Food allergies or food intolerances affect the lives of virtually everyone at some point. People often may have an unpleasant reaction to something they ate and wonder if they have a food allergy. Almost one out of three people either say that they themselves 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 is, in part, due to reactions that are termed "food intolerances" and not 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.

Allergy Symptoms

There is a wide range of food allergy symptoms. This variability of symptoms stems from the large number of tissues in the body which can be affected by an immune reaction to food.

Frequently the first part of the body to react to food is the gastrointestinal tract. The allergic reaction in this portion of the body can cause vomiting, abdominal pain, and diarrhea. As the immune response to food affects other areas of the body, a person may develop hives (urticaria), swelling, sneezing and a runny nose, asthma or difficulty breathing.

The most severe food allergy reaction is anaphylaxis--a systemic, life-threatening shock that can occur minutes after a person eats a food to which they are allergic. Anaphylactic reactions to food probably result in as many as 50 deaths a year in the United States. One of the characteristic features of this kind of reaction is trouble breathing caused by edema (swelling) of the throat or bronchi; it can also cause severe asthma, hives, a drop in blood pressure and loss of consciousness, and death, if not treated immediately.

Eczema due to food allergy is a different kind of reaction in which the target organ is the skin, which becomes crusty, red, scaly, and itchy. In children, eczema is frequently due to foods, but it can also be a preexisting condition made worse by certain foods. Probably fewer than one in twenty adults with eczema has an associated food allergy.

How Allergic Reactions Work

The mechanism behind 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 feature of the immune response 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, finally, they can induce hives or eczema. 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, which is one of the chief foods to cause severe anaphylactic reactions; tree nuts, such as walnuts; fish; and egg.

In children, the pattern is somewhat different. The most common food allergens that cause problems in children are egg, milk, and peanuts.

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 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, in particular 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.

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.

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.

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 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 server 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 a number of 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.

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.
  
• However, in an allergic reaction, only the person allergic to the fish becomes ill.
  
• How much did the patient eat before experiencing a reaction? The doctor will want to know how much you ate each time and try to relate it to the severity of the reaction.
  
• 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. The doctor may also 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, a diagnosis can almost be made. 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 suggest 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 diagnosis of food allergy is made only when a patient has a positive skin test to a specific allergen and the history of their reactions also suggests an allergy to the same food.

In some extremely allergic patients who have severe anaphylactic reactions, skin testing can't 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 one of two blood tests called RAST and 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 immediately available. 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 into vogue over the last few years as the "gold standard" of allergy testing. For this food challenge, 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 one strong 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. However, someone with a history of severe reactions cannot be tested this way. In addition, this testing is expensive because it takes a lot of time to perform. Multiple food allergies are also difficult to evaluate with this procedure.

Consequently, double-blind food challenges are not done often. 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 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

There is at least one situation where more than the simple ingestion of a food to which a person is sensitive is required to provoke a reaction, and that is in 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 a full-blown allergic reaction such as hives. 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 wouldn't 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, consequently, 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 (1/44,000 of a peanut kernel for example) can prompt an allergic reaction. Other less sensitive people may be able to tolerate small amounts of a food to which they're allergic.

Patients with severe food allergies must be prepared to treat an inadvertent exposure. Even people who are very knowledgeable about what they are sensitive to occasionally make a mistake. In order 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 also 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.

There are several medications that can be taken to relieve food allergy symptoms that aren't 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 unproven 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. Allergy shots have not yet been proven to relieve food allergies.

Infants and Children

Milk and soy allergies are particularly common in infants and 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 diagnosis is based in part on 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, parents should try feeding the baby with elemental formulas, which are processed proteins (basically sugars and amino acids). There are few if any allergens within these materials. Corticosteroids are also sometimes used 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 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 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 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 cerebral allergy is not currently recognized by allergists.

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 this behavioral disorder has only been suggested to be associated with food additives occasionally in children, 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 control. Also, children who are on anti-allergy medicines that can cause drowsiness may get sleepy in school or at home.

