Up to 8% of children less than 3 years and approximately 2% of the adult population
experience food-induced allergic disorder.
Hippocrates, often hailed as the "Father of Medicine", first described
adverse reaction to food over 2000 years ago, but it is only recently that the medical
community has accepted and started investigating these disorders.
What do we mean by the term "food allergy"?
Here are some common terms used relating to food allergy, and what they should mean:
Adverse reaction to foods: any abnormal response to a food or food
additive, whether caused by allergic or non-allergic mechanisms.
Food allergy, hypersensitivity or sensitivity: an immunologic reaction
that involves the bodys immune system overreacting to ordinarily harmless
substances. The word "allergy" is frequently overused and misused to include
any irritating or uncomfortable symptom after eating. Strictly speaking the term
should only be used for the immediate symptoms that develop after eating foods like,
milk, egg, peanut, wheat & seafoods. It involves IgE antibodies and is easily
confirmed with a positive skin prick test or RAST to that food. An example of true
food allergy is hives that develop after eating peanuts or fish or wheezing after
Food Intolerance: a physical response to a food or food additive.
This response is not necessarily immunologic, e.g., lactose (a milk-sugar) intolerance,
which occurs when an individual lacks the enzymes to break down the milk sugar for
Food Anaphylaxis: the most severe allergic reaction following food
that can be fatal. The commonest food causing fatality is peanuts.
Food poisoning, toxicity: an adverse reaction that does not involve
the immune system. It can be caused by food that has been contaminated with toxins
(poisons or bacteria), microorganisms or parasites. An example is scombroid fish
poisoning, which can mimic anaphylaxis, but is due to excessive histamine in spoilt
Pharmacologic food reaction: an adverse reaction in which a chemical
found in a food or food additive produces a drug-like (pharmacologic) effect e.g.,
caffeine causing "the jitters".
A psychologically based food
intolerance, where a conditioned response is elicited by the recognition, appearance,
smell or taste of a particular food. Panic Attacks is an example of food aversion
presenting as anaphylaxis.
Natural History of Food Allergy
The natural history of food allergy depends on three principal factors: age at onset,
type of food(s) involved, and severity of the initial reaction. As a rule, the later
the onset of food allergy, the less likely that clinical sensitivity will be lost.
Up to 85% of infants sensitive to cow's milk can tolerate reintroduction by 3 years
and up to 80% of infants with egg allergy are able to consume egg by 5 years of
age. Children diagnosed as having a food allergy after the age of 3 years are less
likely to outgrow the problem. Children are more likely to outgrow milk, eggs, soy,
and wheat before the age of 5, whereas reactions to peanut and fish are more often
severe and persistent. In a recent study 21.5% of peanut allergic children appeared
to have lost their sensitivity over time. Most of the patients who outgrew their
peanut allergy had milder reactions to begin with.
Those parts of the food causing allergic reactions are usually proteins and are
called allergens. Most of these allergens can still cause reactions even after they
are cooked or have undergone digestion in the intestines. Exceptions are fruits
& vegetables, which seem to be more allergenic when fresh. Numerous food proteins
have been studied to establish allergen content.
The most common food allergens – responsible for up to 90% of all allergic
reactions – are the proteins in cows milk, eggs, peanuts, wheat, soy,
fish, shellfish and tree nuts.
All foods come from either a plant or animal source, and foods are grouped into
families according to their origin. Peanut, peas, beans & Soya are some of the
members of the legume family. In some food groups, especially tree nuts and seafood,
an allergy to one member of a food family may result in the person being allergic
to all members of the same group. This is known as cross-reactivity.
However some people will be allergic to peanuts & tree nuts, which are from
different food families: these allergies are called coincidental allergies,
because they are not related.
Symptoms of IgE mediated "true" Food Allergy
Skin: The most common allergic skin reaction to a food is hives
(urticaria). Hives are red, very itchy, swollen
areas of the skin. They arise suddenly and can disappear quickly.
Foods most commonly incriminated in adults include fish, shellfish, nuts, and peanuts,
and those in children include eggs, milk, and peanuts, although reactions to various
seeds (eg, sesame, and poppy) and fruits (eg, kiwi) are becoming more common.
