Food Allergy - General Facts
Most people think that allergies are commonplace, with around one in five of the population suffering from some type of food allergy. However, food allergy is actually much rarer with about 1-2 in a hundred of the adult population actually having adverse reactions to foods.
Whilst uncommon, allergic reactions to foods are real and can take a number of forms. The commonly experienced symptoms are described below and when experienced in association with the consumption of particular foods it is likely that an individual is suffering from a food allergy.
Anyone who suspects they have a food allergy should see a medical doctor, and seek a referral to an allergy clinic.
This webpage has information, together with links to other websites with useful information, which relate to
- What a food Allergy is
- How Food Allergy is diagnosed
- What the main allergenic foods are
- How can you find out if you suffer from a food allergy
Symptoms of Food Allergies
Reactions to foods are usually rapid, appearing within an hour (or sometimes even seconds) of consumption, although in some cases they may be delayed and appear up to four hours after eating.
- Skin rashes, such as nettle rash (also called urticaria or hives) can appear which are generally short lived, disappearing within a few days. Longer lasting, chronic skin reactions (such as scaly patches) can also be experienced. Some of these longer lasting rashes are called atopic dermatitis.
- An itchy nose and eyes, sneezing and a runny nose may be experienced, as can asthmatic symptoms, such as wheezing, breathlessness and coughing. These types of symptom are not seen so often with food allergies.
- Itching and swelling around the lips and mouth may occur on contact with a food. Other symptoms include nausea, cramping pains, bloating, vomiting and diarrhoea.
Our bodies are protected from infections by our immune system. We produce molecules, called antibodies, which recognise the germs causing an infection. There are a number of different sorts of antibody, and the one which causes an allergic reaction is called IgE.
We think that IgE molecules are normally produced in response to infections caused by parasites, like those that cause malaria. We do not understand why, but some people make IgE to other things like pollen, giving rise to hay fever, and to some foods, giving rise to food allergies.
The IgE acts like a tag, sticking to molecules in food or pollen called allergens. When someone who has an allergy eats a problem food the IgE attaches to the allergens, setting off an allergic reaction. One of the common effects that IgE triggers is the release of histamine, which causes the changes we see in our bodies as symptoms, like nettle rash or wheezing.
Allergens are usually proteins, and there are generally several kinds of allergen in each food. We do not know what makes some proteins, and not others, food allergens.
Food Allergy or Food Intolerance?
There is another collection of symptoms that people report suffer from when they eat certain foods, such as headaches, muscle and joint aches and pains, and tiredness, which are often collectively known as food intolerance. This collection of symptoms is less well defined and poorly understood, and hen
ce is generally much harder to diagnose than classical allergy. The exceptions are the gluten intolerance syndrome, known as Coeliac's disease, and lactose intolerance. Coeliac's disease is triggered by eating wheat, barley and rye-containing foods, whilst lactose (a sugar found in milk) can cause intolerance to milk in certain individuals who lack the ability to break it down. Lactose intolerance is not an allergy, but causes some of the same symptoms as milk allergy, like cramping pains in the stomach and diarrhoea.
An uncommon allergic reaction, which can be life-threatening, is called anaphylaxis. It can be triggered by consuming very small amounts of food (just a bite is enough) and the symptoms usually appear within minutes and last for several hours. Asthma, skin rashes, nausea, vomiting and diarrhoea are among the symptoms which can be experienced. An unusual from of this condition can be triggered by eating problem foods within 2-3h of vigorous exercising and is called exercise-induced anaphylaxis. Prompt administration of adrenaline after eating suspected problem foods has helped minimise life-threatening episodes.
If you have a food allergy and go to a medical practitioner how can they tell you have a food allergy? These information sheets explain what is involved and have been written by David Reading (Anaphylaxis Campaign) and Kate Grimshaw (a dietician working with food allergy patients at Southampton Hospital).
The diagnosis of food allergy starts with a combination of an investigation into the patient's clinical history, a clinical examination and a test for the detection of IgE antibodies. Skin prick testing and blood tests are used for this purpose.
The presence of specific IgE does not necessarily lead to symptoms. Therefore it is sometimes necessary to perform a provocation or challenge test with the suspect food, which involves introducing the food to the patient, first by touch, then by ingestion of gradually increasing amounts. Challenge tests must be performed in a hospital or clinic, where any serious reactions can be safely managed.
Which test to use to get an accurate diagnosis is made by considering the case history, the suspected food, the severity of the symptoms and the availability of diagnostic reagents.
