies
Reactions to foods are usually rapid,
appearing within an hour (or sometimes even seconds) of
consumption, although in some cases they may be delayed and
appear up to four hours after eating.
- Skin rashes, such as nettle rash
(also called urticaria or hives) can appear which are
generally short lived, disappearing within a few
days. Longer lasting, chronic skin reactions (such as
scaly patches) can also be experienced. Some of these
longer lasting rashes are called atopic dermatitis.
- An itchy nose and eyes, sneezing
and a runny nose may be experienced, as can asthmatic
symptoms, such as wheezing, breathlessness and
coughing. These types of symptom are not seen so
often with food allergies.
- Itching and swelling around the
lips and mouth may occur on contact with a food.
Other symptoms include nausea, cramping pains,
bloating, vomiting and diarrhoea.
What is Food Allergy?
Our bodies are protected from infections by
our immune system. We produce molecules, called
antibodies, which recognise the germs causing an infection. There
are a number of different sorts of antibody, and the one which
causes an allergic reaction is called IgE.
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An IgE
induced Mast cell |
We think that
IgE molecules are normally produced in response to infections
caused by parasites, like those that cause malaria. We do not
understand why, but some people make IgE to other things like
pollen, giving rise to hay fever, and to some foods, giving rise
to food allergies.
The IgE acts like a tag, sticking to
molecules in food or pollen called allergens. When someone
who has an allergy eats a problem food the IgE attaches to the
allergens, setting off an allergic reaction. One of the common
effects that IgE triggers is the release of histamine, which
causes the changes we see in our bodies as symptoms, like nettle
rash or wheezing.
Allergens are usually proteins, and there
are generally several kinds of allergen in each food. We do not
know what makes some proteins, and not others, food allergens.
Food Allergy or Food
Intolerance?
There is another collection of symptoms
that people report suffer from when they eat certain foods, such
as headaches, muscle and joint aches and pains, and tiredness,
which are often collectively known as food intolerance. This
collection of symptoms is less well defined and poorly
understood, and hen
ce is generally much harder to diagnose than
classical allergy. The exceptions are the gluten intolerance
syndrome, known as Coeliac's disease, and lactose
intolerance. Coeliac's disease is triggered by eating wheat,
barley and rye-containing foods, whilst lactose (a sugar found in
milk) can cause intolerance to milk in certain individuals who
lack the ability to break it down. Lactose intolerance is not an
allergy, but causes some of the same symptoms as milk allergy,
like cramping pains in the stomach and diarrhoea.
Website: http://www.celiac.org
Anaphylaxis
An uncommon allergic reaction, which can be
life-threatening, is called anaphylaxis. It can be
triggered by consuming very small amounts of food (just a bite is
enough) and the symptoms usually appear within minutes and last
for several hours. Asthma, skin rashes, nausea, vomiting and
diarrhoea are among the symptoms which can be experienced. An
unusual from of this condition can be triggered by eating problem
foods within 2-3h of vigorous exercising and is called exercise-induced
anaphylaxis. Prompt administration of adrenaline after eating
suspected problem foods has helped minimise life-threatening
episodes.
Website: http://www.anaphylaxis.org.uk/book.html
How is a diagnosis of food allergy made?
If you have a food allergy and go to a medical practitioner how can they tell
you have a food allergy? These information sheets explain what is involved and
have been written by David Reading (Anaphylaxis Campaign) and Kate Grimshaw (a
dietician working with food allergy patients at Southampton Hospital).
For those
with a more scientific/medical background click here
for a more technical
description of food allergy diagnosis.
The diagnosis of food allergy starts with a combination of an
investigation into the patient's clinical history, a clinical examination and
a test for the detection of IgE antibodies. Skin prick testing and blood tests
are used for this purpose.
The presence of specific IgE does not necessarily lead to
symptoms. Therefore it is sometimes necessary to perform a provocation or
challenge test with the suspect food, which involves introducing the food to the
patient, first by touch, then by ingestion of gradually increasing amounts.
Challenge tests must be performed in a hospital or clinic, where any serious
reactions can be safely managed.
Which test to use to get an accurate diagnosis is made by
considering the case history, the suspected food, the severity of the symptoms
and the availability of diagnostic reagents.
In general, although progress has been made in the diagnosis
of food allergy, many limitations still exist and more sensitive diagnostic
tests are required. The inadequacies of current testing methods sometimes
encourages the use of unproven methods for the diagnosis of food allergy, an
approach that should be avoided.
How can food allergy be managed?
If a food allergy or intolerance is suspected then a visit to
the GP is the place to start as he can refer you to an allergy specialist who
can make a correct diagnosis (as detailed earlier). Once the diagnosis of food
allergy has been made, avoidance of the causative food is essential.
Occasionally complete elimination of the food for 1-2 years may result in a loss
of clinical symptoms, but allergies to fish, peanuts usually persists for life.
Complete avoidance of the offending food is often difficult
due to the presence of very small quantities in commercially manufactured foods.
Progress towards comprehensive labelling of food allergens has led to better
management of allergies, but cases of malnutrition resulting from the
mismanagement of diets due to fear and lack of knowledge have been reported.
There is therefore a need for proper dietetic planning which should be given by
a health professional with a specialised knowledge in this area (usually a
dietitian or nutritionist). Advice given would ensure that a diet is
nutritionally adequate (with the use of nutritional supplements if necessary)
with specific advice of what foods are likely to contain the offending foods.
Sometimes food allergens are labelled using names that the consumer is not
accustomed to. Education of the allergy patient is therefore vital, but it is
not where food allergy management ends, in fact it is where it starts. Armed
with knowledge the food allergic person can be constantly vigilant about what
they eat, and they need to be!