Atopic dermatitis is an occasionally transient,
but more likely chronic, itchy inflammation of the skin often occurring in individuals
with personal or family histories of allergic rhinitis or asthma. Eczema is usually
the first manifestation of Allergies & it usually starts about 4 – 6 months
of age, around the time of weaning onto milk formulae. Cows milk is the commonest
food allergen causing eczema, closely followed by eggs. In a recent study 35% to
40% of children with moderate- to-severe atopic eczema first seen by a university-based
dermatologist were found to be allergic to food.
It is important to know that food allergens from the mother's diet are passed into
breast milk and can cause eczema during breast-feeding.
Respiratory Reactions caused by foods
Both upper (allergic rhinoconjunctivitis) and lower
(bronchospasm and asthma) respiratory reactions have been
provoked in blinded food challenges, although respiratory symptoms in the absence
of skin or gastrointestinal symptoms appear to be rare. In a survey of 323 patients
with chronic rhinitis attending an allergy clinic, only 2 patients (0.6%) had nasal
symptoms reproduced during blinded food challenges. Despite the belief that milk
ingestion " is mucous producing" and frequently leads to nasal congestion,
only 0.08% to 0.2% of infants in 3 epidemiologic surveys were found to have nasal
symptoms after a milk challenge. In one study by Bock of 480 children referred for
evaluation of adverse food reactions, about 16% experienced respiratory symptoms
(sneezing, runny nose, nasal obstruction, wheezing, cough or eye signs) during DBPCFCs, but only 2% of symptoms were confined to the respiratory
tract. Approximately 25% of 112 patients with histories of adverse food reaction
starting after 10 years of age were found to develop respiratory symptoms after
oral challenge, with the majority being nasal symptoms caused by fruit or vegetable
sensitivities. In surveys of children with asthma attending chest clinics, food-induced
respiratory reactions were demonstrated in about 6% to 8% of children. Bock found
that 25% of 279 children referred for evaluation with histories of food-induced
wheezing/asthma actually experienced wheezing as one of their symptoms during DBPCFCs.
In Hugh Sampsons studies of children with atopic dermatitis, nasal symptoms
typically developed within 15 to 90 minutes of initiating the DBPCFC and lasted
about 0.5 to 2 hours. Itchy nose & itchy eyes are commonly followed by prolonged
bursts of sneezing and profuse runny nose. Similarly, a study with 88 children with
atopic dermatitis and asthma revealed acute bronchospasm (dyspnea, cough and wheezing)
in 15% of patients during DBPCFCs, with 8% demonstrating greater than 20% fall in
Asthmatic reactions caused by airborne food allergens have been reported in cases
where susceptible individuals are exposed to vapours or steam emitted from cooking
food (eg, fish, molluscs, crustacean, eggs, and garbanzo beans). Symptoms include
rhino-conjunctivitis, urticaria, laryngeal oedema, bronchospasm, and rarely hypotensive
Gastrointestinal food hypersensitivity (IgE) reactions
Symptoms caused by immediate gastrointestinal hypersensitivity typically develop
within minutes to 2 hours of eating the responsible food allergen and consist of
nausea, abdominal pain, colic, vomiting, and/or diarrhoea. In young infants immediate
vomiting is not always a consistent finding, and some infants are first seen with
intermittent vomiting and failure to thrive. In children with atopic dermatitis
and food allergy, the repeated ingestion of a food allergen induces partial desensitisation
of gastrointestinal mast cells, resulting in subclinical reactions. Generally these
children are seen with complaints of poor appetite, poor weight gain, and intermittent
The oral allergy syndrome (OAS) appears
to have become more prevalent in the past decade, but this may be due to increased
awareness. It is estimated that OAS affects up to 40% of adults with pollen allergy,
especially to birch pollens.
OAS is a form of contact allergy that is confined almost exclusively to the inside
of the mouth (oropharynx) and rarely affects other target organs. Local IgE-mediated
mast cell activation provokes the rapid onset of pruritus (itching); tingling and
swelling of the lips, tongue, palate, and throat; and occasionally a sensation of
pruritus in the ears, tightness in the throat, or both. Symptoms are generally short-lived
and are most commonly associated with eating various fresh fruits and vegetables.
