Although many people believe that foods are a major source of allergies, the fact is that the extent of the problem is still unknown. Some experts claim that true food allergies are relatively rare, while others contend that they are quite common. The statistics can be confusing. For example, some estimates place the incidence of food allergies in children somewhere between less than one-third of 1 percent to as high as 7.5 percent, while others put it at approximately 40 percent. And in adults, the numbers are no more definite; some experts estimate the incidence to be about 15 percent.

Nonallergic Adverse Reactions To Foods
Regardless of the precise figures, true allergies to foods that is, those in which the immune system is involved do indeed exist. The problem is that food allergies must be separated from a variety of other adverse reactions people can have to foods, those that are not immunologically related. These include food intolerances, druglike adverse reactions, and food poisoning. Because the signs and symptoms of any of these conditions can overlap with those of true allergic reactions to foods, diagnosing the cause of a suspected food-related problem can sometimes be difficult.

Food intolerances represent metabolic abnormalities related to the digestion of food rather than immunologic problems. Lactose intolerance is probably the best-known example of this type of reaction. Owing to a shortage of lactase, the intestinal enzyme that digests lactose found in milk and milk products, individuals with lactose intolerance suffer from abdominal cramping and diarrhea when they consume dairy products. African-Americans are especially prone to the condition, which is also prevalent in Mediterraneans and Asians. Symptoms result from the bacterial breakdown of the unmetabolized lactose in the bowel.

Druglike reactions to foods, or more correctly, pharmacologic reactions, occur whenever a particular substance in a food acts on the body-like a drug. Caffeine, which is found in tea and coffee, is an excellent example of a food chemical that works in the body as a drug to keep you awake and more alert; when consumed in excessive quantities, it can make you "edgy." Nevertheless, the effect is purely chemical, not allergic in nature. Similarly, prunes, soybeans, and onions can trigger all kinds of gastrointestinal disturbances in some people on a purely chemical basis, as can the druglike chemicals in avocados, bananas, pineapple, and tomatoes. There are even substances in strawberries that are capable of nonallergically provoking hives and occasionally even life-threatening respiratory and blood pressure problems in susceptible people.

A toxic reaction, or food poisoning, is a third type of nonallergic adverse reaction to foods. Linked to the presence of germs in the food itself, this reaction may result from either the direct effects of the disease-producing germs themselves or from the specific druglike chemicals secreted by the germs. In either case, unlike a true allergic (immunologic) reaction that affects only susceptible individuals, a toxic reaction will affect all those who consume the affected food. Salmonella poisoning, which is the result of eating improperly cooked poultry, is a common example of the first type of toxic reaction. Victims typically suffer from nausea, vomiting, fever, and diarrhea.

Histamine poisoning, which is the result of consuming improperly refrigerated mackerel, swordfish, sardines, anchovies, and herring, is an example of the second type of reaction. Histamine is secreted by the bacteria that contaminate these foods, and rather than the germs themselves, the histamine is responsible for the flushing, hives, headache, and gastrointestinal problems that result.

True Food Allergy
True food allergies involve the immune system and are of two types. By and large, they are of the immediate hypersensitivity variety that typically occurs within minutes of ingesting the culprit food and involves IgE antibodies, mast cells, basophils, and the release of histamine (as well as other mediators). The onset of these reactions may be so fast, in fact, that they may occur while the food is still in the mouth. Far less commonly, the food allergies are of the delayed hypersensitivity variety that involves T lymphocytes and takes many hours to develop.