- Risk Factors
What is a food allergy?
Either food allergy or food intolerance affects nearly everyone at some point. When people have an unpleasant reaction to something they ate, they often think that they have an allergy to the food. Actually, however, research shows that only about 3% of adults and 6%-8% of children have clinically proven true allergic reactions to food.
This difference between the prevalence of clinically proven food allergy and the public's perception of the problem is due primarily to misinterpreting food intolerance or other adverse reactions to food as a food allergy. A true food allergy is an abnormal response to food (known as a food allergen) that is triggered by a specific reaction in the immune system and expressed by certain, often characteristic, symptoms.
Other kinds of reactions to foods that are not food allergies include food intolerances (such as lactose or milk intolerance), food poisoning, and toxic reactions. Food intolerance also is an abnormal response to food, and its symptoms can resemble those of food allergy.
Food intolerance, however, is far more prevalent, occurs in a variety of diseases, and is triggered by several different mechanisms that are distinct from the immunological reaction responsible for food allergy. People who have food allergies must identify and prevent them because, although usually mild and not severe, these reactions can cause devastating illness and, in rare instances, can be fatal.
What is exercise-induced food allergy?
Exercise can induce an allergic reaction to food. The usual scenario is that of a person eating a specific food and then exercising. As he exercises and his body temperature increases, he begins to itch, gets lightheaded, and soon develops the characteristic allergic reactions of hives, asthma, abdominal symptoms, and even anaphylaxis.
This condition has been referred to as food-dependent exercise-induced anaphylaxis (FDEIA) and is most common in teens and young adults. The cure, actually a preventive measure, for exercise-induced food allergy, is simple -- not eating for at least two hours before exercising.
What is cross-reactivity?
Cross-reactivity is the occurrence of allergic reactions to foods or other substances that are chemically or otherwise related to foods known to cause allergy in an individual. If someone has a life-threatening reaction to a certain food, the doctor will counsel that patient to avoid related foods due to possible cross-reactivity, which also might induce the same reaction. For example, if a person has a history of a severe allergy to shrimp, he or she can also possibly be allergic to crab, lobster, and crayfish.
What is oral allergy syndrome?
The oral allergy syndrome is another type of cross-reactivity. This syndrome occurs in highly sensitive people, for example, ragweed or birch pollen. During the seasons that these allergens pollinate, the affected individual may find that when he or she tries to eat fruits, chiefly melons, and apples, a rapid onset of itching is experienced in the mouth and throat, and the fruit cannot be eaten.
The oral allergy syndrome is also known as pollen-food allergy syndrome and is thought to be a type of contact allergy related to the presence of proteins in certain foods that cross-react with allergy-causing pollen proteins.
Oral allergy syndrome occurs in up to 50% of those who have allergic rhinitis caused by pollen.
Symptoms are immediate upon ingestion of fresh or uncooked foods and include:
- Mild swelling of the lips, tongue, palate, and throat
Cooked fruits and vegetables usually do not cause the reaction. The symptoms usually go away within minutes, although up to 10% of people will develop systemic (body-wide) symptoms, and a small number (1%-2%) may experience anaphylactic shock.
Tree nuts and peanuts tend to cause more severe reactions than other foods.
IMAGESSee pictures of allergic skin disorders such as eczema, contact dermatitis and more caused by allergies See Images
What are the most common food allergies?
- Cow's milk
- Peanuts or groundnuts
- Tree nuts
In adults, the most common foods that cause allergic reactions are shellfish, such as shrimp, crayfish, lobster, and crab; nuts from trees, such as walnuts; fish; eggs; and peanuts, a legume that is one of the chief foods that cause serious anaphylactic reactions.
In highly allergic people, even minuscule amounts of a food allergen (for example, 1/44,000 of a peanut kernel) can evoke an allergic reaction. Less sensitive people, however, may be able to tolerate small amounts of food to which they are allergic.
In children, the pattern is somewhat different from adults, and the most common foods that cause allergic reactions include:
- Tree nuts
- Fruits, particularly tomatoes and strawberries
Children sometimes outgrow their allergies, but adults usually do not lose theirs. Also, children are more likely to outgrow allergies to cow's milk or soy formula than allergies to peanuts, fish, or shrimp.
