Health Matters.

True food allergy

What are the causes? In children, common allergy provoking foods include cow's milk
protein, egg white from hens, wheat, soya bean, codfish and peanuts. In adults, nuts
(including brazilnuts, almond, hazelnut, peanut and walnut), fruits (such as peach, apple,
strawberry and citrus fruit), and vegetables (such as celery, tomato, onion, garlic and
parsley) are common allergens. Seafood such as fish, mussels, crab, prawn, shrimp and
squid may also cause allergic reactions.

What are the symptoms? Typically, an immediate type of food allergic reaction
involves the immune system. Traces of the offending food rapidly lead to symptoms of
generalised rash, itching, body swelling, breathing difficulties and even collapse. Peanut
anaphylaxis is a good example where traces of the food are absorbed in the mouth or
intestine. This leads to the rapid release of histamine from cells and allergic tissue
swelling.
Some people with the oral allergy syndrome get a localised red itchy mouth and throat
Some people with the oral allergy syndrome get a localised red itchy mouth and throat on
eating certain fruit, vegetables and nuts. Delayed reactions to food are becoming more
common and this may be the basis of eczema in infants. Coeliac disease occurs due to a
delayed immune reaction to gluten in wheat. This causes intestinal membrane damage
with resultant diarrhoea, abdominal bloating and malabsorption resulting in anaemia, for
example.

Food intolerance
What are the causes? Food Intolerance - for example lactose intolerance - may be
caused by the lack of a specific digestive enzyme. This is a remarkably common
condition. Natural histamine may be too rapidly absorbed from food in the diet and
effectively lead to a histamine 'rush' that mimics an allergy.

Then there are adverse reactions to chemical preservatives and additives in food
such as seen with sulphites, benzoates, salicylates, monosodium glutamate, caffeine,
aspartame and tartrazine.

What are the symptoms? These adverse food reactions are of slower onset, do not
involve the immune system and aren't usually life threatening. Reactions are usually dose
related. With small amounts of the food being tolerated but larger amounts leading to
reactions such rashes, flushing, abdominal pain, vomiting, diarrhoea and palpitations.
These are by far the most common adverse food reaction seen in general practice.

The lack of a specific enzyme in the body may lead to the build up of toxic by-products
and histamine, which then mimic the symptoms of an allergy. This is called a
'pseudo-allergic' reaction.

Food toxicity and aversion
Poisons may naturally occur in foods such as mushrooms and potatoes. Bacteria in
putrefying fish can cause toxic food poisoning called 'schromboid toxicity'. These
reactions occur in all people who consume the toxin and don't involve a digestive
intolerance or an immune reaction.

Some people have a food aversion, and convince themselves - with no sound basis - that
they're 'food allergic' and will vomit if given the particular food. If the food is concealed or
hidden they consume it with no ill effect. Their reaction is psychological, and it's often
difficult to convince these people that they're not allergic.

Diagnosing food allergy
Food allergy can be diagnosed by means of skin-prick tests to various foods or by RAST
testing. Skin testing with fresh food extracts is more accurate. The gold standard in food
allergy is the Double Blind Placebo Controlled Food Challenge (DBPCFC) test under
careful supervision in a hospital.

If no food can be identified, but an allergic reaction is strongly suspected, a two- to
four-week elimination diet is performed. The person lives on a limited number of foods,
which are unlikely to cause allergies, such as lamb, rice, pears and sweet potato. Then
once the allergic symptoms settle, foods are slowly reintroduced one at a time to identify
the offending food. This should only be done under the supervision of a dietician as a
child can end up in a state of malnutrition on a prolonged few-food diet.

Food Intolerance is very difficult to diagnose as there are no reliable blood or skin tests
available. The suspected food has to be eliminated from the diet and a clinical
improvement documented. The offending food should then be reintroduced again for a
short period to confirm the adverse reaction and hence the intolerance.

Food allergy prevention
For high-risk families (those families with severely allergic parents or siblings) it's
recommended that the pregnant mother avoid cigarette smoking and highly allergenic
foods in the last half of pregnancy. She should try to exclusively breastfeed the child and
continue the avoidance of allergenic foods. If the mother is unable to breastfeed then a
hypoallergenic formula-milk should be considered.

Delay the introduction of solid foods until six months of age and then restrict first foods to
lamb meat, chicken, rice, sweet potatoes, carrots and pears. Avoid cow's milk, eggs,
wheat, fish, soya, citrus for the first year and introduce nuts and peanuts only in the third
year. Food labels need to be carefully read to take account of contents in tinned and
processed food. Try wherever possible to avoid foods containing additives and
preservatives.

Once food allergy has been confirmed, the most effective preventative treatment is
complete avoidance of that food. If the food cannot be completely avoided, oral sodium
chromoglycate may help to prevent adverse reactions.

This article was last medically reviewed by Dr Rob Hicks in October 2005.
First published in September 1999.