In general, although progress has been made in the diagnosis of food allergy, many limitations still exist and more sensitive diagnostic tests are required. The inadequacies of current testing methods sometimes encourages the use of unproven methods for the diagnosis of food allergy, an approach that should be avoided.
How can food allergy be managed?
If a food allergy or intolerance is suspected then a visit to the GP is the place to start as he can refer you to an allergy specialist who can make a correct diagnosis (as detailed earlier). Once the diagnosis of food allergy has been made, avoidance of the causative food is essential. Occasionally complete elimination of the food for 1-2 years may result in a loss of clinical symptoms, but allergies to fish, peanuts usually persists for life.
Complete avoidance of the offending food is often difficult due to the presence of very small quantities in commercially manufactured foods. Progress towards comprehensive labelling of food allergens has led to better management of allergies, but cases of malnutrition resulting from the mismanagement of diets due to fear and lack of knowledge have been reported. There is therefore a need for proper dietetic planning which should be given by a health professional with a specialised knowledge in this area (usually a dietitian or nutritionist). Advice given would ensure that a diet is nutritionally adequate (with the use of nutritional supplements if necessary) with specific advice of what foods are likely to contain the offending foods. Sometimes food allergens are labelled using names that the consumer is not accustomed to. Education of the allergy patient is therefore vital, but it is not where food allergy management ends, in fact it is where it starts. Armed with knowledge the food allergic person can be constantly vigilant about what they eat, and they need to be!
Communicating to others is an important part of food allergy management, including in emergncy situations. Medical bracelets and wristbands are available to provide vital information on the wearer's life-threatening allergies in emergencies, or if the wearer is unconcious or otherwise unable to communicate.
Where there is an indication that a food allergy may have been outgrown, an effort to safely introduce the offending food in the allergic individual's diet is done by a careful challenge procedure in a setting where any serious reactions can be properly managed (i.e. in hospital). It needs to be established whether the food is safe in all forms (e.g. raw and or cooked). Appropriate advice on what can be introduced into the diet needs to be given by an appropriately qualified individual.
Food allergy -even when severe -can be managed perfectly well. What is required is sound medical guidance and a commitment by the patient to remain vigilant and always carry prescribed medication.
There are a number of groups of foods that are responsible for causing the majority of food allergies. The list below gives some information on these allergens. The list of foods for which allergies have been reported is much longer. A more comprehensive list can be obtained from Food Allergy Network http://foodallergy.org
Two out of a hundred infants under one year old suffer from cow's milk allergy, making it the most common food allergy of childhood. In general children lose this sensitivity as they grow up with nine out of ten losing it by the age of three; it is unusual for adults to suffer from this allergy.
Symptoms are frequently vomiting and diarrhoea in children, with 30-50% also having skin rashes of some type. A small number of children have an anaphylactic reaction to milk which tends to be lifelong.
The major allergens in milk are the caseins and the whey protein b -lactoglobulin. People are usually allergic to more than one kind of milk protein.
The proteins from cow's milk are very similar to those from goats and sheep, and can cause the same sorts of reaction in cow's milk-allergic subjects. Thus goat's or sheep's milk cannot be used as a cow's milk substitute in allergic individuals.
Allergy to eggs is usually observed in young children rather than adults, and like cow's milk allergy, fades with time. Occasionally children suffer from a severe form of allergy which is not outgrown.
The main allergens are the egg white proteins ovomucoid, ovalbumin, and ovotransferrin.
The eggs of other poultry, such as ducks, are very similar to those of hens and can cause reactions in egg-allergic individuals.
Fish and shellfish:
Allergies to shellfish are unusual in children, mostly being experienced by adults. Reactions to fish are found in children and adults. The incidence of seafood allergy is higher in those countries with a high consumption of fish and shellfish.
Severe reactions are more frequently found with these foods, including anaphylaxis.
Cooking does not destroy the allergens in fish and shellfish, and some individuals maybe allergic to the cooked, but not raw, fish.
The major allergens in fish are flesh proteins called parvalbumins which are very similar in all kinds of fish. This is why people allergic to cod tend to be allergic to fish such as hake, carp, pike, and whiting as well.
Shellfish allergens are usually found in the flesh and are part of the muscle protein system, whilst in foods such as shrimps, allergens have also been found in the shells.
In general allergic reactions to fruits and vegetables are mild, and are often limited to the mouth, and are called the oral-allergy syndrome (OAS).
Around four out of ten people having OAS are also allergic to tree and weed pollens. Thus people who are allergic to birch pollen are much more likely to be allergic to apples.
There allergens in fruits and vegetables are not as complicated as other foods. Many of them are very like the allergens in pollens, which is why people with pollen allergies are also allergic to certain fruits.
