Food Allergy and Intolerances, NIAID Fact Sheet: NIAID
Article title: Food Allergy and Intolerances, NIAID Fact Sheet: NIAID
Conditions: Food Allergy, food intolerance, allergic reaction, MSG allergy, lactase deficiency, sulfite allergy, exercise-induced food allergy (type of Food allergies), peanut allergy, milk allergy, soy allergy, migraine, osteoarthritis, rheumatoid arthritis, allergic tension fatigue syndrome, hyperactivity
Source: NIAID
Food Allergy and Intolerances
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 population.
This difference
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.
How Allergic
Reactions Work
Common Food
Allergies
Cross
Reactivity
Differential
Diagnoses
Diagnosis
Exercise-Induced
Food Allergy
Treatment
Infants and
Children
Controversial
Issues
Controversial
Diagnostic Techniques
Controversial
Treatments
Summary
Resources
How Allergic
Reactions WorkAn 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
gastrointestinal tract.
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
parent.
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 AllergiesIn 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.
Cross ReactivityIf
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 cross-reactivity.
Another interesting
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."
Differential Diagnoses
A differential 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, for example.
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.
Gluten
intolerance 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.
DiagnosisTo 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
elimination diet.
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, usually within an hour after eating the food?
- Was allergy treatment successful? (Antihistamines should
relieve hives, for example, if they stem from a food allergy.)
- Is the reaction always associated with a certain food?
- Did anyone else get sick? For example, if the person has eaten
fish contaminated with histamine, everyone who ate the fish should
be sick. In an allergic reaction, however, only the person
allergic to the fish becomes ill.
- How much did the patient eat before experiencing a reaction?
The severity of the patient’s reaction is sometimes related to the
amount of food the patient ate.
- How was the food prepared? Some people will have a violent
allergic reaction only 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 thoroughly, they can eat it with
no allergic reaction.
- Were other foods ingested at the same time of the allergic
reaction? Some foods may delay digestion and thus delay the onset
of the allergic reaction.
Sometimes a 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.
Consequently, 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 reaction.
Exercise-Induced Food
AllergyAt 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.
TreatmentFood 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 allergic.
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.
Special
precautions 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 food allergies.
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 used.
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.
Exclusive breast 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
allergy-free period.
Controversial IssuesThere 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.
Another 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.
Controversial
Diagnostic TechniquesOne controversial
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.
Another 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 foods.
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.
Controversial
TreatmentsControversial 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 allergic reaction.
Summary 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.
Resources
HOTLINE:National Jewish
Medical and Research Center in Denver.
Nurses available to
answer questions
1/800/222-LUNG
http://www.njc.org/ALLERGY
REFERRALS:American Academy of Allergy, Asthma and
Immunology
611 East Wells Street
Milwaukee, WI
53202
1/800/822-2762.
http://www.aaaai.org/scripts/find-a-doc/main.aspEXTRACTS
FOR ALLERGY TESTING:U.S. Food and Drug
Administration
Center for Biologics Evaluation and
Research
1/800/835-4709
http://www.fda.gov/cber/index.htmlECZEMA:National
Arthritis, Musculoskeletal and Skin Diseases Information
Clearinghouse
One AMS Circle
Bethesda, MD
20892-3675
301/495-4484
http://www.nih.gov/niams/American
Academy of Dermatology
930 N. Meacham Rd.
Schaumburg, IL
60173
1/888/462-DERM
http://www.aad.org/Eczema
Association
1221 S.W. Yamhill, Suite 303
Portland, OR
97205
503/228-4430
LACTOSE INTOLERANCE
and
CELIAC SPRUE:National Digestive Diseases
Information Clearinghouse
Box NDDIC
Bethesda, MD
20892
301/654-3810
http://www.niddk.nih.gov/health/digest/pubs/lactose/lactose.htmhttp://www.niddk.nih.gov/health/digest/pubs/celiac/index.htmFOOD
CONTENTS: U.S. Department of Agriculture
Food and
Nutrition Information Center
301/436-7725
http://www.nalusda.gov/fnic/index.htmlRECIPES:
American Dietetic Association
216 W. Jackson
Boulevard
Chicago, IL 60606-6995
1/800/877-1600
http://www.eatright.org/RESOURCES:Food
Allergy and Anaphylaxis Network
10400 Eaton Place, Suite
107
Fairfax, VA 22030
1/800/929-4040
http://www.foodallergy.org/American
College of Allergy, Asthma and Immunology
85 W. Algonquin Road,
Suite 550
Arlington Heights, IL 60005
1/800/842-7777
http://allergy.mcg.edu/Asthma
and Allergy Foundation of America
1125 15
th Street,
N.W., Suite 502
Washington, DC 20036
1/800/7-ASTHMA
http://www.aafa.org/
NIAID, a component of the National Institutes
of Health, supports research on AIDS, tuberculosis and other
infectious diseases as well as allergies and
immunology.
Prepared by:
Office of Communications and
Public Liaison
National Institute of Allergy and Infectious
Diseases
National Institutes of Health
Bethesda, MD 20892
Public Health Service
U.S. Department of Health and
Human Services
June 2001
» Next page: Foodborne Diseases, NIAID Fact Sheet: NIAID
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