FOOD ALLERGY AND INTOLERANCES
A comprehensive look at food
allergies and intolerances, their symptoms, causes, diagnosis, and
Institute of Allergy and Infectious Diseases
Food allergies or food
intolerances affect nearly everyone at some point. People often have an
unpleasant reaction to something they ate and wonder if they have a
food allergy. One out of three people either say that they have a food
allergy or that they modify the family diet because a family member is
suspected of having a food allergy. But only about three percent of
children have clinically proven allergic reactions to foods. In adults,
the prevalence of food allergy drops to about one percent of the total
between the clinically proven prevalence of food allergy and the public
perception of the problem is in part due to reactions called "food
intolerances" rather than food allergies. A food allergy, or
hypersensitivity, is an abnormal response to a food that is triggered
by the immune system. The immune system is not responsible for the
symptoms of a food intolerance, even though these symptoms can resemble
those of a food allergy.
It is extremely
important for people who have true food allergies to identify them and
prevent allergic reactions to food because these reactions can cause
devastating illness and, in some cases, be fatal.
An allergic reaction
involves two features of the human immune response. One is the
production of immunoglobulin E (IgE), a type of protein called an
antibody that circulates through the blood. The other is the mast cell,
a specific cell that occurs in all body tissues but is especially
common in areas of the body that are typical sites of allergic
reactions, including the nose and throat, lungs, skin, and
The ability of a
given individual to form IgE against something as benign as food is an
inherited predisposition. Generally, such people come from families in
which allergies are common -- not necessarily food allergies but
perhaps hay fever, asthma, or hives. Someone with two allergic parents
is more likely to develop food allergies than someone with one allergic
Before an allergic
reaction can occur, a person who is predisposed to form IgE to foods
first has to be exposed to the food. As this food is digested, it
triggers certain cells to produce specific IgE in large amounts. The
IgE is then released and attaches to the surface of mast cells. The
next time the person eats that food, it interacts with specific IgE on
the surface of the mast cells and triggers the cells to release
chemicals such as histamine. Depending upon the tissue in which they
are released, these chemicals will cause a person to have various
symptoms of food allergy. If the mast cells release chemicals in the
ears, nose, and throat, a person may feel an itching in the mouth and
may have trouble breathing or swallowing. If the affected mast cells
are in the gastrointestinal tract, the person may have abdominal pain
or diarrhea. The chemicals released by skin mast cells, in contrast,
can prompt hives.
Food allergens (the
food fragments responsible for an allergic reaction) are proteins
within the food that usually are not broken down by the heat of cooking
or by stomach acids or enzymes that digest food. As a result, they
survive to cross the gastrointestinal lining, enter the bloodstream,
and go to target organs, causing allergic reactions throughout the body.
The complex process
of digestion affects the timing and the location of a reaction. If
people are allergic to a particular food, for example, they may first
experience itching in the mouth as they start to eat the food. After
the food is digested in the stomach, abdominal symptoms such as
vomiting, diarrhea, or pain may start. When the food allergens enter
and travel through the bloodstream, they can cause a drop in blood
pressure. As the allergens reach the skin, they can induce hives or
eczema, or when they reach the lungs, they may cause asthma. All of
this takes place within a few minutes to an hour.
Common Food Allergies
In adults, the most
common foods to cause allergic reactions include: shellfish such as
shrimp, crayfish, lobster, and crab; peanuts, a legume that is one of
the chief foods to cause severe anaphylaxis, a sudden drop in blood
pressure that can be fatal if not treated quickly; tree nuts such as
walnuts; fish; and eggs.
In children, the
pattern is somewhat different. The most common food allergens that
cause problems in children are eggs, milk, and peanuts. Adults usually
do not lose their allergies, but children can sometimes outgrow them.
Children are more likely to outgrow allergies to milk or soy than
allergies to peanuts, fish, or shrimp.
The foods that
adults or children react to are those foods they eat often. In Japan,
for example, rice allergy is more frequent. In Scandinavia, codfish
allergy is more common.
If someone has a
life-threatening reaction to a certain food, the doctor will counsel
the patient to avoid similar foods that might trigger this reaction.
For example, if someone has a history of allergy to shrimp, testing
will usually show that the person is not only allergic to shrimp but
also to crab, lobster, and crayfish as well. This is called
example of cross-reactivity occurs in people who are highly sensitive
to ragweed. During ragweed pollination season, these people sometimes
find that when they try to eat melons, particularly cantaloupe, they
have itching in their mouth and they simply cannot eat the melon.