Many fruit allergens are destroyed by cooking, and thus cooked fruits are often safe for fruit allergic people to eat.
Allergies to latex gloves, especially amongst health professionals, are increasing. As many of the latex allergens are like those found in certain tropical fruits, such as bananas, these people can get an allergic reaction to handling or eating these foods .
This group of foods includes soya beans and peanuts. Peanuts are one of most allergenic foods and frequently cause very severe reactions, including anaphylaxis.
Allergy to peanuts is established in childhood and usually maintained throughout life.
Both these foods have multiple allergens which are present in the raw and cooked foods.
Peanut allergy can be so severe that only very tiny amounts of peanut can cause a reaction. Thus the traces of nuts found in processed oils, or the carry over of materials on utensils used for serving foods, can be enough in some individuals, to cause a reaction.
The main allergens in peanuts and soya are the proteins used by the seed as a food store for it to grow into a seedling. One of the allergens in soya bean is very similar to a major allergen from dust mites, a common environmental allergen. We aren't sure yet whether this means there is a link between dust allergy and soya allergy.
This group includes true tree nuts, such as Brazil nuts, hazelnuts, walnut and pecan.
Whilst not as intensively studied as peanuts, indications are that tree nuts can cause symptoms as severe which can occasionally be fatal.
Children who become sensitised to tree nuts tend to remain allergic throughout life.
Hazelnut and almond allergies are more like those people get to fruit, and are linked to pollen allergies.
Nut allergens can be both destroyed by, or resistant, to cooking and we think that roasting may actually create new allergens.
The allergens can be the seed storage proteins, or other molecules which are also found in pollen.
Suffered by children and adults alike, wheat allergy appears to be particularly associated with exercise-induced anaphylaxis.
The more of a cereal (wheat, rye, barley, oats, maize or rice) we eat the more likely we are to suffer an allergy. Thus rice allergy is found more frequently in populations eating ethnic diets.
Seed storage proteins (such as wheat gluten) and other proteins present in grain to protect it from attack by moulds and bacteria, have been found to be major allergens.
How can you find out if you suffer from a food allergy?
Signs and symptoms
The clinical manifestations of IgE-mediated allergy to foods are extremely diverse. They may involve the skin (urticaria), the respiratory system (asthma), and the oral mucosa (swelling, redness) as well as the gastro-intestinal tract (nausea, vomiting, pain and discomfort). Atopic dermatitis is also associated with food allergy.
Reactions can occur minutes to hours after exposure to the allergen. The most severe of these reactions is anaphylactic shock, which combines several of those symptoms. Typical signs can include peripheral vasodilatation leading to circulatory collapse, accompanied by swelling of the skin and mucosal tissues (angioedema), which results in breathing difficulties. Untreated anaphylactic shock can rapidly result in death. A second severe, and potentially fatal, manifestation is Quincke's oedema, the typical signs are swelling of the deep layers of the skin and mucous membranes, such as those of the throat. Technically, it has been described as localised anaphylaxis.
Diagnosis of food allergy requires a combination of clinical history, laboratory (specific IgE measurements) or outpatient (skin prick tests) tests and challenges with the food.
Diagnosis of an allergy involves a clinical doctor drawing together three different kinds of information.
- A detailed history of past allergic reactions and other allergic conditions, such as asthma, eczema and hayfever, and consideration of any seasonal or environmental symptoms.
- A thorough medical examination, involving peak flow measurements if the patient is asthmatic, and a close look to see if there are allergic symptoms in the skin, eyes and nose. This information will help the specialist decide which tests are appropriate.
- Results of allergy tests -usually skin prick testing or blood testing.
So what do these tests involve? Is skin prick testing dangerous? At what age can testing be given? Are the tests that are offered reliable? Here, we attempt to answer these and other common questions.
Skin Prick Testing
This is suitable for any age group. Even babies under a year old are tested at some clinics in this way. A tiny prick is made with a lancet through a drop of allergen placed on the skin, usually on the forearm.
A positive reaction will be indicated by itching within a few minutes. The site where the allergen was introduced then becomes red and swollen, with a raised weal in the centre that looks like a nettle sting. The weal enlarges and reaches its maximum size within 15-20 minutes, when the measurements of the weal are recorded. The reaction fades within an hour.
This method introduces such a small amount of allergen into the skin that testing is usually safe, even in cases of severe nut allergy. Dr. Bill Frankland, consultant allergist at the London Allergy Clinic, knows of only three cases reported in the world medical literature where skin prick testing caused anaphylaxis. This is out of millions of tests performed.