Patients allergic to birch pollen may have symptoms after eating raw potatoes, carrots,
celery, apples, hazelnuts and kiwi. Cross-reactivity between birch pollen and various
fruits and vegetables is due to homology among various pathogenesis-related proteins,
which are important in the defence against plant diseases. For example Mal d 1,
the major apple allergen, is 63% homologous to the major birch pollen allergen,
Bet v 1. Oral allergy syndrome has been described among several fruits in the Prunoideae
subfamily (peach, apricot, cherry, and plum) and Brazil nuts. Patients with OAS
generally can eat these foods in the cooked form without difficulty.
Mixed IgE- and non-IgE mediated disorders of the GIT due to foods
Allergic eosinophilic esophagitis, gastritis, or gastroenteritis are characterized
by infiltration of the oesophagus, stomach, and/or intestinal wall with eosinophils
and increased eosinophils in the blood in 50% of patients. The eosinophilic infiltrate
leads to thickening and rigidity of the bowel walls.
Allergic eosinophilic oesophagitis is seen most frequently during infancy through
adolescence and presents with chronic reflux (gastrointestinal reflux), intermittent
vomiting, food refusal, abdominal pain, difficulty swallowing, irritability, sleep
disturbance, and failure to respond to conventional reflux medication. One study
of children less than 1 year of age with gastrointestinal reflux found 40% had a
cows milk-induced reflux.
Allergic eosinophilic gastroenteritis may occur at any age and may appear with symptoms
similar to oesophagitis, gastritis, or both. Weight loss or failure to thrive is
a hallmark of this disorder. Up to 50% of these allergic eosinophilic disorders
are atopic, and food-induced IgE-mediated reactions have been implicated in a minority
Dietary protein enterocolitis syndrome is a disorder most frequently seen in the
first several months of life in which infants are first seen with irritability,
protracted vomiting, and diarrhoea, not infrequently resulting in dehydration. Vomiting
generally occurs 1 to 3 hours after feeding and continued feeding may result in
bloody diarrhoea, anaemia & failure to thrive. Symptoms are most commonly provoked
by cows milk or soy formulae but occasionally result from food protein passed
in maternal breast milk. A similar enterocolitis syndrome is seen in older infants
and children, which is caused by egg, wheat, rice, oat, peanut, chicken, and fish
In adults shellfish (eg, shrimp, crab, and lobster) sensitivity may provoke a similar
syndrome with severe nausea, abdominal cramps, and protracted vomiting. Stools often
contain occult blood. Skin prick Test responses to the suspected foods are negative.
Coeliac disease is a T cell-induced inflammation of the small
intestines, due to gliadin present in wheat, oats, barley & rye. The severity
varies from a debilitating malabsorption syndrome to a "silent" subclinical
Life-threatening Food Allergy or Anaphylaxis
Food allergies are the single most common cause of generalized anaphylaxis seen
in hospital emergency departments. It is estimated that about 100 fatal cases of
food-induced anaphylaxis occur in the USA each year (compared to about 50 cases
from bee sting). In addition to the skin, respiratory, and gastrointestinal symptoms
mentioned earlier, patients might have cardiovascular symptoms, including hypotension
and vascular collapse, caused by massive mast cell mediator release. However, most
food-induced anaphylactic reactions are not associated with major increase in serum
In a series of 12 fatal or near-fatal anaphylactic reactions, all patients experienced
severe respiratory compromise, 10 of 12 experienced nausea and vomiting, and only
7 of 12 patients experienced skin symptoms. About one-third of patients had a biphasic
reaction and one quarter experienced prolonged symptoms (eg, up to 3 weeks)
Factors associated with severe reactions include, concomitant asthma, history of
previous severe reactions, denial of symptoms, and failure to give adrenaline early.
Food-associated exercise-induced anaphylaxis
This is an increasingly recognized form of anaphylaxis. It occurs only when the
patient exercises within 2 to 4 hours of eating a food, but in the absence of exercise
the patient can eat the food without any apparent reaction. Patients are usually
atopic and have a positive skin prick test response to the food that provokes their
symptoms. Occasionally they have a history of reacting to the food when they were
younger. This disorder appears to be more prevalent in the late teens to mid-thirties.
In my experience, males are more commonly seen with this disorder, but other studies
report females to be affected twice as commonly as males. The exact mechanism(s)
involved in this disorder are unknown. It is speculated that the food hypersensitivity
is subthreshold for the mast cells to degranulate & exercise is required as
a co-factor to achieve the correct threshold. Wheat is the most common food (in
my experience) associated with this disorder, however several others have been implicated,
including shellfish, fish, celery, fruit, and milk.
