Food allergy affects up to 6 to 8 percent of children under the age of three and 2 percent of adults. If you have an unpleasant reaction to something you have eaten, you might wonder if you have a food allergy. One out of three people either believe they have a food allergy or modify their or their family's diet. Thus, while food allergy is commonly suspected, health care providers diagnose it less frequently than most people believe.
Food allergy is an abnormal response to a food triggered by the body's immune system.
Allergic reactions to food can cause serious illness and, in some cases, death. Therefore, if you have a food allergy, it is extremely important for you to work with your health care provider to find out what food(s) causes your allergic reaction.
Sometimes, a reaction to food is not an allergy at all but another type of reaction called "food intolerance."
Food intolerance is more common than food allergy. The immune system does not cause the symptoms of a food intolerance, though these symptoms can look and feel like those of a food allergy.
In adults, the foods that most often cause allergic reactions include
- Shellfish such as shrimp, crayfish, lobster, and crab
- Tree nuts such as walnuts
The most common foods that cause problems in children are
Tree nuts and peanuts are the leading causes of deadly food allergy reactions called anaphylaxis.
Adults usually keep their allergies for life, but children sometimes outgrow them. Children are more likely to outgrow allergies to milk or soy, however, than allergies to peanuts or shrimp. The foods to which adults or children usually react are those foods they eat often. In Japan, for example, rice allergy is more frequent. In Scandinavia, codfish allergy is more common.
An immediate allergic reaction involves two actions of your immune system.
- Your immune system produces immunoglobulin E (IgE), a type of protein that works against a specific food. This protein is called a food-specific antibody, and it circulates through the blood.
- The food-specific IgE then attaches to mast cells, cells found in all body tissues. They are more often found in areas of your body that are typical sites of allergic reactions. Those sites include your nose, throat, lungs, skin, and gastrointestinal (GI) tract.
Generally, your immune system will form IgE against a food if you come from a family in which allergies are common—not necessarily food allergies but perhaps other allergic diseases such as hay fever or asthma. If you have two allergic parents, you are more likely to develop food allergy than someone with one allergic parent.
If your immune system is inclined to form IgE to certain foods, you must be exposed to the food before you can have an allergic reaction.
- As this food is digested, it triggers certain cells in your body to produce a food-specific IgE in large amounts. The food-specific IgE is then released and attaches to the surfaces of mast cells.
- The next time you eat that food, it interacts with food-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 you to have various symptoms of food allergy.
Food allergens are proteins within the food that enter your bloodstream after the food is digested. From there, they go to target organs, such as your skin or nose, and cause allergic reactions.
An allergic reaction to food can take place within a few minutes to an hour. The process of eating and digesting food affects the timing and the location of a reaction.
- If you are allergic to a particular food, you may first feel itching in your mouth as you start to eat the food.
- After the food is digested in your stomach, you may have GI symptoms such as vomiting, diarrhea, or pain.
- When the food allergens enter and travel through your bloodstream, they may cause your blood pressure to drop.
- As the allergens reach your skin, they can cause hives or eczema.
- When the allergens reach your lungs, they may cause asthma.
Food allergy is treated by avoiding the foods that trigger the reaction. Once you and your health care provider have identified the food(s) to which you are sensitive, you must remove them from your diet. To do this, you must read the detailed ingredient lists on each food you 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.
FDA requires ingredients in a packaged food to appear on its label. You can avoid most of the things to which you are sensitive if you read food labels carefully and avoid restaurant-prepared foods that might have ingredients to which you are allergic.
If you are highly allergic, even the tiniest amounts of a food allergen (for example, a small portion of a peanut kernel) can prompt an allergic reaction.
If you have severe food allergies, you must be prepared to treat unintentional exposure. Even people who know a lot about what they are sensitive to occasionally make a mistake. To protect yourself if you have had allergic reactions to a food, you should
- Wear a medical alert bracelet or necklace stating that you have a food allergy and are subject to severe reactions
- Carry a syringe of adrenaline (epinephrine), obtained by prescription from your health care provider, and be prepared to give it to yourself if you think you are getting a food allergic reaction
- Seek medical help immediately by either calling the rescue squad or by getting 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 GI discomfort.
