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
An allergic reaction involves two features of the human immune response. 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 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. 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. When the food allergens enter and travel through the bloodstream, they can cause a drop in blood pressure. 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. The most common food allergens that cause problems in children are eggs, milk, and peanuts. 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.
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. 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. 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. 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. 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.
Sulfites can occur naturally in foods or are added to enhance crispness or prevent mold growth.
There are several other diseases that share symptoms with food allergies including ulcers and cancers of the gastrointestinal tract. Some people may have a food intolerance that has a psychological trigger. 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. 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.
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?
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. 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.
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. If the scratch test is positive, the patient has IgE on the skin's mast cells that is specific to the food being tested. 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. 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 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.
Consequently, double-blind food challenges are done infrequently.
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. 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. 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. 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. 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. There are several medications that a patient can take to relieve food allergy symptoms that are not part of an anaphylactic reaction. 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. 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. The doctor diagnoses food allergy partly by changing the child's diet. Rarely, food challenge is used. If soy formula causes an allergic reaction, the baby may be placed on an elemental formula. The doctor will sometimes prescribe corticosteroids to treat infants with severe food allergies. 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. 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. It is controversial, for example, whether migraine headaches can be caused by food allergies. The more difficult issue is whether food allergies actually cause migraines in such people. 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. There is no evidence that such problems are due to food allergies.
Some people believe hyperactivity in children is caused by food allergies.
Controversial Diagnostic Techniques
One controversial diagnostic technique is cytotoxicity testing, in which a food allergen is added to a patient's blood sample. 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. To date, no one has conclusively shown that this test correlates with allergies to foods. 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.
Summary
Food allergies are caused by immunologic reactions to foods.
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.
