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TOPIC: Food Allergies Symptoms and Causes
Food Allergies Symptoms and Causes 09 Nov 2008 22:51 #554
Do you want to know the real difference between a food allergy and food sensitivity?
The following article from Today's Dietitian explains.
Food Allergy Symptoms and Causes
By Dale Ames Kline, MS, RD, CNSD
Vol. 7 No. 8 P. 10
True food allergies are not as common as popularly thought. There is much debate over the incidence level, the types of adverse reactions actually caused by sensitivities, diagnostic methods, and treatment.
Part of the problem is terminology. Both immunologically and nonimmunologically triggered reactions to food are called food sensitivities, but both are not true allergies.
A food allergy is an immunologically triggered reaction to a food component or additive, with clinical symptoms appearing immediately or over a period of hours or days. Allergy usually involves the antibody immunoglobulin E (IgE) binding to the allergen.
Food intolerances are another category of food sensitivities, which are a nonimmunological physiologic response to food that can be triggered by enzymatic disorders, food additives or contaminants, or pharmacological agents in food.
Separating fact from fiction becomes difficult when a patient believes the cause of his or her symptoms is food and shows symptoms when exposed to that food. We should regard such reactions skeptically, however, when a double-blind challenge does not link the reaction to the food and should consider that additives found in food, rather than the foods themselves, may be to blame.
However, foods are usually the cause of an adverse food reaction.1 Chart 1 shows the differences between immunologic and nonimmunologic reactions to food.
The Allergic Reaction
A true food allergy is an immunologic reaction, triggered by a digested food antigen (usually a protein or peptide) reaching the circulatory system. The �foreign� food antigen sets off an immune reaction in tissues or organs mediated by IgE, mast cells and inflammatory mediators released from the mast cells, and other granulocytes. This is the classic food allergy and is known as Type I hypersensitivity.
The mediators then interact with end organs, resulting in clinically observable symptoms. Mast cells and basophils are involved in the allergic reaction. Mast cells are found below the skin surface and below mucous surfaces in the eyes, nose, mouth, respiratory tract, and intestine, while basophils are found in the bloodstream. Sensitized IgE binds to the mast cells, which attach to the antigen.
Immune cells that can recognize foreign antigens are macrophages, dendritic cells, or B cells. The allergic reaction starts when immune cells bind to the antigen, then �present� the antigen to T cells, which is why these cells are also called antigen-presenting cells (APCs). The T cells then bind the antigen fragment to determine which specific cells and cytokines need to be produced, thereby determining the direction of the immune response.
In the case of a Type I hypersensitivity reaction, the T cells produce interleukin-2 (IL-2), which stimulates the production of B cells and T helper 2 (Th2) cells. The Th2 cells then produce IL-4, signaling the B cells to produce IgE antibodies. IgE antibodies have an affinity for mast cell receptors, binding to them; then they circulate in the body together.
In contrast, had the T cells signaled the body to produce Th1 cells instead of Th2 cells, the B cells would get a signal to produce immunoglobulin G (IgG) instead of IgE, so no allergic reaction would occur.
When the food antigen enters the body, it binds to the IgE, which is bound to mast cells. When a food antigen binds to two adjacent IgE antibodies, it is known as cross-linking, triggering the release of granules inside the mast cells into the surrounding fluids.
Contained in the granules are histamine and bradykinin, which dilate small blood vessels and constrict smooth muscles, particularly in the bronchial tubes. The granules also contain heparin (which prevents blood coagulation), enzymes that break down proteins, chemotatic factors that attract eosinophils and neutrophils, and substances that cause platelets to adhere to blood vessel walls and constrict arteries.
The stimulated mast cells produce prostaglandins and leukotrienes that cause capillaries to leak, smooth muscles to contract, granulocytes to move actively, and platelets to become sticky. The result of an antigen-IgE-mast cell reaction is acute inflammation with dilation of the blood vessels and local seepage of fluid from those vessels, later followed by an influx of granulocytes. The granulocytes that move from the inflammation site can release their own inflammatory mediators hours after the initial reaction.
If the antigen enters the eyes and nose, it causes swelling and redness of the linings of the eyelids and nasal passages, secretion of tears and mucus, and sneezing�all symptoms of hay fever. If the antigen penetrates the lungs, the linings of the bronchial tubes swell and secrete mucus while the muscles constrict, narrowing the tubes and making breathing difficult�symptoms of asthma. If an antigen gets into the bloodstream and interacts with basophils as it does with mast cells, the release of the chemical mediators causes systemic reactions such as hives and rashes. The reaction is shown to the right.
