All About Food Allergies
Allergies develop in stages. When the immune system first encounters an allergen (or antigen)—a substance that it mistakenly sees as a harmful foreign invader—it signals specialized cells to make antibodies, or immunoglobulins, against it. There is no allergic reaction in that first exposure; however, if the substance again enters the body, the antibodies programmed to mount an attack against it will go into action. In some instances, the response will not produce symptoms; but the stage will have been set for a future antigen-antibody reaction and an allergic response.
There are many symptoms of food allergies, including nausea, vomiting, diarrhea, constipation, indigestion, headaches, skin rashes or hives, itching, shortness of breath (including asthma attacks), and, in severe cases, widespread swelling of the skin and mucous membranes. Swelling in the mouth or throat is potentially fatal because it can block the airways to the lungs. In the most severe cases, anaphylactic shock—a life-threatening collapse of the respiratory and circulatory system—may develop.
Allergens usually provoke the same symptoms each time, but many factors affect intensity, including how much of the offending food was eaten, and how it was prepared. Some people can tolerate small amounts of an offending food; others are so hypersensitive that they react to even a minute trace.
Some allergens are easily identified because symptoms will develop immediately after eating the offending food. The most allergenic foods in infancy are egg, milk, peanut, wheat, and soya (about 85 per cent of children lose their sensitivity within the first 3 to 5 years of life), whereas in older children and adults tree nuts, peanuts, and seafood are the most likely to cause severe reactions. Many people have mild allergies to various fruits and vegetables. Cooking can often reduce the allergenic potential of foods as proteins responsible for allergies are degraded by heat. This, however, is not always the case. Roasting of peanuts produces heightened allergenicity.
Allergens are not always readily identified. It may be necessary to keep a carefully documented diary of the time and content of all meals and the appearance and timing of subsequent symptoms. After a week or two, a pattern may emerge. If so, eliminate the suspected food from the diet for at least a week, and then try it again. If symptoms develop, chances are you have identified the offending food.
In more complicated cases, allergy tests may be required. One or more of the following may be used:
Skin test: The most common test, where food extracts are placed on the skin, which is then scratched or pricked, allowing the penetration of a small amount of the extract. Development of a hive or itchy swelling usually indicates an allergic response.
RAST (radioallergosorbent test) blood study: Small amounts of the patient’s blood are mixed with food extracts and then analyzed for signs of antibody action. This test may be safer for hypersensitive people, who may have a severe reaction to the skin test.
Medically supervised elimination diet and challenge tests: The patient is put on a hypoallergenic diet of foods that are unlikely to cause allergies for 7 to 10 days, at which time all allergic symptoms should completely disappear. (If they do not, a reaction to something other than food should be suspected.) The doctor then administers small amounts of food or food extracts to see if an allergic response occurs.
Living with Food Allergies
Once allergens have been identified, eliminating those foods from the diet should solve the problem. But this can be more complicated than it sounds. Some of the most common food allergens are hidden ingredients in many processed foods. Also, many foods are chemically related; thus, a person allergic to lemons may also be allergic to oranges and other citrus fruits. In some cases, the real culprit may be a contaminant or an accidental additive in food. For example, some people who are allergic to orange juice and other citrus juices may actually be able to tolerate the peeled fruits themselves, since it is limonene, the oil in citrus peels, that often produces the allergic reaction.
Allergies and Genetically Modified Foods
Genetic modification involves the introduction of novel proteins into foods and raises the question of increased allergenicity. Researchers, for example, have examined the possibility of introducing a gene from the arctic flounder into a tomato to prevent the tomato from freezing. This raises the question of someone with a fish allergy having a reaction to a tomato. No such tomatoes have been introduced and none will be until the question of transferred allergens is resolved. Traditional crossbreeding can also introduce novel proteins for which there are no allergy testing requirements. Also, in the case of genetically modified foods, the new proteins are not necessarily consumed. Canola oil, for example, contains none of the proteins that were introduced into the plant for improved agricultural performance.