Similarly, people who have severe birch pollen allergy also may react
to the peel of apples. This is called the "oral allergy syndrome."
diagnosis means distinguishing food allergy from food intolerance or
other illnesses. If a patient goes to the doctor's office and says, "I
think I have a food allergy," the doctor has to consider the list of
other possibilities that may lead to symptoms that could be confused
with food allergy.
One possibility is
the contamination of foods with microorganisms, such as bacteria, and
their products, such as toxins. Contaminated meat sometimes mimics a
food reaction when it is really a type of food poisoning.
There are also
natural substances, such as histamine, that can occur in foods and
stimulate a reaction similar to an allergic reaction. For example,
histamine can reach high levels in cheese, some wines, and in certain
kinds of fish, particularly tuna and mackerel. In fish, histamine is
believed to stem from bacterial contamination, particularly in fish
that hasn't been refrigerated properly. If someone eats one of these
foods with a high level of histamine, that person may have a reaction
that strongly resembles an allergic reaction to food. This reaction is
called histamine toxicity.
Another cause of
food intolerance that is often confused with a food allergy is lactase
deficiency. This most common food intolerance affects at least one out
of ten people. Lactase is an enzyme that is in the lining of the gut.
This enzyme degrades lactose, which is in milk. If a person does not
have enough lactase, the body cannot digest the lactose in most milk
products. Instead, the lactose is used by bacteria, gas is formed, and
the person experiences bloating, abdominal pain, and sometimes
diarrhea. There are a couple of diagnostic tests in which the patient
ingests a specific amount of lactose and then the doctor measures the
body's response by analyzing a blood sample.
Another type of food
intolerance is an adverse reaction to certain products that are added
to food to enhance taste, provide color, or protect against the growth
of microorganisms. Compounds that are most frequently tied to adverse
reactions that can be confused with food allergy are yellow dye number
5, monosodium glutamate, and sulfites. Yellow dye number 5 can cause
hives, although rarely. Monosodium glutamate (MSG) is a flavor
enhancer, and, when consumed in large amounts, can cause flushing,
sensations of warmth, headache, facial pressure, chest pain, or
feelings of detachment in some people. These transient reactions occur
rapidly after eating large amounts of food to which MSG has been added.
Sulfites can occur
naturally in foods or are added to enhance crispness or prevent mold
growth. Sulfites in high concentrations sometimes pose problems for
people with severe asthma. Sulfites can give off a gas called sulfur
dioxide, which the asthmatic inhales while eating the sulfited food.
This irritates the lungs and can send an asthmatic into severe
bronchospasm, a constriction of the lungs. Such reactions led the U.S.
Food and Drug Administration (FDA) to ban sulfites as spray-on
preservatives in fresh fruits and vegetables. But they are still used
in some foods and are made naturally during the fermentation of wine,
There are several
other diseases that share symptoms with food allergies including ulcers
and cancers of the gastrointestinal tract. These disorders can be
associated with vomiting, diarrhea, or cramping abdominal pain
exacerbated by eating.
is associated with the disease called gluten-sensitive enteropathy or
celiac disease. It is caused by an abnormal immune response to gluten,
which is a component of wheat and some other grains.
Some people may have
a food intolerance that has a psychological trigger. In selected cases,
a careful psychiatric evaluation may identify an unpleasant event in
that person's life, often during childhood, tied to eating a particular
food. The eating of that food years later, even as an adult, is
associated with a rush of unpleasant sensations that can resemble an
allergic reaction to food.
To diagnose food
allergy a doctor must first determine if the patient is having an
adverse reaction to specific foods. This assessment is made with the
help of a detailed patient history, the patient's diet diary, or an
The first of these
techniques is the most valuable. The physician sits down with the
person suspected of having a food allergy and takes a history to
determine if the facts are consistent with a food allergy. The doctor
asks such questions as:
What was the timing of the reaction? Did the reaction come on quickly,
within an hour after eating the food?
allergy treatment successful? (Antihistamines should relieve hives, for
example, if they stem from a food allergy.)
the reaction always associated with a certain food?