In a milk allergy, the milk protein initiates an immune response. In the intestines, the release of mediators from mast cells causes diarrhea, cramps, vomiting, bleeding, inflammation, or protein-losing enteropathies. The localized reaction increases permeability of the gastrointestinal (GI) tract, making it likely that the antigen will enter the body and end up in the bloodstream, causing systemic manifestations that can range in severity from hives to anaphylaxis. (Anaphylaxis is an immediate response to an antigen, causing a drop in blood pressure and difficulty breathing. If not treated, it can be fatal.
Why some people get allergies and others do not appears to be related to the patient�s genetic predisposition, exposure to food and other allergens, and the state of the GI barrier. Food allergies appear to be an excessive response of the immune system. Some people are more predisposed to producing IL-4, which increases IgE production and the sensitization of mast cells.
Some people may not develop allergies until there are changes in the structure of function of the GI tract. Any disruptions of the GI barrier�caused by an immature GI tract, inflammation, stress and destruction of villi and microvilli, viral gastroenteritis, or the presence of malnutrition�increase GI permeability, allowing antigens to enter the body.2 The antigens are able to enter the lymphatic vessels, gaining entry to the entire body, where they create an allergic reaction.
Types of Allergies
There are four types of immunological allergic reactions, shown at the left.
Type I, an immediate hypersensitivity reaction, is IgE-mediated. Known as atopy or anaphylactic hypersensitivity, this is most common and best understood. It is described above in the example of a milk allergy.
Type II reactions do not usually occur with food antigens. They are antibody-mediated and cytotoxic. The activation of complement causes much of the damage. This type of reaction occurs in blood transfusions where the recipient has antibodies to the transfused red cells.
Type III reactions involve immune complexes, with damage caused by the activation of complement in response to antigen-antibody complexes that have been deposited in the tissues or walls of the blood vessels. These immune complexes settle into the vascular wall and harm the tissue. The damage results from the complement and the influx and activation of neutrophils and macrophages which release bactericidal enzymes and IL-1. This type of reaction may play a larger role than once thought in the development of allergic reactions that may not directly involve IgE.
Type IV reactions, T cell-mediated delayed hypersensitivity reactions, involve T cells interacting directly with antigens. Host cells can be injured due to the release of chemicals from cytotoxic cells and the production of cytokines, resulting in inflammation and scarring.
Prevalence of Allergies
Accurate statistics on food allergy are difficult to obtain. As many as one-third of consumers in North America and Europe believe they have food allergies.3 However, the literature suggests that true food allergy (defined as an immediate, IgE-mediated Type I hypersensitivity reaction) is uncommon.
Based on double-blind, placebo-controlled food challenges, skin tests, and blood tests to detect antifood antibodies, the consensus seems to be that food allergy affects up to 8% of children under the age of 5 and 1% to 2% of the adult population.4,5 Food allergy is more common in children than adults, and the majority of food-allergic children experience symptoms of food-related allergy during the first year of life. Most food-allergic children outgrow their early food allergies, especially to cow�s milk and egg proteins, by roughly the age of 5, but some IgE-mediated food allergies may persist throughout life.5
It is estimated that 30,000 individuals require emergency department treatment for food allergies and 150 die from allergic reaction to foods each year.6 In 2004, it was estimated that 2.3% of the general population (approximately 6.6 million Americans) had either physician-diagnosed and/or convincing seafood allergy.7
The most severe allergic response�life-threatening anaphylactic reaction, mediated by a Type I hypersensitivity reaction�is rare. Roughly 100 fatal cases of food-related anaphylaxis occur in the United States each year.8
Statistical studies of the incidence of food allergy may not be representative of the incidence of adverse reactions to foods that are mediated by mechanisms other than Type I hypersensitivity. In adults, non�immune-mediated intolerance seems to be much more common than food allergy. Due to the lack of definitive objective tests, it is not possible to provide reliable statistics. Some practitioners estimate that up to 50% of the total population may experience some degree of food-related reactions.
Although any food protein can be potentially allergenic, relatively few are known to cause most allergic reactions. In addition, an allergenic protein can only induce an allergic reaction in an atopic person who has been sensitized to it.
