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Sorry, I'm allergic to it.

Are you really? Surveys show that about one-third of us think we're allergic to some kind of food, yet true food allergy probably affects less than 2 percent of the population. In the first years of life, the percentage is doubtless higher--as much as 5 percent perhaps--but much of this allergy is eventually outgrown.

What we often call food allergy is really only intolerance to certain foods (i.e., those that don't agree with us for reasons other than an allergic response). True allergy involves the body's immune system. In allergic individuals, it reacts against certain food components, producing antibodies against them to rid the body of what it mistakenly perceives as harmful substances.

The most common allergens (food components that generate antibodies) are proteins, rather than carbohydrates or fats, and a single food may contain multiple allergens. Proteins in milk, eggs, fish, shellfish, soy, wheat, peanuts, and tree nuts (e.g., walnuts) are the usual offenders.

When an allergic individual eats an offending food, the histamine release can cause swelling or itching of the lips, mouth, or throat. As the food enters the stomach, histamine can produce nausea, vomiting, and cramping. In the lower bowel, it produces diarrhea. So widespread is histamine production that the lungs and skin may also be involved, producing sneezing, running nose, breathing difficulties, itching, hives, eczema, or redness of the skin.

Although all these symptoms usually do not occur, in rare cases many of them occur simultaneously, causing a threat to life. This so-called anaphylaxis or anaphylactic shock may be severe enough to produce swollen throat, breathing difficulties, rapid fall in blood pressure, unconsciousness, and even death. Treatment of anaphylaxis is the immediate intramuscular injection of epinephrine (adrenaline) and a powerful antihistamine such as Benadryl.

Fortunately, most allergic reactions to food are not nearly so dramatic, and although most food allergies are genetic, some people develop them despite no family history of allergy. If both parents have allergies, their offspring have twice the risk of being allergic than if only one parent is allergic.

Food intolerance

The more common cases of food intolerance involve reactions not associated with the immune system. The intolerance may be because of the lack of a digestive enzyme, such as lactase. Food poisoning is another type of intolerance, caused by a substance that directly affects the intestinal tract lining, producing nausea, vomiting, or diarrhea. Some food poisoning may mimic an allergic reaction, such as certain contaminated fish that produce histamine within themselves before they are eaten.

Food additives are often blamed for allergies, but few persons are sensitive to them; in such cases, the response is usually one of intolerance, not allergy. MSG is a common additive that uncommonly causes mild and short-lived problems. Aspartame has also been blamed for intolerance reactions, but after 10 years of monitoring by the FDA for possible adverse reactions, no convincing evidence has yet emerged.

Sulfites can cause anaphylaxis

Sulfites, on the other hand, can produce violent and even fatal reactions in a small number of persons. As a result, the FDA has banned their use on fresh fruits and vegetables usually eaten raw. Packaged and processed foods must show the presence of sulfites on the label.

If you suspect that you have a true food allergy, consult a physician. If your medical history, physical examination, and skin or laboratory tests suggest it, your doctor may give increasing doses of the suspected food until symptoms develop to desensitize you. Such challenges are not usually used, however, in persons with a history of severe food reactions.

Such individuals are advised to carry syringes of epinephrine whenever they eat out. (See MU, May 1993, for description of the Epi-Pen.) An oral histamine may be used to treat unexpected reactions--and some persons have successfully used an oral histamine before eating favorite foods to which they have had a mild reaction. Nonetheless, avoidance of an offending food is the preferred--and safest--way to prevent a reaction.
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Title Annotation:what is food allergy and what causes it
Publication:Medical Update
Date:Sep 1, 1993
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