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Wednesday, May 14, 2008
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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 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.
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
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