Schools and day care centers must have plans in place to address any food allergy emergency. Parents and caregivers should take special care with children and learn how to
- Protect children from foods to which they are allergic
- Manage children if they eat a food to which they are allergic
- Give children epinephrine
There are several medicines that you can take to relieve food allergy symptoms that are not part of an anaphylactic reaction. These include
- Antihistamines to relieve GI symptoms, hives, or sneezing and a runny nose
- Bronchodilators to relieve asthma symptoms
You should take these medicines if you have accidentally eaten a food to which you are allergic. They do not prevent an allergic reaction when taken before eating the food. No medicine in any form will reliably prevent an allergic reaction to that food before eating it.
Food Allergy or Food Intolerance?
If you go to your health care provider and say, "I think I have a food allergy," your provider has to consider other possibilities that may cause symptoms and could be confused with food allergy, such as food intolerance. To find out the difference between food allergy and food intolerance, your provider will go through a list of possible causes for your symptoms. This is called a "differential diagnosis." This type of diagnosis helps confirm that you do indeed have a food allergy rather than a food intolerance or other illness.
Types of Food Intolerance
- Food poisoning
One possible cause of symptoms like those of food allergy is foods contaminated with microbes, such as bacteria, and bacterial products, such as toxins. Contaminated meat and dairy products sometimes cause symptoms, including GI discomfort, that resemble a food allergy when it is really a type of food poisoning.
- Histamine toxicity
There are substances, such as histamine present in certain foods, that cause a reaction like an allergic reaction. For example, histamine can reach high levels in cheese, some wines, and certain kinds of fish such as tuna and mackerel.
In fish, histamine is believed to come from contamination by bacteria, particularly in fish that are not refrigerated properly. If you eat one of these foods with a high level of histamine, you could have a reaction that strongly resembles an allergic reaction to food. This reaction is called "histamine toxicity."
- Lactose intolerance
Another cause of food intolerance confused with a food allergy is lactose intolerance or lactase deficiency. This common food intolerance affects at least one out of ten people.
- Lactase is an enzyme that is in the lining of the gut.
- Lactase breaks down lactose, a sugar found in milk and most milk products.
- There is not enough lactase in the gut to digest lactose.
- Lactose, instead, is used by bacteria to form gas which causes bloating, abdominal pain, and sometimes diarrhea.
There are tests your health care provider can use to find out whether your body can digest lactose.
- Food additives
Another type of food intolerance is a reaction to certain products that are added to food to enhance taste, provide color, or protect against the growth of microbes. Several compounds, such as MSG (monosodium glutamate) and sulfites, are tied to reactions that can be confused with food allergy.
MSG is a flavor enhancer, and, when taken in large amounts, can cause some of the following signs.
- Sensations of warmth
- Chest discomfort
- Feelings of detachment
These passing reactions occur rapidly after eating large amounts of food to which MSG has been added.
Sulfites occur naturally in foods or may be added to increase 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 that the asthmatic inhales while eating the sulfited food. This irritates the lungs and can send an asthmatic into severe bronchospasm, a tightening of the lungs.
The Food and Drug Administration (FDA) has banned sulfites as spray-on preservatives in fresh fruits and vegetables. Sulfites are still used in some foods, however, and occur naturally during the fermentation of wine.
- Gluten intolerance
Gluten intolerance is associated with the disease called "gluten-sensitive enteropathy" or "celiac disease." It happens if your immune system responds abnormally to gluten, which is a part of wheat and some other grains.
- Psychological causes
Some people may have a food intolerance that has a psychological trigger. If your food intolerance is caused by this type of trigger, a careful psychiatric evaluation may identify an unpleasant event in your life, often during childhood, tied to eating a particular food. Eating that food years later, even as an adult, is associated with a rush of unpleasant sensations.
- Other causes
There are several other conditions, including ulcers and cancers of the GI tract, that cause some of the same symptoms as food allergy. These problems include vomiting, diarrhea, and cramping abdominal pain made worse by eating.
