Food Allergies and Food Intolerance

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 5% of children have clinically proven allergic reactions to foods. In teens and adults, food allergies occur in about 4% 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 Food Allergies Work


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 food allergy symptoms. 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, vomiting, or diarrhea. The chemicals released by skin mast cells, in contrast, can prompt hives.


The complex process of digestion affects the timing and the location of an allergic 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 bronchospasm (wheezing or constriction of the lungs). All of this takes place within a few minutes to an hour.


Which Food Allergies Are Most Common?

In adults, the most common foods allergies include:

  • tree nuts such as walnuts
  • shellfish such as shrimp, crayfish, lobster, and crab
  • milk
  • eggs


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 Food Allergies


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 For Food Allergies

A differential diagnosis means distinguishing a 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 a 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 allergy 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, also known as lactose intolerance. 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 and other dairy products. If a person does not have enough lactase, the body cannot digest the lactose in most dairy 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 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.


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.

Diagnosing Food Allergies

To diagnose a 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 doctor 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.




Also thought to be helpful in diagnosing food allergies is an oral food challenge. This is done under strict medical supervision









The Exercise-Induced Food Allergy



Treatment for Food Allergies

The main treatment for food allergies is 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 two syringes 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.

Even if they are unclear that it is an allergic reaction, they should give the injection as a precaution because it will not hurt them and could save their life.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.





Food Allergies in Infants and Children







Controversial Food Allergy 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 in cheese or red wine 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, fatigue, 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 theory 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 allergy drugs that can cause drowsiness may get sleepy in school or at home.

Controversial Food Allergy Diagnostic Techniques



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.



Controversial Food Allergy Treatments


Summary of Food Allergies

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 doctors.





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