What is food allergy ?
A food allergy is where the body reacts in a clear and abnormal way after ingestion of a food. The food sets off an immunological cascade resulting in the release of chemicals that are responsible for the allergy. There are two kinds.
- Immediate reactions, also called IgE reactions where a food protein causes a reaction within minutes.
- Delayed reactions – or Non IgE reactions, occur predominantly in infants and cause gastrointestinal symptoms.
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 serious reaction seen in infants and children.
Usually this occurs in infants and toddlers who ingest a newish protein. There are two possible reactions
- Skin only – Within seconds to minutes a rash is noted around the mouth, there maybe swelling of the lips, and face, occasionally vomiting, and itchiness. This settles after a short period.
- Anaphylaxis – usually there is the above skin reaction, which progresses to cough, noisy breathing, possible wheeze, pallor and respiratory distress.
- Infants and toddlers will often self resolve following anaphylaxis. However treatment is advisable
- Risk of death from Anaphylaxis increases once over the age of 10 years
- Adolescents with peanut allergy, and asthma, are the highest risk of life threatening anaphylaxis.
Delayed (hours to days) food allergy (non IgE food allergy)
Gastrointestinal Infant Reactions.
This can be divided into several subtypes.
Food Protein proctocolitis
- Infants appear well, but have mucousy blood stained bowel actions when exposed to cow’s milk or soy protein either in breast milk or in formula. Usually occurs in the first 3-6 months
Food protein induced Enteropathy
- Infants have trouble feeding, poor weight gain, distressing colicky symptoms, usually due to cow’s or soy milk protein, ingested either via breast milk or formula.
Food Protein Induced Enterocolitis Syndrome (FPIES)
- Sudden onset severe vomiting within 2hours of ingesting a new protein such as rice, oats, or meat. There maybe pallor and diarrhoea and is sometimes difficult to diagnose.
- Older infants and toddler who have problems swallowing certain foods, which can lead to poor weight gain. This is fare and diagnosis requires biopsy of the eosophagus.
Food and Eczema
In some infants with eczema, food can be one of the triggers. Usually the infant will have moderate to severe eczema which significantly increases in severity following exposure to certain proteins, including egg, dairy and soy. Removing this food will lessen the severity but will not result in an eczema cure.
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
The most important assessment is a careful history from a medical practitioner trained in allergies, usually a paediatrician who specialises in this area of medicine. Blood tests or skin prick tests can aid the diagnosis, but are best performed by specialists as results can be misleading.
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.