Where there is an indication that a food allergy may have
been outgrown, an effort to safely introduce the offending food in the allergic
individual's diet is done by a careful challenge procedure in a setting where
any serious reactions can be properly managed (i.e. in hospital). It needs to be
established whether the food is safe in all forms (e.g. raw and or cooked).
Appropriate advice on what can be introduced into the diet needs to be given by
an appropriately qualified individual.
Food allergy -even when severe -can be managed
perfectly well. What is required is sound medical guidance and a commitment by
the patient to remain vigilant and always carry prescribed medication.
Allergenic Foods
There are a number of groups of foods that
are responsible for causing the majority of food allergies. The
list below gives some information on these allergens. The list of
foods for which allergies have been reported is much longer. A
more comprehensive list can be obtained from Food Allergy Network
http://foodallergy.org
Cow's Milk:
Two out of a hundred infants
under one year old suffer from cow's milk
allergy, making it the most common food allergy of
childhood. In general children lose this sensitivity
as they grow up with nine out of ten losing it by the
age of three; it is unusual for adults to suffer from
this allergy.
Symptoms are frequently
vomiting and diarrhoea in children, with 30-50% also
having skin rashes of some type. A small
number of children have an anaphylactic reaction to
milk which tends to be lifelong.
The major allergens in milk are
the caseins and the whey protein b -lactoglobulin.
People are usually allergic to more than one kind of
milk protein.
The proteins from cow's
milk are very similar to those from goats and
sheep, and can cause the same sorts of reaction in
cow's milk-allergic subjects. Thus
goat's or sheep's milk cannot be used as a
cow's milk substitute in allergic individuals.
Eggs:
Allergy to eggs is usually
observed in young children rather than adults, and
like cow's milk allergy, fades with time.
Occasionally children suffer from a severe form of
allergy which is not outgrown.
The main allergens are the egg
white proteins ovomucoid, ovalbumin, and
ovotransferrin.
The eggs of other poultry, such
as ducks, are very similar to those of hens and can
cause reactions in egg-allergic individuals.
Fish and shellfish:
Allergies to shellfish are
unusual in children, mostly being experienced by
adults. Reactions to fish are found in children and
adults. The incidence of seafood allergy is higher in
those countries with a high consumption of fish and
shellfish.
Severe reactions are more
frequently found with these foods, including
anaphylaxis.
Cooking does not destroy the
allergens in fish and shellfish, and some individuals
maybe allergic to the cooked, but not raw, fish.
The major allergens in fish are
flesh proteins called parvalbumins which are very
similar in all kinds of fish. This is why people
allergic to cod tend to be allergic to fish such as
hake, carp, pike, and whiting as well.
Shellfish allergens are usually
found in the flesh and are part of the muscle protein
system, whilst in foods such as shrimps, allergens
have also been found in the shells.
Fruits:
In general allergic reactions
to fruits and vegetables are mild, and are often
limited to the mouth, and are called the oral-allergy
syndrome (OAS).
Around four out of ten people
having OAS are also allergic to tree and weed
pollens. Thus people who are allergic to birch pollen
are much more likely to be allergic to apples.
There allergens in fruits and
vegetables are not as complicated as other foods.
Many of them are very like the allergens in pollens,
which is why people with pollen allergies are also
allergic to certain fruits.
Many fruit allergens are
destroyed by cooking, and thus cooked fruits are
often safe for fruit allergic people to eat.
Allergies to latex gloves,
especially amongst health professionals, are
increasing. As many of the latex allergens are like
those found in certain tropical fruits, such as
bananas, these people can get an allergic reaction to
handling or eating these foods .
Legumes:
This group of foods includes
soya beans and peanuts. Peanuts are one of most
allergenic foods and frequently cause very severe
reactions, including anaphylaxis.
Allergy to peanuts is
established in childhood and usually maintained
throughout life.
Both these foods have multiple
allergens which are present in the raw and cooked
foods.
Peanut allergy can be so severe
that only very tiny amounts of peanut can cause a
reaction. Thus the traces of nuts found in processed
oils, or the carry over of materials on utensils used
for serving foods, can be enough in some individuals,
to cause a reaction.
The main allergens in peanuts
and soya are the proteins used by the seed as a food
store for it to grow into a seedling. One of the
allergens in soya bean is very similar to a major
allergen from dust mites, a common environmental
allergen. We aren't sure yet whether this means
there is a link between dust allergy and soya
allergy.
Tree nuts:
This group includes true tree
nuts, such as Brazil nuts, hazelnuts, walnut and
pecan.
Whilst not as intensively
studied as peanuts, indications are that tree nuts
can cause symptoms as severe which can occasionally
be fatal.
Children who become sensitised
to tree nuts tend to remain allergic throughout life.
Hazelnut and almond allergies
are more like those people get to fruit, and are
linked to pollen allergies.
Nut allergens can be both
destroyed by, or resistant, to cooking and we think
that roasting may actually create new allergens.
The allergens can be the seed
storage proteins, or other molecules which are also
found in pollen.
Cereals:
Suffered by children and adults
alike, wheat allergy appears to be particularly
associated with exercise-induced anaphylaxis.
The more of a cereal (wheat,
rye, barley, oats, maize or rice) we eat the more
likely we are to suffer an allergy. Thus rice allergy
is found more frequently in populations eating ethnic
diets.
Seed storage proteins (such as
wheat gluten) and other proteins present in grain to
protect it from attack by moulds and bacteria, have
been found to be major allergens.
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