Adults and children tend to react to the foods they eat more often. For example, in Japan, allergy to rice, and in Scandinavia, allergy to codfish is more common than elsewhere.
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What causes allergic reactions to food?
Both heredity and environmental factors may play a role in the development of food allergy. The allergens in food are those ingredients that are responsible for inciting an allergic reaction. They are proteins that usually resist the heat of cooking, the acid in the stomach, and the intestinal digestive enzymes. As a result, the allergens survive to cross the gastrointestinal lining and enter the bloodstream causing (protein-induced) allergic reactions throughout the body. The mechanism of food allergy involves the immune system and heredity.
- Immune system: An allergic reaction to food involves two components of the immune system.
- One component is a type of protein, an allergy antibody called immunoglobulin E (IgE), which circulates through the blood.
- The other is the mast cell, a specialized cell that stores up histamine and is found in all tissues of the body.
- The mast cell is particularly found in areas of the body that are typically involved in allergic reactions, including the nose and throat, lungs, skin, and gastrointestinal tract.
- Heredity: The tendency of an individual to produce IgE against something seemingly as innocuous as food appears to be inherited.
- Generally, people with allergies come from families in which allergies are common -- not necessarily to food but perhaps allergies to pollen, fur, feathers, or drugs.
- Thus, a person with two allergic parents is more likely to develop food allergies than someone with one allergic parent.
- Mechanism: Food allergy is a hypersensitivity reaction, meaning that before an allergic reaction to an allergen in food can occur, a person needs to have been exposed previously, or "sensitized," to the food.
- At the initial exposure, the allergen stimulates lymphocytes (specialized white blood cells) to produce the IgE antibody that is specific to the allergen.
- This IgE then is released and attaches to the surface of the mast cells in different tissues of the body.
- The next time the person eats that particular food, its allergen hones in on the specific IgE antibody on the surface of the mast cells and prompts the cells to release chemicals such as histamine.
- Depending upon the tissue in which they are released, these chemicals cause the various symptoms of food allergy.
What are food allergy risk factors?
Both adults and children may develop food allergies.
Factors that increase one's risk of having a food allergy include the following:
- Young age: Food allergies are most common in infants and toddlers.
- Having a past food allergy as a child or an allergy to another food: Those who are allergic to one type of food are more likely to develop other food allergies. Adults who have outgrown food allergies they had as children are more likely to develop subsequent food allergies than people who have never had them.
- Family or personal history of allergy, eczema, asthma, or hay fever: This increases the chances of developing a food allergy.
What are the symptoms of a food allergy?
The complex process of digestion affects the timing, location, and particular symptoms of an allergic reaction to food. All of the symptoms of food allergy occur within a few minutes to an hour of eating.
A food allergy can initially be experienced as itching in the mouth and difficulty swallowing and breathing. Then, during digestion of the food in the stomach and intestines, symptoms such as nausea, vomiting, diarrhea, and abdominal pain can start. Incidentally, the gastrointestinal symptoms of food allergy are those that are most often confused with the symptoms of different types of food intolerance.
As mentioned previously, the allergens are absorbed and enter the bloodstream. When they reach the skin, allergens can induce hives or eczema, and when they reach the airways, they can cause asthma. As allergens travel through the blood vessels, they can cause lightheadedness, weakness, and anaphylaxis, which is a sudden drop in blood pressure.
Anaphylactic reactions are severe even when they start with mild symptoms, such as a tingling in the mouth and throat or discomfort in the abdomen. They can be fatal if not treated quickly.
What conditions have mistakenly been attributed to food allergy?
Research studies have shown that individuals who are prone to migraines can have their headaches brought on by histamine, which is one of the compounds that mast cells produce in an allergic reaction. The theory that food allergies can cause migraine headaches, however, is unproven.
There is also inadequate scientific research evidence to support the claims that food allergies can cause or aggravate rheumatoid arthritis, osteoarthritis, tension-fatigue syndrome, cerebral allergy (headaches and difficulty concentrating), environmental-toxic reactions, or hyperactivity in children.
