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 Work
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 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 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.
Cross
Reactivity
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 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.
Diagnosis
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 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 Allergy
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.
Treatment
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 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
Issues
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.
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 Techniques
One 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
Treatments
Controversial 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.asp
EXTRACTS 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.html
ECZEMA:
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.htm
http://www.niddk.nih.gov/health/digest/pubs/celiac/index.htm
FOOD CONTENTS:
U.S. Department of Agriculture
Food and Nutrition Information Center
301/436-7725
http://www.nalusda.gov/fnic/index.html
RECIPES:
American Dietetic Association
216 W. Jackson Boulevard
Chicago, IL 60606-6995
1/800/877-1600
http://www.eatright.org
RESOURCES:
Food Allergy 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 15th 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
January 1999
Last updated September 22, 1999 (dlb) |
