How to diagnose food allergies

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 soy (about 85 percent of children lose their sensitivity within the first three to five 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. Here are some tips on how to recognize and diagnose food allergies.

• 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 seven 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.

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.