Perhaps of all allergic reactions, the public, clinicians, and medical personnel mis-understand food allergy most. As little as 1 percent of the U.S. population has true food allergies compared with 20 percent who are perceived to have them. True food allergy is a typical, and sometimes catastrophic, type I, IgE-mediated reaction, which preferably must be proven by in vitro or in vivo testing.
The gold standard of food allergy test-ing is the double-blinded, placebo-con-trolled food challenge, which will establish whether the patient is truly allergic to a specific food. Because this method of test-ing for food allergies is time consuming, expensive, and potentially dangerous, in vivo testing is usually performed in food allergy centers. Occasionally, straight food challenges may be tried when the chances of reactions are minimal and the serum IgE (radioallergosorbent test, or RAST) specific for the allergen is low or negative.
Approximately 20 percent of the U.S. population may perceive food as causative reaction in food allergies, whereas the true prevalence of food allergies is approxi-mately 1 percent. Food allergies are usually found in individuals with a strong per-sonal and family history of allergies. Atopic responses are associated with many foods. In children under the age 2, 90 percent of the incriminating foods are eggs, milk, legumes (such as peanuts, which are not nuts), and soy. In adults, fish, shellfish, fruits, and tree nuts might be added to the list.
Case 9.1 Peanut Allergy
N.G. is a two-year-old female who was well until the age of seventeen months when she developed a watery, runny nose that persisted throughout the spring but was gone by the sum-mer. She responded well to conventional antihistamines and did well during the summer. In the middle of August, she broke out in hives and wheezing soon after eating a peanut butter sandwich, but the symptoms were reversed with the antihistamine. She had never had peanut butter before that event, but her
mother was a self-described “peanut but-ter addict” and consumed large amounts during her pregnancy with and breast-feeding of N.G. N.G.’s subsequent skin prick test with peanut revealed a 20 mm by 30 mm wheal and flair response with a markedly elevated RAST to the peanut protein. She has remained peanut free since that time. She is able to eat tree nuts (e.g., almonds and cashews) without a problem because peanuts are not nuts.
True allergic reactions (IgE mediated) to food must be differentiated from food intol-erance, which is rarely multisystemic, not necessarily found among atopic patients, and with no positive skin prick tests or in vitro responses (RAST). A nonallergic reac-tion that typifies food intolerance would be an acute gastrointestinal response in a lactose-intolerant individual. This patient does not have the enzyme lactase and, therefore, cannot break down sugar lactose in milk and other foods into glucose and galactose. Profound diarrhea or vomiting usually results without any other system involved (respiratory or skin, for example). Food avoidance is the treatment of choice, as it is with food allergy, and over time, food intolerance may resolve. Lactaid is a brand of milk in which the enzyme lactase is added so that the milk may be consumed.