People with food allergies have an allergic reaction when they come in contact with certain foods. This happens because their immune system overreacts to the proteins in that food. Twelve million people in the United States have food allergies.
The differential diagnosis of food allergy mainly is distinguishing it from other abnormal responses to food, that is, from food intolerance, which can occur in a variety of other illnesses. If a patient says to the doctor, "I think I have a food allergy," the doctor has to consider a number of diagnoses. The possibilities include not only food allergy, but also any other diseases that have symptoms brought on by food. The differential diagnosis includes reactions to certain chemicals in food for example, histamine or food additives, food poisoning, several other gastrointestinal diseases, and psychological symptoms.
Histamine toxicity: Some natural substances (for example, histamine) in foods can cause reactions resembling allergy. Histamine can reach high levels in cheese, some wines, and certain fish, particularly tuna and mackerel. In fish, the histamine is believed to stem from bacterial contamination, especially in fish that has not been refrigerated properly. Remember that mast cells release histamine in an allergic reaction. If a person eats a food that contains a high level of histamine, therefore, he may develop histamine toxicity, a response that strongly resembles an allergic reaction to food.
Food Additives: Another type of food intolerance is an adverse reaction to certain compounds that are added to food to enhance taste, provide color, or protect against the growth of microorganisms. Consumption of large amounts of these additives can produce symptoms that mimic the entire range of allergic symptoms. (Although some doctors attribute hyperactivity in children to food additives, the evidence is not compelling, and the cause of this behavioral disorder remains uncertain.) The compounds most frequently tied to adverse reactions that can be confused with food allergy are yellow dye number 5, monosodium glutamate (MSG), and sulfites. Yellow dye number 5 can cause hives, although rarely. MSG enhances flavor, but when consumed in large amounts, can cause flushing, sensations of warmth, lightheadedness, headache, facial pressure, pain in the chest, and feelings of detachment. These symptoms occur soon after eating large amounts of food containing added MSG, and are temporary.
Sulfites occur naturally in some foods and are added to others to enhance crispness or prevent the growth of mold. In high concentrations, sulfites can pose problems for people with severe asthma. The sulfites emit a gas called sulfur dioxide, which the asthmatic inhales while eating the food containing sulfites. This gas irritates the lungs and can induce in an asthmatic a severe constriction of the air passages to the lungs (bronchospasm), making breathing very difficult. Such reactions led the U.S. Food and Drug Administration (FDA) to ban the use of sulfites as spray-on preservatives for fresh fruits and vegetables. Sulfites, however, are still added to some foods, and they also form during the fermentation of wine.
Food poisoning: Eating food that is contaminated with microorganisms, such as bacteria, and their products, such as toxins, is the usual cause of food poisoning. which certainly qualifies as a form of food intolerance. Thus, the ingestion of contaminated eggs, salad, milk, or meat can produce symptoms that mimic food allergy.