Most severe allergic reactions to foods occur in response to a surprisingly small number of foods. The foods most commonly associated with allergic reactions in children are milk, egg, wheat, soy, peanut, tree nuts, fish, and shellfish.9 The incidence of peanut allergies is significantly on the rise, causing concern in schools. Allergies to milk, egg, wheat, and soy are usually outgrown in early childhood. Adults often experience allergic reactions to the foods that tend to persist as allergens beyond infancy; these are peanuts, tree nuts, eggs, shellfish, and certain species of fish.9
Lists of the most allergenic foods vary according to the source of the data. In general, the �top eight� are peanut products; soy products; egg products; milk products; tree nut products�the most allergenic of these are almond, Brazil nut, cashew, filbert (hazelnut), macadamia, pecan, pine nut, pistachio, and walnut; fish products (some species); shellfish�crustaceans (shrimp, prawn, lobster, crab, crayfish) and mollusks (clams, mussels, oysters, scallops); and wheat products.10,11
However, the severity of reactions associated with these foods varies. For example:
� Peanuts, tree nuts, shellfish, fish, milk, and egg account for most reported cases of anaphylactic reactions in children and adults.
� Soy is less frequently reported as a highly allergenic food, although it is often associated with severe cases of allergy and atopic dermatitis in childhood.
� Gluten allergy (quite distinct from gluten-sensitive enteropathy or celiac disease, which is not considered an allergic condition) is usually mild and is omitted from many �top allergen� lists.
Other allergenic foods, present on some lists and absent on others, include sesame seed and products containing sesame seed, mustard seed, cod, and corn.
It is known that more adults are becoming truly allergic to foods during their middle and later years�why is still unclear. In adults, the most serious allergies are caused by shellfish and nuts. Other foods that may cause allergies include tomato, mushroom, carrot, celery, corn, chicken, orange, pineapple, kiwi, garlic, yeast, food colors, and preservatives. This is by no means a complete list.
Diagnostic and statistical difficulties arise from the differing opinions as to what symptoms should be classified as allergic in origin.
Atopy is an inherited set of symptoms, involving IgE, that includes upper respiratory problems, rhinitis (dripping sinuses), and skin rashes. A child�s chances of inheriting atopy are 58% when both parents are atopic, 38% when one is atopic, and 12.5% if neither parent is atopic.
Symptoms from a food allergy may appear immediately following ingestion or up to five days after eating an allergic food, which further complicates diagnosis. Many symptoms involve the GI tract, particularly the intestine, as it comes in contact with the antigens. These symptoms�diarrhea, constipation, cramps, bleeding, ulcerations, inflammation�may or may not be an allergic reaction. GI disturbances occur in 70% of patients with allergies.
Skin rashes, including eczema and hives, are common food allergy symptoms and occur in 24% of all patients. Twelve percent of school-age children suffer from eczema due to food allergies. Respiratory symptoms, including asthma and rhinitis, appear in only 4% of patients with food allergies.
Researchers have found that food allergies can induce asthma. In a small number of people, food allergies cause bronchoconstriction and increased airway hyperresponsiveness, both symptoms of asthma.12 Treating the food allergies appropriately may help control asthma.
Diagnosing Food Allergies
It is a misconception that a positive reaction to a skin test is a positive diagnosis for a food allergy. A positive test may not be associated with symptoms.2 Included in the assessment for food allergies are health history, history of the symptoms and the suspected foods, physical examination that includes anthropometric assessment of growth and clinical symptoms of allergies, two-week food and symptom diary, immunological testing involving skin tests or radioallergosorbent tests (RASTs), trial elimination diet, and food challenges.
A thorough history includes suspected foods, symptoms, when the symptoms occur and how much food it takes to bring them on, and a health history, with emphasis on a family history of allergies. A physical assessment, including anthropometric assessment of nutritional status, growth and development in children, and allergic symptoms is next. A two-week food diary is kept, including foods eaten, the amount, time of appearance of symptoms, and any medications taken.
Once the list of suspected foods is drawn up from the diet diary, specific tests can be done. The most reliable tests are skin prick, RAST, and enzyme-linked immunosorbent assay (ELISA). The skin prick test is the least expensive and preferred method to use, since the results of the RAST and ELISA test correlate with the skin prick test. A RAST or ELISA test is done if there is risk for an anaphylactic reaction that could endanger the person�s life. A negative skin-test response rules out IgE-mediated allergic reactions.
The skin prick test is done by scratching the skin with the suspected allergen and looking for a red wheal, indicating an immune response. This test may overdiagnose food allergies and should be followed by a food challenge. In a RAST, small samples of blood are mixed with food extracts and washed with radioactively labeled IgE. If the person is allergic to the food, measurable levels of IgE antibodies can be detected. The ELISA test is similar to the RAST, except it does not use radioactively labeled material.
The foods that test positive are included in a food challenge. All foods known or suspected to cause an allergic reaction are eliminated from the diet. After two to four weeks, when all symptoms have disappeared, foods are reintroduced at a rate of one per week to determine the allergenic foods.