anyone else get sick? For example, if the person has eaten fish
contaminated with histamine, everyone who ate the fish should be
In an allergic reaction, however, only the person allergic to the
much did the patient eat before experiencing a reaction? The
of the patient's reaction is sometimes related to the amount of food
was the food prepared? Some people will have a violent allergic reaction
to raw or undercooked fish. Complete cooking of the fish destroys those
allergens in the fish to which they react. If the fish is cooked
eat it with no allergic reaction.
other foods ingested at the same time of the allergic reaction?
foods may delay digestion and thus delay the onset of the
diagnosis cannot be made solely on the basis of history. In that case,
the doctor may ask the patient to go back and keep a record of the
contents of each meal and whether he or she had a reaction. This gives
more detail from which the doctor and the patient can determine if
there is consistency in the reactions.
The next step some
doctors use is an elimination diet. Under the doctor's direction, the
patient does not eat a food suspected of causing the allergy, like
eggs, and substitutes another food, in this case, another source of
protein. If the patient removes the food and the symptoms go away, the
doctor can almost always make a diagnosis. If the patient then eats the
food (under the doctor's direction) and the symptoms come back, then
the diagnosis is confirmed. This technique cannot be used, however, if
the reactions are severe (in which case the patient should not resume
eating the food) or infrequent.
If the patient's
history, diet diary, or elimination diet suggests a specific food
allergy is likely, the doctor will then use tests that can more
objectively measure an allergic response to food. One of these is a
scratch skin test, during which a dilute extract of the food is placed
on the skin of the forearm or back. This portion of the skin is then
scratched with a needle and observed for swelling or redness that would
indicate a local allergic reaction. If the scratch test is positive,
the patient has IgE on the skin's mast cells that is specific to the
food being tested.
Skin tests are
rapid, simple, and relatively safe. But a patient can have a positive
skin test to a food allergen without experiencing allergic reactions to
that food. A doctor diagnoses a food allergy only when a patient has a
positive skin test to a specific allergen and the history of these
reactions suggests an allergy to the same food.
In some extremely
allergic patients who have severe anaphylactic reactions, skin testing
cannot be used because it could evoke a dangerous reaction. Skin
testing also cannot be done on patients with extensive eczema.
For these patients a
doctor may use blood tests such as the RAST and the ELISA. These tests
measure the presence of food-specific IgE in the blood of patients.
These tests may cost more than skin tests, and results are not
available immediately. As with skin testing, positive tests do not
necessarily make the diagnosis.
The final method
used to objectively diagnose food allergy is double-blind food
challenge. This testing has come to be the "gold standard" of allergy
testing. Various foods, some of which are suspected of inducing an
allergic reaction, are each placed in individual opaque capsules. The
patient is asked to swallow a capsule and is then watched to see if a
reaction occurs. This process is repeated until all the capsules have
been swallowed. In a true double-blind test, the doctor is also
"blinded" (the capsules having been made up by some other medical
person) so that neither the patient nor the doctor knows which capsule
contains the allergen.
The advantage of
such a challenge is that if the patient has a reaction only to
suspected foods and not to other foods tested, it confirms the
diagnosis. Someone with a history of severe reactions, however, cannot
be tested this way. In addition, this testing is expensive because it
takes a lot of time to perform and multiple food allergies are
difficult to evaluate with this procedure.
double-blind food challenges are done infrequently. This type of
testing is most commonly used when the doctor believes that the
reaction a person is describing is not due to a specific food and the
doctor wishes to obtain evidence to support this judgment so that
additional efforts may be directed at finding the real cause of the
At least one
situation may require more than the simple ingestion of a food allergen
to provoke a reaction: exercise-induced food allergy. People who
experience this reaction eat a specific food before exercising. As they
exercise and their body temperature goes up, they begin to itch, get
light-headed, and soon have allergic reactions such as hives or even
anaphylaxis. The cure for exercised-induced food allergy is simple --
not eating for a couple of hours before exercising.
Food allergy is
treated by dietary avoidance. Once a patient and the patient's doctor
have identified the food to which the patient is sensitive, the food
must be removed from the patient's diet. To do this, patients must read
lengthy, detailed ingredient lists on each food they are considering
eating. Many allergy-producing foods such as peanuts, eggs, and milk,
appear in foods one normally would not associate them with. Peanuts,
for example, are often used as a protein source and eggs are used in
some salad dressings. The FDA requires ingredients in a food to appear
on its label. People can avoid most of the things to which they are
sensitive if they read food labels carefully and avoid
restaurant-prepared foods that might have ingredients to which they are
In highly allergic
people even minuscule amounts of a food allergen (for example, 1/44,000
of a peanut kernel) can prompt an allergic reaction. Other less
sensitive people may be able to tolerate small amounts of a food to
which they are allergic.
