Food Allergies and Intolerances
What is a food allergy?
Some people react with what is known as a hypersensitivity reaction (an allergic reaction) when eating certain foods, or additives. For most people with food allergies, commonly eaten foods cause the reaction. For a food allergy to be confirmed, the symptoms must specifically appear when a person eats the problem food, disappear or reduce when a person avoids the problem food, and reappear when the person eats the problem food once more.
In children, foods that commonly provoke an allergy include cow's milk protein, egg white from hens, wheat, soya bean, cod fish and peanuts. In adults, nuts and seafood such as fish, mussels, crab, prawn, shrimp and squid may also cause allergic reactions.
What are the usual symptoms of food allergy?
Because an immediate food allergic reaction involves the immune system of the body, symptoms develop quickly. Within minutes of eating the problem food, generalised rashes, itching, diarrhoea, vomiting, swelling of the lips and soft tissues, breathing difficulties and even shock can all develop. The most severe form of food allergy leads to anaphylaxis, where histamine (a natural chemical in the body) is rapidly released and causes a major and potentially fatal allergic reaction.
What is a food intolerance?
Food intolerance reactions occur less quickly than allergic reactions, don't involve the immune system and aren't usually life threatening. They are sometimes known as pseudo-allergic reactions, and a good example would be lactose intolerance. Here, the body is unable to process and digest the cow's milk sugar lactose, caused by a deficiency of the sugar-digesting enzyme lactase in the gut. This causes diarrhoea, pain and bloating after drinking cow's milk or taking in dairy products.
Some people can also suffer from natural histamine being absorbed too quickly from food in their diet into their body, causing a ‘rush’ of histamine that leads to headaches, palpitations and flushing. These symptoms can mimic those of a true allergy very closely. Some people may also react to modern chemical preservatives and additives in food, such as sulphites, sodium benzoate, salicylate, monosodium glutamate (MSG), caffeine and tartrazine. The symptoms this can cause are usually linked to how much of the chemical or additive has been eaten, with small amounts being tolerated or causing only minimal symptoms, but with larger amounts leading to symptoms such as itching, flushing, abdominal pain, vomiting and diarrhoea.
How are food allergies and intolerances diagnosed?
The usual way of diagnosing food allergy is by means of skin-prick tests to various foods or by a RAST (radioallergosorbent test) on a blood sample. A more accurate guide is skin-prick testing with fresh food extracts, and the very best is a double-blind placebo-controlled food challenge (DBPCFC) under careful supervision in a hospital. However, this is time consuming, expensive, and not always readily available.
If a food allergy is strongly suspected but it proves difficult to identify a particular food, then an elimination diet may be considered. This lasts two to four weeks and involves eating only a limited number of foods that are unlikely to cause allergies, such as lamb, rice, pears and sweet potato. Once any allergic symptoms have settled, then foods are slowly reintroduced one at a time to identify any food causing allergic symptoms. This should only be done under the supervision of a dietician. Food intolerances are much harder to diagnose as there are no reliable blood or skin tests available for these.
How can I prevent food allergies?
Breastfeeding appears to reduce the incidence of allergies – especially allergic infantile eczema - feeding only by breastfeeding for the first four to six months is best here if possible. Avoiding certain basic foods offers no benefit to the allergy-prone child unless he or she has a diagnosed food allergy; but it is sensible to introduce new foods slowly and one at a time into a baby's diet. If any adverse reaction is noted (such as rashes, swelling or vomiting) then discuss this with your doctor.
Once a food allergy has been confirmed, the most effective preventive treatment is to completely avoid that food. There are also two main types of medication that can be used to relieve the symptoms of an allergic reaction to foods; antihistamines, which can be used to treat mild to moderate allergic reaction and adrenaline, which can be used to treat severe allergic reactions (anaphylaxis)
Antihistamines work by blocking the effects of histamine, a protein responsible for most of the symptoms of an allergic reaction. Many antihistamines are available from your pharmacist without prescription. Adrenaline works by narrowing the blood vessels to work against the effects of low blood pressure, and by opening up the airways to help ease breathing difficulties. If you are thought to have a potential risk of anaphylaxis or have had a previous episode of anaphylaxis, you can be given an auto-injector of adrenaline to use in case of emergencies.