What is food allergy ?
The first thing to clear up is the definitions. A food allergy is where the body reacts in a clear and abnormal way after ingestion of a food. The food is ingested and this sets off a cascade of immune system response that results in the release of body chemicals that are responsible for the allergy. In immediate reactions and antibody called IgE reacts with the protein of a food setting off the cascade. In delayed sensitivity the reaction is more complex and involves other components of the immune system. The most common foods for these kinds of reactions include cow and soy milk protein, egg, fish and shellfish, and peanuts. Sometimes salicylates in citrus fruit and tomatoes can cause reactions around the mouth as a contact dermatitis. This is not really a food allergy, as eating the food does not cause any problems.
Food allergies sometimes go hand in hand.
For example, in individuals allergic to:
- Cow’s milk: ~90% will be allergic to goat’s milk.
- Cashew: almost all will be allergic to pistachio.
- Walnut: most will be allergic to pecan.
- Fish: ~75% will be allergic to other fish.
- Prawn: most will be allergic to other crustaceans (e.g. crab, lobster).
- Peanut: ~5% are allergic to another legume (e.g. soy).
Immediate (within 30minutes) – IgE allergy – this is the most common reaction seen in infants and children.
The obvious example here is a clear skin reaction following the ingestion of a protein such as peanut. Shortly after the peanut is ingested there can be a rash, lip swelling, sometimes vomiting and sometimes anaphylaxis. The definition of anaphylaxis is where there is involvement of the cardiorespiratory tract. However anaphylaxis is exceedingly rare under the age of 2 years. In this age group the symptoms are usually confined to the skin, and perhaps lip swelling.
Delayed (hours to days) food allergy (non IgE food allergy)
Sometimes food is a trigger for eczema, particulalry in infants and toddlers. This is part of the delayed reaction. About 15 – 20 % of those who have moderate to severe eczema will have a food component. Other reactions include….
Food protein induced enterocolitis syndrome – a disorder of young infants due to cow or soy milk. Present with delayed onset (1 -3 hours after allergen ingestion) of protracted vomiting and sometimes floppiness and pallor that can be misdiagnosed as infection.
Food protein induced enteropathy
– presents with milder form of diarrhoea, vomiting and poor weight gain.
So food allergic reactions are quite specific contrary to what parents report. Often what is being reported is food intolerance.
This is where a food will cause an undesirable effect in an infant or child but it is not a true allergic reaction. For instance some juices can cause signficant diarrhoea and cows milk can cause constipation. Lactose is a disaccharide, which requires the presence of lactase in the gut to digest this. Some people do not have much lactase so the lactose will end up in the large intestine where it is digested by bacteria. This process will result in gas and byproducts that can lead to explosive diarrhoea. This is called lactose intolerance and is not an allergy. The same process can occur if some people ingest too much of a particular food – such as bread. Contrary to popular belief it is not gluten that causes abdominal symtpoms but fructans, a carbohydrate associated with bread.
Diagnosing food allergy
This can be challenging. A history of rash within minutes to hours of ingesting a particular food can be proven by either blood tests of skin prick tests. The latter are easier more convenient but need to be interpreted cautiously.
Skin Prick tests
Skin Prick tests involve placing a drop of liquid on the forarm and scratching the skin so the liquid is analysed by the dermis. If the liquid contains the suspected material (allergen) then the area tested will result in a small lump called a weal that looks like a mosquito bite. If positive then this is suggestive of a food allergy if backed up by history. If this area is negative then it makes food allergy highly unlikely.
The other test is a food challenge. This should be done under medical supervision
If there has been a definite diagnosis of food allergy then avoidance is the most important treatment. Accidental exposure is managed with medical intervention. It is impossible to predict whether or not someone will have an anaphylactic reaction so often an adrenaline injector such as an epipen is prescribed. See ASCIA for guidelines
What about future siblings ?
There has been a great deal of interest in siblings of those with allergies. Unfortunately avoidance of such proteins during subsequent pregnancies and early life have not been found to be affective and this is not recommended. Indeed there is emerging evidence that the opposite should be considered and infants around 4 – 6 month be exposed to these proteins. This is quite safe. See article on introducing solids.