Preventing Food Sensitisation & Allergy
This is a very controversial area at the present time, as there are conflicting
reports on the effects of maternal diet during pregnancy & breast-feeding in
There is some evidence to suggest that breast-feeding in infancy, up to 6 months
may result in delayed or lower rate of allergies. The nursing mother should probably
also avoid foods that may cause her infant to become allergic. However no nursing
mother should make alterations in her own diet without first consulting her physician.
Unsupervised dietary eliminations could potentially result in nutritional deficiencies,
which could affect both the mother and the unborn child.
If breastfeeding is not possible, or if a supplement to breastfeeding is desired,
use hypoallergenic formulae. Soy-based formula or goats milk formula is not
recommended for allergic or potentially allergic infants.
An allergist diagnoses food Allergy by taking a detailed and painstaking medical
history. All events surrounding the allergic reactions and everything
ingested ("everything that is put inside the mouth") should be recorded,
along with symptoms experienced, in a daily diary. The allergist will note the frequency,
seasonality, severity and nature of the symptoms. It's important to note the time
elapsed between eating a food and the reaction. After a thorough physical examination
the allergist decides on the investigations that will confirm his/her suspicions.
Skin Prick Test
In the evaluation of IgE-mediated (true) food allergy, the skin prick test identifies
specific IgE antibodies to the suspected food.
The test is usually done on the forearm, after the patient has been off antihistamines
for at least 72 hours. It is a very good test for diagnosing allergy to milk, eggs,
wheat, peanuts, nuts, seafood, and soy. The test is not very good for fruits &
vegetables when commercial extracts are used.
Skin prick tests are most valuable when they are negative because the negative predictive
value of these tests is very high (over 95%). Unfortunately, the positive predictive
value is in the order of only 50%. Thus, a positive skin test in isolation cannot
be considered proof of clinically relevant hypersensitivity, whereas a negative
test virtually rules out IgE-mediated food allergy to the food in question.
In Vitro Testing (RAST)
Blood tests for specific IgE (radioallergosorbent test [RAST]) are generally less
sensitive than skin prick tests. As with skin tests, a negative result on RAST testing
is very reliable in ruling out an IgE-mediated reaction to a particular food, but
a positive result has a low positive predictive value.
The CAP-RAST is a newer and more sensitive assay than the RAST. It correlates very
well with clinical reactivity to certain foods (milk, egg, peanut, fish).
Because most patients with food allergy are sensitive to only a few foods and a
small number of foods are responsible for most reactions, it is usually inappropriate
to test for allergies to an extensive number of foods. In the context of a detailed
history, selective testing is more likely to reveal causal foods.
Unproven (Unorthodox) Tests for Food Allergy
Measurement of immunoglobulin G4 (IgG4) antibody, provocation-neutralization, cytotoxicity,
applied kinesiology, hair analysis and other unproven methods are not useful.
When patients have a history suggestive of food-related illness and tests for IgE
antibody to food are positive, the first course of action is to eliminate the food
from the diet. Further testing is usually not needed in patients with food-induced
anaphylaxis. However, if symptoms are chronic (atopic dermatitis, asthma) and/or
many foods are implicated, diagnostic oral food challenges may be necessary.
Oral Food Challenges
Double-blind, placebo-controlled food challenges (DBPCFC)
are considered the gold standard for diagnosing food allergy. The procedure is labour
intensive but can be modified for an office setting. Patients avoid the suspected
food(s) for at least 2 weeks, antihistamines are discontinued, and doses of asthma
medications are reduced as much as possible. After intravenous access is obtained,
graded doses of either a challenge food or a placebo food are administered. The
food is hidden either in another food or in opaque capsules.
Medical supervision and immediate access to emergency medications, including adrenaline,
antihistamines, inhaled beta agonists (eg, Ventolin nebuliser) and steroid, and
equipment for cardiopulmonary resuscitation are required because reactions can be
severe. Challenges are terminated when a reaction becomes apparent, and emergency
medications are given as needed. Patients are also observed for delayed reactions.
If allergy to only a few foods is suspected, single-blind
or open challenges may be used to screen for reactivity.
Negative challenges are always confirmed with open feeding of a larger, meal-sized
portion of the food. Oral challenges should not be performed in patients with a
clear history of reactivity or a severe reaction.