Do infants and children have problems with food allergy?
Most allergies to foods begin in the first or second year of life. While some of these reactions may resolve over time (such as allergies to cow's milk or eggs), other food allergies acquired in infancy (such as allergies to nuts or shellfish) typically persist throughout life. Allergies to milk or soy formula (a milk substitute made from soybeans) sometimes occur in infants and young children. These early allergies sometimes do not involve the usual hives or asthma but rather can cause symptoms in babies resembling infantile colic, and perhaps blood in the stool, or poor growth. The clinical picture of infantile colic, which usually starts within one month of birth, is that of a crying child who sleeps poorly at night. The cause of colic is uncertain.
A variety of psychosocial and dietary factors have been implicated, however, and allergy to milk or soy has been proposed as a cause of colic in a minority of infants with colic.
In infants, food allergy is usually diagnosed by observing the effect of changing the infant's diet; rarely, by using a food challenge.
- If the baby is on cow's milk, the doctor will suggest a change to soy formula or breast-feeding only, if possible.
- If the soy causes an allergic reaction, the baby can be placed on an elemental formula. These formulas are processed proteins and carbohydrates, basically amino acids and sugars, and contain few, if any, allergens.
Other factors that influence an infants or child allergy to food includes the following:
- Food protein-induced enterocolitis syndrome (FPIES): A bowel condition that affects children and is sometimes referred to as a delayed food allergy.
- It often occurs in infants who are beginning to eat solid foods.
- Unlike a typical food allergy, the symptoms come on hours after a food is consumed.
- It is a serious illness accompanied by vomiting and diarrhea.
- In severe cases, dehydration and shock can result due to the fluid lost from diarrhea and vomiting.
- Milk, soy, and grains are the foods that most often trigger FPIES.
- Research has suggested that FPIES results from a different mechanism than the production of IgE antibodies seen with typical allergic reactions.
- Breastfeeding: Exclusive breastfeeding, that is, excluding all other foods, for at least the first four months of life, appears to help protect high-risk children against milk allergy and eczema in the first two years of life.
- Breast milk contains less protein that is foreign to the infant and, therefore, is less allergenic than cow's milk or soy formula.
- Exclusive breastfeeding should be a consideration, therefore, especially in infants who are predisposed to food allergies.
- Some children are so sensitive to a certain food, however, that if the mother eats that food, sufficient quantities enter the breast milk to cause a reaction to the food in the child.
- In this situation, the mothers themselves must avoid eating those foods to which the baby is allergic.
- No conclusive evidence has been obtained that suggests that breastfeeding prevents the development of allergies later in life.
- Special considerations in children: An allergic child who itches, sneezes, and wheezes a lot can feel miserable and, therefore, sometimes misbehave or appear hyperactive.
- At the other extreme, children who are on allergy medicines that can cause drowsiness may become sleepy in school or at home.
- Parents and caregivers must understand these different behaviors, protect the children from the foods that induce their allergies, and know how to manage an allergic reaction, including how to administer epinephrine.
- Also, schools need to have plans in place to address emergencies, including anaphylactic shock.
What types of healthcare specialists diagnose and treat food allergies?
Allergists (including pediatric allergists) are medical doctors who have had special education and training in the diagnosis and treatment of allergies, including food allergies.
How do healthcare professionals diagnose food allergies?
To diagnose food allergy, a health care professional first must determine if the patient is having an adverse reaction to specific foods. A history of foods consumed and resulting symptoms is key information for the physician in the early stages of diagnosis. The doctor assesses with the help of a detailed history from the patient, the patient's dietary diary, or an elimination diet. He or she then confirms the diagnosis by the more objective skin tests, blood tests, or food challenges.
The health history usually is the most important diagnostic tool in diagnosing food allergy. The physician interviews the patient to determine if the facts are consistent with a food allergy.
The healthcare professional may ask the following questions:
- What was the timing of the reaction? Did the reaction come on quickly, usually within an hour after eating the food?
