Allergies, Food Intolerance and Other Adverse Reactions to Food
Overview of adverse reactions to food.
By Margaret Lahey and Shari Rosen
In an international survey, an average of 12% of respondents reported food allergy or intolerance with differences ranging from 4.6% in Spain to 19% in Australia (Woods, Abramson, Bailey, & Walters, 2001). This survey also documented that those with an adverse reaction to food were likely to report allergic reactions to non-food substances as well (e.g., pollen). The use of the term allergy varies among researchers and writers. In general, the term allergy or hypersensitivity is used when the adverse reactions to food involve the immune system; other types of adverse reactions that do not involve the immune system may result from metabolic, pharmacologic, or toxic reactions to substances in food (e.g., Atkins, 1986); such reactions will be referred to here as "other adverse reactions" or "food intolerance."
The term allergy or hypersensitivity is generally considered to be an abnormal response of the immune system to some substance. In an allergic reaction, the body's immune system produces increased amounts of immunoglobulin E antibody (IgE), a type of protein that circulates in the blood stream. The IgE produced attaches to the surface of mast cells that occur throughout the tissues in the body but are most common in the skin and in the respiratory and gastrointestinal tracts. These cells then release chemicals (e.g., histamine) causing allergic symptoms, which vary with the location of the tissue where they are released (e.g., stomach pain, diarrhea, swelling, itching). Histamine in its bonded form is pharmacologically inert and found in all body tissues. When released it can enhance permeability of capillaries, trigger smooth muscle contractions and increase mucosal secretion; the location of the allergic reaction is related to the individual and not to the particular food allergen (Pace, 1995). It is assumed that genetic factors are involved in allergies; allergies tend to run in families and concordance rates are higher for monozygotic than dizygotic twins although different family members may be allergic to different substances and have reactions at different locations (e.g., Strachan, Wong, & Spector, 2001). Food allergies are frequently associated with atopic dermatitis and allergic reactions to pollens (e.g., Niggemann, Reibel, Roehr et al., 2001; Pastorello, Pravettoni, Incorvaia & Bellanti, 1999; Van Bever, Doex, & Stevens, 1989).
Allergic reactions to foods typically appear shortly after ingesting the problem food and reactions can range from mild (e.g., sneezing, skin rash, digestive discomfort, hives) to life threatening anaphylactic shock (e.g., Ebo & Stevens, 2001). Skin rashes and intestinal symptoms are the most common symptoms of food allergies although respiratory symptoms (e.g., runny nose, wheezing) are sometimes found; anaphylactic shock is rare. Foods associated with an allergic reaction include animal products (e.g., eggs, cow's milk, fish, shellfish) and non-animal products (e.g., peanuts, soy products, wheat, nuts, oranges, tomatoes). Children are most likely to be allergic to cow's milk, eggs, wheat and soy (Ebo & Stevens, 2001). However, any food could cause an allergic reaction. Foods that cause allergic reactions are frequently those that are eaten most often. In fact, some suggest that food allergies may develop through excessive exposure to a particular food. Reactions to foods can also be delayed and occur hours to days after ingestion making identification of the offending allergen more difficult.
A diagnosis of a food allergy involves examining the history of symptoms experienced after the ingestion of certain foods often involving a daily food-diary listing foods eaten and symptoms. Direct testing includes a skin test where a diluted amount of the suspected food extract is placed on the skin and then the skin is lightly punctured. A positive reaction usually appears within 15 to 20 minutes as a raised bump surrounded by redness. Such a reaction indicates the presence of allergic antibodies to the suspected food. Its usefulness is considered good for egg, milk, peanut, and wheat, but not for soy (Eigenmann & Sampson, 1998). In some cases blood test are used to examine the amount of IgE; the results are not available for one or two weeks. Some doctors also use the RAST test where a sample of the patient's blood is mixed with an extract of the suspected food. The blood is then examined to see if antibodies to that food are found. Two other in vitro tests involving examination of the blood are the ELISA and the MAST, which are used to detect enzymes involved in the digestion of particular foods. Further tests help confirm the presence of an allergy. In one, the suspected food is eliminated from the diet for a couple of weeks and then reintroduced with a careful watch over symptoms. In the second, referred to as an oral challenge, small portions of the suspected food are ingested in gradually increasing portions over a certain period of time to see if an allergic reaction occurs; this is usually carried out under the supervision of a physician in case reactions are serious. In a double-blind food challenge, various foods are placed in capsules and the patient is asked to swallow each and is watched for symptoms; neither the doctor nor the patient know the contents of each capsule. If there is a reaction to only one of the capsules, the diagnosis can be confirmed. If multiple allergies exist, this is more difficult. Moreover, it cannot be administered if reactions are severe. A double-blind, placebo-controlled food challenge (DBPCFC) is the gold standard for determining allergies (e.g., Bock & Atkins, 1990).
Two to eight percent of children and one to two percent of adults in the United States have confirmed food allergies (e.g., Pastorello, et al., 1999; Sampson & Burks, 1996). Cross reactivity also exists with many food allergies. For example, those allergic to one fish may find they are allergic to a number of different types of fish. Also cross-reactivity can exist with pollens and food such as birch pollen and apples or ragweed and melons. Treatment of a food allergy generally involves eliminating the offending food or food additive. This means careful reading of ingredient labels on food products and extra caution when eating in restaurants to be sure small amounts of the offending substance are not present. Strict adherence to elimination of offending substances sometimes results in a tolerance for the substance. This is particularly so for children and for non-severe allergies to eggs, cows' milk and soy, but not for nuts or fish.
Many complaints about adverse reactions to food do not involve an immune response (Crowe, 2001); that is, blood tests will not indicate an increase in IgE. Such non-allergic adverse reactions to food can involve many mechanisms and because there is no clear test to diagnose the problem (e.g., as presence of immune response) it is a controversial subject (Brostoff, 1990; Pace, 1995). In contrast to a food allergy, those suffering from such adverse reactions to food can often tolerate small amounts of the food although larger amounts of the food worsen the symptoms. Two factors recognized as contributing to adverse reactions to food include enzyme deficiencies that interfere with the digestion of particular foods and histamines in certain foods that trigger release of histamines in some, but not all, people.
Reactions to food can be caused by a direct release of histamine rather than as a result of a release of histamine caused by antigen-antibody reactions as happens with allergies. Some food products are histamine releasers (e.g., fish, tomatoes, egg white, strawberries and chocolate); other foods are themselves high in histamines (e.g., fermented and preserved foods as sauerkraut, dried sausage, fermented cheeses). Usually, histamine is more of a problem if the intestinal mucosa is permeable to histamine. Contaminated food can also cause adverse reactions that are actually a type of food poisoning. Finally, some individuals may have adverse reactions to substances that are added to foods such as color, sulfites, and monosodium glutamate. As with allergies, a DBPCFC is the gold standard for determining if an individual has adverse reactions to a food or additive and elimination of the offending substance from the diet is the solution to the problem.
Many food intolerances are the result of digestive problems that are related to enzyme deficiency. Lactose intolerance is a well-known and a common type of food intolerance. For example, prevalence in France is estimated to be from 5 to 100% of infants following weaning (Olives & Breton, 1998). The intolerance results from a deficiency in the enzyme lactase, which is necessary for digesting the lactose sugars in dairy products and converting them into substances (e.g., glucose) that can be absorbed. In the absence of this enzyme, lactose is converted to organic acids that result in abdominal pain, flatulence and possibly diarrhea. Some other adverse reactions to foods may be related to psychological factors associated with particular foods, although when the offending substance is removed the psychological triggers may no longer produce the symptom (Brostoff, 1990).