Patients with severe
food allergies must be prepared to treat an inadvertent exposure. Even
people who know a lot about what they are sensitive to occasionally
make a mistake. To protect themselves, people who have had anaphylactic
reactions to a food should wear medical alert bracelets or necklaces
stating that they have a food allergy and that they are subject to
severe reactions. Such people should always carry a syringe of
adrenaline (epinephrine), obtained by prescription from their doctors,
and be prepared to self-administer it if they think they are getting a
food allergic reaction. They should then immediately seek medical help
by either calling the rescue squad or by having themselves transported
to an emergency room. Anaphylactic allergic reactions can be fatal even
when they start off with mild symptoms such as a tingling in the mouth
and throat or gastrointestinal discomfort.
are warranted with children. Parents and caregivers must know how to
protect children from foods to which the children are allergic and how
to manage the children if they consume a food to which they are
allergic, including the administration of epinephrine. Schools must
have plans in place to address any emergency.
There are several
medications that a patient can take to relieve food allergy symptoms
that are not part of an anaphylactic reaction. These include
antihistamines to relieve gastrointestinal symptoms, hives, or sneezing
and a runny nose. Bronchodilators can relieve asthma symptoms. These
medications are taken after people have inadvertently ingested a food
to which they are allergic but are not effective in preventing an
allergic reaction when taken prior to eating the food. No medication in
any form can be taken before eating a certain food that will reliably
prevent an allergic reaction to that food.
There are a few
non-approved treatments for food allergies. One involves injections
containing small quantities of the food extracts to which the patient
is allergic. These shots are given on a regular basis for a long period
of time with the aim of "desensitizing" the patient to the food
allergen. Researchers have not yet proven that allergy shots relieve
Infants and Children
Milk and soy
allergies are particularly common in infants and young children. These
allergies sometimes do not involve hives and asthma, but rather lead to
colic, and perhaps blood in the stool or poor growth. Infants and
children are thought to be particularly susceptible to this allergic
syndrome because of the immaturity of their immune and digestive
systems. Milk or soy allergies in infants can develop within days to
months of birth. Sometimes there is a family history of allergies or
feeding problems. The clinical picture is one of a very unhappy colicky
child who may not sleep well at night. The doctor diagnoses food
allergy partly by changing the child's diet. Rarely, food challenge is
If the baby is on
cow's milk, the doctor may suggest a change to soy formula or exclusive
breast milk, if possible. If soy formula causes an allergic reaction,
the baby may be placed on an elemental formula. These formulas are
processed proteins (basically sugars and amino acids). There are few if
any allergens within these materials. The doctor will sometimes
prescribe corticosteroids to treat infants with severe food allergies.
Fortunately, time usually heals this particular gastrointestinal
disease. It tends to resolve within the first few years of life.
feeding (excluding all other foods) of infants for the first 6 to 12
months of life is often suggested to avoid milk or soy allergies from
developing within that time frame. Such breast feeding often allows
parents to avoid infant-feeding problems, especially if the parents are
allergic (and the infant therefore is likely to be allergic). There are
some children who are so sensitive to a certain food, however, that if
the food is eaten by the mother, sufficient quantities enter the breast
milk to cause a food reaction in the child. Mothers sometimes must
themselves avoid eating those foods to which the baby is allergic.
There is no
conclusive evidence that breast feeding prevents the development of
allergies later in life. It does, however, delay the onset of food
allergies by delaying the infant's exposure to those foods that can
prompt allergies, and it may avoid altogether those feeding problems
seen in infants. By delaying the introduction of solid foods until the
infant is 6 months old or older, parents can also prolong the child's
There are several
disorders thought by some to be caused by food allergies, but the
evidence is currently insufficient or contrary to such claims. It is
controversial, for example, whether migraine headaches can be caused by
food allergies. There are studies showing that people who are prone to
migraines can have their headaches brought on by histamines and other
substances in foods. The more difficult issue is whether food allergies
actually cause migraines in such people. There is virtually no evidence
that most rheumatoid arthritis or osteoarthritis can be made worse by
foods, despite claims to the contrary. There is also no evidence that
food allergies can cause a disorder called the allergic tension fatigue
syndrome, in which people are tired, nervous, and may have problems
concentrating, or have headaches.