- Was allergy treatment successful? For example, if hives stem from a food allergy, antihistamines should relieve them.
- Is the reaction always associated with a certain food?
- Did anyone else get sick? For example, if the person has eaten fish contaminated with histamine, everyone who ate the fish should be sick. In an allergic reaction, however, only the person allergic to the fish becomes ill.
- How much did the patient eat before experiencing a reaction? The severity of the patient's reaction can sometimes relate to the amount of the suspect food eaten.
- How was the food prepared? Some people will have a violent allergic reaction only to raw or undercooked fish. A thorough cooking of the fish destroys those allergens in the fish to which they react so that they then can eat it with no allergic reaction.
- Were other foods eaten at the same time as the food that caused the allergic reaction? Fatty foods can delay digestion and thus delay the onset of the allergic reaction.
What skin and blood tests diagnose the causes of food allergies?
- Skin tests: In a percutaneous skin prick-puncture skin test, a dilute extract of the suspected food is placed on the skin of the forearm or back. This portion of the skin then is punctured or pricked with a needle and observed for swelling or redness, which would signify a local allergic reaction to the food. A positive prick-puncture test indicates that the patient has the IgE antibody that is specific to the food being tested on the skin's mast cells. Skin tests are rapid, simple, and relatively safe.
- A person can have a positive skin test for a food allergen, however, without experiencing allergic reactions to that food. A doctor diagnoses a food allergy only when the patient has a positive skin test for a specific allergen and the history suggests an allergic reaction to the same food. In some highly allergic people, however, especially if they have had anaphylactic reactions, skin tests should not be done because they could provoke another dangerous reaction. Skin tests also cannot be done in patients with extensive eczema.
- Blood tests: In those situations where skin tests cannot be done, a doctor may use blood tests such as the RAST, ImmunoCap, and ELISA. These tests measure the presence of food-specific IgE antibodies in the blood of patients, but they cost more than skin tests, and the results are not available immediately. As with positive skin tests, positive blood tests make the diagnosis of a specific food allergy only when the clinical history is compatible.
What types of diets help diagnose the cause of food allergies?
- Dietary diary: Sometimes, a history alone cannot determine the diagnosis. In that situation, the doctor may ask the patient to keep a record of the contents of each meal and whether reactions occurred that are consistent with allergy. The dietary diary provides more details than the oral history so that the doctor and patient can better determine if there is a consistent relationship between food and allergic reactions.
- Elimination diet: The next step that some doctors use is an elimination diet. Under the doctor's direction, the patient does not eat a food suspected of causing the allergy (for example, eggs) and substitutes another food (in this instance, another source of protein). If after the patient removes the food, the symptoms go away, the doctor almost always can make a diagnosis of food allergy. If the patient then resumes eating the food (still under the doctor's direction) and the symptoms return, this sequence confirms the diagnosis. The patient should not resume eating the food, however, if the allergic reactions have been severe because this re-challenge is too risky. This technique is also not suitable if the allergic reactions have been infrequent.
If the patient's history, dietary diary, or elimination diet suggests that a specific food allergy is likely, the doctor then will use tests, such as skin tests, blood tests, and a food challenge, which can more objectively confirm an allergic response to food.
What is the oral food challenge used to diagnose food allergies?
The double-blind food challenge has become the gold standard for objective allergy testing. (Some physicians prefer the term double-masked, rather than double-blind.)
- In this test, various foods, some of which are suspected of inducing an allergic reaction, are placed in individual opaque capsules. Both the patient and the doctor are blinded, so that neither of them knows which capsules contain the suspected allergens. (The capsules are prepared by another medical worker.)
- The patient swallows a capsule and the doctor then observes whether an allergic reaction occurs. This process is repeated with each capsule. Alternatively, the food to be tested may be disguised in another type of food to which the person is not allergic.
- The advantage of a food challenge is that if the patient has an allergic reaction only to the suspected foods and not to the other foods tested, the diagnosis of food allergy is confirmed.
- Just as with a re-challenge after the elimination diet and with the skin tests, however, someone having a history of severe reactions should not be tested with a food challenge because of the danger of inducing another severe reaction.