A negative response usually means the patient is not sensitive to that allergen. But skin prick testing for food allergens may be unreliable and "false negatives" can occur where the reaction to food is not immediate. A negative response may occur if the patient is taking antihistamines. These should be stopped five days before testing. Another cause is the fact that some allergens are very unstable; thus for many fruits and vegetables a doctor might use a "prick-prick" test, where a lancet is used to pierce the fruit or vegetable and then used to make the skin prick.
A positive response usually means the patient is allergic to that allergen. However, a patient may have a positive skin test but suffer no symptoms when coming into contact with the allergen. Positive skin tests may also occur before an individual has experienced allergic symptoms to a food. People may also still have positive skin tests to foods and inhalants, even when they have grown out of the allergy and no longer have an allergic reaction on eating a problem food.
RAST (Radioallergosorbent test) and CAP-RAST are the most commonly used blood tests in the UK (the CAP-RAST seems to be superseding the RAST test because it appears to be more reliable and more sensitive). Other immunological blood tests not using radioactive material but enzymes are now superseding the original RAST methodology. They work by detecting the presence of a molecule, called IgE, which is involved in allergic reactions (link to appropriate web page), which can bind a particular allergen.
Blood tests give graded results from 1-6, with 6 being the most positive. Blood tests are not affected by antihistamines, and can be used in patients with severe eczema. A blood sample is taken and sent of to a specialised laboratory for analysis; as a result they are quite costly and the results are not available immediately. These tests also only indicate an allergy if the IgE is present in the blood, and there is not a clear relationship between the presence of IgE and the severity of the allergy. In some cases the blood test is negative, but an individual still experiences symptoms on consuming a food.
(The following was compiled with the help of Dr Jonathan Hourihane).
A patient who has had a genuinely life-threatening allergic reaction, like anaphylaxis, should probably not be challenged with the food that caused it. Past symptoms must be discussed thoroughly with the medical team offering the challenge.
Challenge testing MUST always be given under strict medical supervision, and by a specialist with a high degree of knowledge about allergy. It involves giving a patient increasing doses of the suspected allergenic food, allowing ample time between doses for a response to occur. Challenges are often conducted in a double-blind manner. This is when neither the patient, nor the investigator knowing whether the food being given has any allergen hidden in it or not. A safe challenge involves the following course of action by the medical team performing the challenge:
- They must ensure the patient or person is fit and well before challenge. In particular, there must be an absence of asthma or wheezing. Antihistamines must be avoided during the week leading up to the challenge.
- Careful planning the doses to be used and anticipating the medical responses to reactions before starting. Ensuring resuscitation equipment is standing by.
- Ensuring the patient and if a child the parent understand what is going to happen. They will have to sign a medical consent form.
- Increasing the dose very gradually. For example, in a peanut challenge, the doctor or nurse might choose to start with a small piece of peanut (or peanut butter) rubbed on the lip. If there is no reaction after 10-15 minutes, they can proceed cautiously to the next stage.
- The next stage might be allowing the patient to eat, for example, a tiny smear of peanut butter spread thinly on a small piece of bread.
- Gradually increasing the dose until, for example, 8-16 nuts have been eaten. Many challenges are stopped too early due to anxiety, but it is necessary to proceed if the true picture is to be obtained. A negative challenge is valid only if no symptoms are observed following exposure to a large dose of the problem food.
- Adequate observation for up to four hours after the challenge.
- A nurse or doctor assessing any allergic reaction. Medication may - or may not - be necessary.
A detailed history and examination alongside the chosen test is required in order to give a complete picture, and make a correct diagnosis. As a consequence of the unreliability of many blood tests in predicting allergic reactions, skin prick testing remains the primary tool to confirm an allergic diagnosis, and gives the most reliable results. However, none of these tests are infallible. This is because the sensitivity and predictive ability of skin prick tests and specific IgE measurements varies considerably from food to food. It is very high for fish, for instance, but very poor in the case of apple allergy. Food challenges remain the "gold standard" for diagnosis, except where the patient has suffered an anaphylactic shock.
After food allergy has been diagnosed, the only treatment measure that can be offered is to avoid the offending food. This makes it very important to provide consumers with clear information about the composition of foods. Food avoidance can also have serious nutritional consequences when it removes an important food group from a person's diet. For these reasons someone with a food allergy should consult a trained dietician before implementing a restricted diet.
If you think you have an allergy you need to consult a clinical doctor, to ensure that you receive a proper diagnosis. Anyone who suspects they are at risk of suffering an anaphylactic reaction should be referred to an allergy clinic.