After ruling out food intolerances and other health problems, your health care provider will use several steps to find out if you have an allergy to specific foods.
This technique is the most valuable. Your provider will ask you several questions and listen to your history of food reactions to decide if the facts go with a food allergy.
- What was the timing of your reaction?
- Did your reaction come on quickly, usually within an hour after eating the food?
- Did allergy medicines help? Antihistamines should relieve hives, for example.
- Is your reaction always associated with a certain food?
- Did anyone else who ate the same food get sick? For example, if you ate fish contaminated with histamine, everyone who ate the fish should be sick.
- How much did you eat before you had a reaction? The severity of a reaction is sometimes related to the amount of food eaten.
- How was the food prepared? Some people will have a violent allergic reaction only to raw or undercooked fish. Complete cooking of the fish may destroy the allergen, and they can then eat it with no allergic reaction.
- Did you eat other foods at the same time you had the reaction? Some foods may delay digestion and thus delay the start of the allergic reaction.
Sometimes your health care provider can't make a diagnosis solely on the basis of your history. In that case, you may be asked to keep a record of the contents of each meal you eat and whether you have a reaction. This gives more detail from which you and your provider can see if there is a consistent pattern in your reactions.
The next step some health care providers use is an elimination diet. Under your provider's direction
- You don't eat a food suspected of causing the allergy, such as eggs
- You then substitute another food—in the case of eggs, another source of protein
- Your provider can almost always make a diagnosis if the symptoms go away after you remove the food from your diet
The diagnosis is confirmed if you then eat the food and the symptoms come back. You should do this only when the reactions are not significant and under health care provider direction.
Your provider can't use this technique, however, if your reactions are severe or don't happen often. If you have a severe reaction, you should not eat the food again.
If your history, diet diary, or elimination diet suggests a specific food allergy is likely, your health care provider will then use tests to confirm the diagnosis.
One of these is a scratch skin test, during which an extract of the food is placed on the skin of your lower arm. Your provider will then scratch this portion of your skin with a needle and look for swelling or redness which would be a sign of a local allergic reaction. If the scratch test is positive, it means that there is IgE on the skin's mast cells that is specific to the food being tested. Skin tests are rapid, simple, and relatively safe.
You can have a positive skin test to a food allergen, however, without having an allergic reaction to that food. A health care provider diagnoses a food allergy only when someone has a positive skin test to a specific allergen and the history of reactions suggests an allergy to the same food.
If you are extremely allergic and have severe anaphylactic reactions, your health care provider cannot use skin testing because causing an allergic reaction could be dangerous. Skin testing also cannot be done if you have eczema over a large portion of your body.
In those cases, a health care provider may use blood tests such as the RAST (radioallergosorbent test) or the ELISA (enzyme-linked immunosorbent assay). These tests measure the presence of food-specific IgE in your blood. As with skin testing, positive tests do not necessarily mean you have a food allergy.
Double-Blind Food Challenge
The final method health care providers use to diagnose food allergy is double-blind food challenge. This testing has come to be the "gold standard" of allergy testing.
- Your health care provider will give you individual opaque capsules containing various foods, some of which are suspected of starting an allergic reaction.
- You swallow a capsule and are watched to see if a reaction occurs. This process is repeated until you have swallowed all the capsules.
In a true double-blind test, your health care provider is also "blinded" (the capsules having been made up by another medical person). In that case your provider does not know which capsule contains the allergen.
The advantage of such a challenge is that if you react only to suspected foods and not to other foods tested, it confirms the diagnosis. You cannot be tested this way if you have a history of severe allergic reactions.
In addition, this testing is difficult because it takes a lot of time to perform and many food allergies are difficult to evaluate with this procedure. Consequently, health care providers seldom do double-blind food challenges.
This type of testing is most commonly used if your health care provider thinks the reaction you describe is not due to a specific food and wishes to obtain evidence to support this. If your provider finds that your reaction is not due to a specific food, then additional efforts may be used to find the real cause of the reaction.