Cerebral allergy is
a term that has been applied to people who have trouble concentrating
and have headaches as well as other complaints. This is sometimes
attributed to mast cells degranulating in the brain but no other place
in the body. There is no evidence that such a scenario can happen, and
most doctors do not currently recognize cerebral allergy as a disorder.
controversial topic is environmental illness. In a seemingly pristine
environment, some people have many non-specific complaints such as
problems concentrating or depression. Sometimes this is attributed to
small amounts of allergens or toxins in the environment. There is no
evidence that such problems are due to food allergies.
Some people believe
hyperactivity in children is caused by food allergies. But researchers
have found that this behavioral disorder in children is only
occasionally associated with food additives, and then only when such
additives are consumed in large amounts. There is no evidence that a
true food allergy can affect a child's activity except for the proviso
that if a child itches and sneezes and wheezes a lot, the child may be
miserable and therefore more difficult to guide. Also, children who are
on anti-allergy medicines that can cause drowsiness may get sleepy in
school or at home.
diagnostic technique is cytotoxicity testing, in which a food allergen
is added to a patient's blood sample. A technician then examines the
sample under the microscope to see if white cells in the blood "die."
Scientists have evaluated this technique in several studies and have
not been found it to effectively diagnose food allergy.
controversial approach is called sublingual or, if it is injected under
the skin, subcutaneous provocative challenge. In this procedure, dilute
food allergen is administered under the tongue of the person who may
feel that his or her arthritis, for instance, is due to foods. The
technician then asks the patient if the food allergen has aggravated
the arthritis symptoms. In clinical studies, researchers have not shown
that this procedure can effectively diagnose food allergies.
An immune complex
assay is sometimes done on patients suspected of having food allergies
to see if there are complexes of certain antibodies bound to the food
allergen in the bloodstream. It is said that these immune complexes
correlate with food allergies. But the formation of such immune
complexes is a normal offshoot of food digestion, and everyone, if
tested with a sensitive enough measurement, has them. To date, no one
has conclusively shown that this test correlates with allergies to
Another test is the
IgG subclass assay, which looks specifically for certain kinds of IgG
antibody. Again, there is no evidence that this diagnoses food allergy.
treatments include putting a dilute solution of a particular food under
the tongue about a half hour before the patient eats that food. This is
an attempt to "neutralize" the subsequent exposure to the food that the
patient believes is harmful. As the results of a carefully conducted
clinical study show, this procedure is not effective in preventing an
Food allergies are
caused by immunologic reactions to foods. There actually are several
discrete diseases under this category, and a number of foods that can
cause these problems.
After one suspects a
food allergy, a medical evaluation is the key to proper management.
Treatment is basically avoiding the food(s) after it is identified.
People with food allergies should become knowledgeable about allergies
and how they are treated, and should work with their physicians.
National Jewish Medical
and Research Center in Denver
Nurses available to
American Academy of
Allergy, Asthma and Immunology
611 East Wells Street
Milwaukee, WI 53202
Extracts for Allergy
U.S. Food and Drug
Center for Biologics
Evaluation and Research
Musculoskeletal and Skin Diseases Information Clearinghouse
One AMS Circle
Bethesda, MD 20892-3675
American Academy of
930 N. Meacham Rd.
Schaumburg, IL 60173
1221 S.W. Yamhill,
Portland, OR 97205
and Celiac Sprue
Diseases Information Clearinghouse
Bethesda, MD 20892
U.S. Department of
Food and Nutrition
216 W. Jackson Boulevard
Chicago, IL 60606-6995
Food Allergy Network
10400 Eaton Place,
Fairfax, VA 22030
American College of
Allergy, Asthma and Immunology
85 W. Algonquin Road,
Arlington Heights, IL
Asthma and Allergy
Foundation of America
1125 15th Street, N.W.,
Washington, DC 20036
Institute of Allergy and Infectious Diseases of The National Institutes
of Health. Fact Sheet: Food Allergy and Intolerances. April 1993. Last
revised January 1999. (Online)