Food allergy is caused when the immune system (IS), responsible
to keep us clean from infections thinks that proteins of certain foods
are of those of harmful things e.g. of bacteria.
The IS reaction is to release toxins to 'destroy' these proteins, but they end up giving us the allergy reaction.
The information below is adapted from the research funded by the European Union, co-ordinated by Dr Clare Mills at the Institute of Food Research, Norwich, UK.
Presently a home kit exists to help you identify any of several food allergens. The Yorktest is well known in UK and is scientifically proven to help identify any of 113 food allergens. There are several kits starting from under £20 and it is worth trying them. Visit their website where you can see their different kits and read about them.
Every time a person reacts to food, it is assumed that an allergy reaction has taken place. Some 20% of people believe they are allergic to one food or another, however, the truth is that food allergy is actually rarer with about 1-2 in a hundred of the adult population actually having adverse reactions to foods. (This may, however, be up to 10% in children)
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.
Symptoms of Food Allergy
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.
What is Food Allergy?
Our bodies are protected from infections by our immune system. We produce a type of proteins, called antibodies, which recognise the germs causing an infection. There are a number of different sorts of antibodies, and the one which causes an allergic reaction is called IgE.
Figure 1: Free IgE antibodies (green) cleave to food antigens (red stars) e.g. milk proteins. This cleavage forces the IgE ab to coat the mast cell signalling a danger. The mast cell releases granules of chemical toxins. These granules burst and release these potent chemicals which bring on the allergy attack.
It is thought that IgE antobodies are normally produced in response to infections caused by parasites, like those that cause malaria. It is not understood 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. It is not fully understood why some foods can cause allergies and others do not, but a theory is that it is probably that some proteins in foods mimic very closely proteins present in viruses and bacteria. Hence, in some persons, ususally this is genetically predisposed, their immune system is not able to distinguish the food protein from the virus or bacteria one, hence it attacks.
Colour is added to sweets to get the children's attention. Avoid them if your child has food allergy because they may make it worse
Food Allergy or Food Intolerance?
There is another collection of symptoms that people report suffer from when they eat certain foods, such as bloating, 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 hence is generally much harder to diagnose than classical allergy. The exceptions are the gluten intolerance syndrome, known as Coeliac disease, and lactose intolerance. Coeliac 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. For a full review and details on lactose intolerance click here. See also Coeliac disease.
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.
How is a diagnosis of food allergy made?
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.
Ketchup with its many additives can easily be blamed for causing an allergy. However, fresh organic red coloured fruits and vegetables may also be the cause an allergy reacion in some children and adults!
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 dietician 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!
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
- Cow's Milk:
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.
If pregnant or trying to be, avoid seafood and shellfish. Some believe that it may cause your child to develop food allergy.
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.
- Tree nuts:
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.