- In addition, this procedure is expensive because it is difficult and requires a lot of time, especially for patients with multiple food allergies.
- This type of test must also be done under the careful supervision of a physician. Consequently, double-blind food challenges are done infrequently.
- They are done most commonly, however, when the doctor wishes to obtain evidence to confirm the suspicion that the patient's symptoms are not due to a food allergy.
- Then, additional efforts may be directed at finding the real cause of the patient's symptoms.
What are food allergy treatments?
Treatment for food allergies may include the following:
- Dietary avoidance: Avoiding the offending allergen in the diet is the primary focus of food allergy treatment.
- Once a food to which the patient is sensitive has been identified, the food must be removed from the diet.
- To do this, affected people need to read lengthy, detailed lists of ingredients on the label for each food they consider eating.
- Many allergy-producing foods such as peanuts, eggs, and milk appear in foods that are not ordinarily associated with them. For example, peanuts often are used as protein supplements, eggs are found in some salad dressings, and milk is in bakery products.
- The FDA requires that the ingredients in food be listed on its label, and careful attention to food labels is critical for anyone with food allergies.
- People can avoid most of the foods to which they are sensitive if they carefully read the labels on foods and when in restaurants, avoid ordering foods that might contain ingredients to which they are allergic.
- Treating an anaphylactic reaction: People with severe food allergies must be prepared to treat an anaphylactic reaction.
- Education about the signs of a reaction and how to manage the reaction is essential.
- Even those who know a lot about their allergies can either make an error or be served food that does not comply with their instructions, including food that may have had cross-contact with allergy-causing foods during cooking or preparation.
- To protect themselves, people who have had anaphylactic reactions to a food should wear medical alert bracelets or necklaces stating that they have a food allergy and that they are subject to severe reactions.
- These individuals also always should carry a syringe of adrenaline (epinephrine [EpiPen, Auvi-Q]), obtained by prescription from their doctors; people with severe food allergies should be prepared to self-administer epinephrine if they think they are developing an allergic reaction.
- They then should immediately seek medical help by either calling the rescue squad or having themselves transported to an emergency room.
- Treating other symptoms of food allergy: Several medications are available for treating the other symptoms of food allergy.
- For example, antihistamines can relieve gastrointestinal symptoms, hives, sneezing, and a runny nose.
- Bronchodilators can relieve the symptoms of asthma. These medications are taken after a person inadvertently has ingested a food to which he is allergic. They are not effective, however, in preventing an allergic reaction when taken before eating the food. No medication in any form is available to reliably prevent an allergic reaction to a certain food before eating that food.
What is the prognosis (outlook) for food allergy?
As described above, avoidance of the trigger food is the primary treatment for food allergy. The outlook is excellent for those who can avoid consuming the allergen and who remain prepared to treat a severe allergic reaction such as anaphylaxis. There are no long-term complications associated with food allergy, other than the risk for severe reactions.
What are complications of food allergies?
The most serious complication, as mentioned earlier, is the development of a life-threatening anaphylactic reaction.
Are allergy shots effective in preventing or decreasing food allergy?
Allergy shots, a form of treatment known as immunotherapy, involve injecting small quantities of substances to which the patient is allergic. The shots are given regularly for a long time to desensitize the patient or get the patient to tolerate the allergen without developing symptoms. This type of therapy is effective in controlling symptoms of allergies related to hay fever, indoor allergens, and insect stings. Researchers, however, have not yet proven that these shots can prevent any allergic reactions to food.
What research is being done on food allergies?
Research is ongoing in many areas of food allergy. Food Allergy Research and Education (FARE) is an organization that awards research grants to investigators who are studying questions such as who gets food allergies and the possible role of factors including diet, hygiene, geography, ethnicity, and others in causing food allergy. FARE also has resources and education for families and patients suffering from food allergies.
Food allergy is caused by immune reactions to foods, sometimes in individuals or families predisposed to allergies. Several foods, especially shellfish, milk, eggs, peanuts, and fruit can cause allergic reactions (notably hives, asthma, abdominal symptoms, lightheadedness, and anaphylaxis) in adults or children. When a food allergy is suspected, a medical evaluation is the key to proper management.