Controversial Diagnostic Techniques

Just as there are controversial food allergy syndromes and treatments there are also controversial ways of diagnosing food allergies. One of these is cytotoxicity testing, in which 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." This technique has been evaluated in a number of studies and has not been found 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, this procedure has not been shown to 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 well 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.

A medical evaluation after one suspects a food allergy is the key to proper management. Treatment is basically avoidance of the other food(s) after they are identified. People with food allergies should become knowledgeable about allergies and how they are treated and should work with their physicians.

The National Institutes of Health supports research on food allergies through grants that it provides to research institutions throughout the world. Understanding the cause of an immune system dysfunction in allergy will ultimately lead to better methods of diagnosing, treating and preventing allergic diseases.

Questions and Answers

Q. Would you discuss what common substances are in both peanuts and other kinds of nuts? The response I often get when I tell people that I am severely allergic to both peanuts and nuts is that I cannot be because peanuts are legume, unlike nuts.

A. First of all, it is possible to be allergic to two distinct foods. It is interesting that both peanuts and nuts are concentrated sources of protein, which is probably one reason why reactions to both these foods are so frequent. But you can have cross reactions between tree nuts and peanuts, or you could develop allergies to both.

Q. I would like to ask about diet during pregnancy. I have heard some talk about avoiding certain foods during your last trimester.

A. There is no evidence that avoidance of foods in the last trimester can prevent food allergies. In fact, some experimental evidence suggest this is harmful.

Q. Would gross swelling of the lips be indicative of a food allergy?

A. Well, it can be, but you can also have something called idiopathic angioedema, which can cause swelling of the lips. This disorder is not caused by food allergies. If you have such a problem, talk it over with your doctor. If there is any chance that it might be a food allergy, the doctor can place you on an elemental diet for 10 days. If the problem does not go away, you have ruled out food allergy.

Q. Is intolerance a disease entity?

A. Food intolerance is not a distinct entity. It is a term used to cover any adverse reaction to a food that doesn't have an immunologic basis.

Q. I have a 14-year old daughter who has developed chronic hives in the past nine months. She has swelling and hives on the bottom of her feet to the point where she cannot walk; her fingers swell and she cannot write. She has been skin tested for food allergies, and it was negative. We are going through the elimination diet process now and seeing some improvement, but not a whole lot. The only thing that controls it is prednisone. What would you suggest as the next step?

A. Sometimes there is a non-specific improvement as you manipulate the diet. But chronic hives and angioedema, especially of that duration, are almost never due to food allergies. Unfortunately, no cause is usually found. There is hope that they will resolve with time. I'd advise you to limit the use of steroids because steroids can do more damage than if you just use antihistamines.

Q. Is there any hope of better management of lactose intolerance than chewing tablets that help dissolve lactose during the meal?

A. Probably not, other than avoidance of foods that have lactose.

Q. What is the best way to diagnose lactose deficiency?

A. There are a couple of test that involve ingesting a specific amount of lactose and then measuring the body's response. Such blood tests are done by physicians.

Q. Can you tell us anything about gluten intolerance?

A. Gluten intolerance is associated with the disease called gluten-sensitive enteropathy or celiac disease. It is due to an abnormal immune response to gluten, which is a component of wheat and some other grains.

Q. Is it possible that many people who were given the diagnosis of irritable bowel syndrome in the past are turning out to actually have allergies?

A. There is no good evidence that irritable bowel syndrome is due to food allergies in most instances.

Q. This summer I had lunch at a fast food restaurant for the first time and I broke out in hives. The doctor seemed to think maybe it was the sulfites in the food. Now you said there had been regulations for salad bars and sulfites, but in the whole food industry are there regulations for the use of sulfites?

A. Yes, there are industry-wide regulations covering the use of sulfites. Now I do not know anything about the restaurant you ate at, but it is more likely that you ate something else you are allergic to because sulfites rarely cause hives.

Information provided by NIH.