The best method to determine a food allergy is a double-blind challenge. The patient is given a capsule, not knowing whether it contains food or a placebo. Up to two-thirds of people who claim adverse reactions to food cannot reproduce the symptoms on a double-blind challenge. While this figure may seem high, it does point to the problem of psychologically induced food allergies. Belief in an allergy can cause symptoms.
Nonimmunologic Reactions to Food
Lactose intolerance is the most common nonimmunologic adverse reaction to food and is often confused with milk allergy. In the intestines, a deficiency of the lactase enzyme, necessary to digest lactose, may cause diarrhea, cramps, flatulence, and bloating.
Pharmacological agents found in food�vasoactive amines�may precipitate an adverse food reaction. Histamine�found in fermented cheeses, other fermented foods such as sauerkraut, anchovies, mackerel, sardines, and canned tuna�can cause headaches, decreased blood pressure, and asthma or allergylike symptoms. Phenylethylamine, found in aged cheeses, red wine, and chocolate, can cause migraine headaches. Histamine-releasing agents, found in shellfish, chocolate, strawberries, tomatoes, peanuts, pork, wine, and pineapple, cause the body to release histamine, causing urticaria, eczema, and other allergylike symptoms.
Additives are more controversial. Whether they cause reactions, and the scope of the problem, is debated. Some additives have been recognized as causing reactions and others have not. Considering the number of additives used and the endless combinations found in foods, you can see the difficulty of determining the effects of a single additive or synergism between additives.
Sulfiting agents used by the food and beverage industry include sodium sulfite, potassium sulfite, sodium metabisulfite, potassium metabisulfite, potassium bisulfite, and sulfur dioxide. They are used as sanitizing agents for food containers and equipment, and as preservatives to reduce or prevent microbial spoilage of food. Foods that may contain sulfites include wine, shrimp and other seafood, potatoes, dried fruits and vegetables, salads, cider, and vinegar.
In August 1986, the FDA banned the use of sulfiting agents in fresh foods. Potatoes did not fall into that category and remain a potential hazard, along with wine, dried fruits such as apricots, lemon juice, maraschino cherries, fruit drinks, and frozen avocados. Reactions to sulfiting agents include asthma, anaphylaxis, and loss of consciousness. Reports of deaths prompted the FDA to ban their use in fresh foods.
The severity of the reaction to a food determines the best treatment. Some people are so allergic that all traces of the food must be eliminated. They need education in label reading to recognize unusual sources of the offending food. For example, an allergy to corn means no corn meal, corn sweeteners, or corn syrup.
If a person is allergic to a food, he or she may be allergic to other foods in the same plant family. An allergy to peanuts may mean an allergy to other foods in the pea (legume) family, such as peas, black-eyed peas, beans (including green beans), and lentils.
If the allergy is moderate or mild, it may be possible to eat small amounts of the offending food without a reaction. In this case, a rotation diet is best. Only one food per day that causes a reaction may be eaten, every four or five days.
Eliminating offending foods from the diet will become easier in 2006, thanks to the Food Allergen Labeling and Consumer Protection Act (FALCPA) passed by Congress in 2004. FALCPA goes into effect in January 2006 and requires manufacturers to clearly label any food that contains one of the eight major food allergens: milk, eggs, fish, shellfish, tree nuts (walnuts, almonds, pecans, etc), wheat, peanuts, and soybeans. The label must contain the common name of the food and indicate any major allergen used in spices, flavorings, additives, and colorings. For instance, a food that contains casein must indicate that the product contains �milk.�
Adequate nutrition can be a problem if too many foods are eliminated. The diet should contain the appropriate amount of protein, carbohydrate, fat, fiber, vitamins, and minerals. If eggs, wheat, milk, and beef are eliminated from the diet, it can be a real challenge to meet daily requirements, especially for children.
The challenge in working with patients with food allergies is to find substitutes that contain equivalent nutrients to the eliminated foods and teach patients how to read labels, substitute foods, and cook with unaccustomed foods.
� Dale Ames Kline, MS, RD, CNSD, is president of Nutrition Dimension, Inc. A former hospital chief clinical dietitian and nutrition educator in the Women, Infants & Children program, she has written and edited continuing education home study courses since 1984.
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Adverse Reactions to Food
Allergy (Immunologic Reaction)
� Type I � Immunoglobin E (IgE)-mediated
� Type II and Type III � IgG-mediated (IgM occasionally)
� Type IV � T cell-mediated
Food Intolerances (Nonimmunologic reaction)
� Metabolic: enzyme deficiencies such as lactase
� Toxic reactions: foodborne illnesses
� Sensitivity to additives or pharmacologic agents in food: MSG, sulfites, histamine, tyramine, and tartrazine and other food colors
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