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September 18, 2000
Prevention and Primary Care - Elizabeth Smoots, M.D.

Food Allergies: Rare but Dangerous

Some people think their intolerance to certain foods, such as dairy products or sugar, may indicate a more dangerous food allergy. In fact, only a small fraction of Americans have food allergies -- 8 in 100 children and 2 in 100 adults. If a person is allergic to a type of food, his or her immune system overreacts to certain proteins in the food by making excess amounts of an allergic antibody called immunoglobulin E, or IgE. This antibody is usually overabundant in people with allergies or with a family history of asthma, hay fever, or eczema. It causes the body to release histamine and other chemicals that cause allergy symptoms.

Signs of a food allergy

Some allergy symptoms are merely annoying; others are life threatening. Symptoms usually appear within 24 hours after eating the food and may last two to three days (or even longer). Typical food-allergy reactions are as follows:

  • Hives, eczema, itchy rash, or swelling of the face or extremities
  • Nausea, vomiting, abdominal cramps, diarrhea, blood in stool
  • Nasal congestion, itching, sneezing, or hay fever
  • Itching, tingling, and swelling of the lips, tongue, or mouth*
  • Shortness of breath, repeated coughing or wheezing, asthma*
  • Itching or tightness in the throat, hoarseness, hacking cough*
  • Weak pulse, low blood pressure, passing out*
  • Migraine headaches+
  • Problems such as hyperactivity, anxiety, and fatigue+
  • Anaphylaxis (severe reaction with severe swelling of the tongue and throat, difficulty breathing, loss of consciousness, and shock)*

*These symptoms can progress to a life-threatening emergency and require immediate medical attention.

+These symptoms have only occasionally been linked to food allergies.

Confirming the diagnosis

If you suspect you have a food allergy, keep a food diary for several weeks. Record everything you eat and your symptoms, and note the length of time after eating that your symptoms occur. Your health care provider can use this information, along with results of some of the lab tests listed below, to help determine if a food allergy is causing your symptoms.

Allergy skin testing. Drops of different food extracts are placed on your arm or back, and the skin underneath is pricked with a needle. Skin wheals (raised areas) measuring more than 3 millimeters across indicate an allergic response. Positive tests are accurate only 50 percent of the time, but negative tests are 95 percent accurate.This is a blood test that detects the IgE antibodies to specific foods. This test is generally less accurate but can be safer and more convenient than skin testing.

Radioallergosorbent (RAST) test. This is a blood test that detects the IgE antibodies to specific foods. This test is generally less accurate but can be safer and more convenient than skin testing.

Food challenge. Dried forms of suspected foods are consumed under the supervision of a health professional who is trained to treat any reactions that occur. A food challenge is the most accurate test for food allergies but should not be performed in patients who are at risk for severe allergic reactions.

Elizabeth Smoots, M.D., F.A.A.F.P., is a board-certified family physician in Seattle, Washington. A fellow of the American Academy of Family Physicians, Dr. Smoots writes about prevention and primary care medicine.

Related Link:

The Food Allergy Network