It is important to distinguish a true food allergy from other abnormal responses to food, that is, food intolerances, which are far more common than food allergy. Once the diagnosis of food allergy is made (primarily by the medical history) and the allergen is identified (usually by skin tests), the treatment is to avoid the offending food.
People with food allergies should work with their physicians and become knowledgeable about allergies and how they are diagnosed and treated.
What is histamine toxicity? Is it a type of food allergy?
There are many conditions that can mimic food allergy. It is critical to distinguish true food allergy from other abnormal responses to food, that is, from food intolerance, which can occur in a variety of other illnesses, or food poisoning, which occurs when contaminated food is ingested. If a patient says to the doctor, "I think I have a food allergy," the doctor has to consider several diagnoses.
The possibilities include not only food allergy but also any other diseases that have symptoms brought on by food. These include 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 have 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.
- Histamine toxicity has been referred to as pseudoallergic fish poisoning and accounts for over one-third of seafood-related food-borne illnesses, according to research from the U.S. Centers for Disease Control and Prevention (CDC).
Are food poisoning and food allergies the same condition? Is lactose deficiency a food allergy?
- 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. Thus, the ingestion of contaminated eggs, salad, milk, or meat can produce symptoms that mimic food allergy. Common microbes that can cause food poisoning include noroviruses, Campylobacter jejuni, Salmonella, Listeria monocytogenes, Vibrio vulnificus, and E. coli 0157:H7.
- Lactase deficiency (lactose intolerance): Another cause of food intolerance, which often is confused with a food allergy, specifically to milk, is lactase deficiency.
- This common food intolerance affects at least one out of 10 people.
- Lactase is an enzyme in the lining of the small intestine. This enzyme digests or breaks down lactose, a complex sugar in milk, into simple sugars, which are then absorbed into the blood.
- If a person has lactase deficiency, he does not have enough lactase to digest the lactose in most milk products. Instead, other bacteria in the intestine use the undigested lactose, thereby producing gas.
- Symptoms of lactose intolerance include bloating, abdominal pain, and diarrhea.
- In a diagnostic test for lactase deficiency, the patient ingests a specific amount of lactose.
- Then, by analyzing a blood sample for simple sugars, the doctor determines the patient's ability to digest the lactose and absorb the simple sugars.
- A lower-than-normal value usually means a lactase deficiency.
Are gluten-sensitive enteropathy, other gastrointestinal diseases, or phychological factors related to food allergies?
- Gluten-sensitive enteropathy: Gluten intolerance occurs in a disease called gluten-sensitive enteropathy, or celiac sprue (also called celiac disease). A unique abnormal immune response to certain components of gluten, which is a constituent of the cereal grains wheat, rye, and barley, causes celiac disease. Although sometimes referred to as an allergy to gluten, research has revealed that this immune response involves a branch of the immune system that is different from the one involved in a classical food allergy. It involves a misdirected immune system, referred to as autoimmunity. The patients have an abnormality in the lining of the small intestine and experience diarrhea and malabsorption, especially of dietary fat. The treatment for this condition involves consuming a gluten-free diet.
- Other gastrointestinal diseases: Several other gastrointestinal diseases produce abdominal symptoms (especially nausea, vomiting, diarrhea, and pain) that are sometimes caused by food. These diseases, therefore, can resemble food allergies. Examples include peptic ulcer, gallstones, non-ulcer dyspepsia (which is a type of indigestion), Crohn's disease (regional enteritis), cancers of the gastrointestinal tract, and a rare condition called eosinophilic gastroenteritis.
- Psychological: Some people have a food intolerance that has a psychological origin. In these people, a careful psychiatric evaluation may identify a traumatic event in that person's life, often during childhood, tied to eating a particular food. The eating of that food years later, even as an adult, is associated with a rush of symptoms that can resemble an allergic reaction to food.
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Food Allergy Research & Education. <https://www.foodallergy.org/home>.
Sicherer, S. H., et al. "Food Allergies." Medscape.com. 27 Jul 2015.
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