Food allergy is a complex area, further complicated by food intolerance, which can mimic the effects of an allergy. We look at the causes and symptoms of both, and steps to prevent food allergies developing.
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 oral allergy syndrome get a localised red itchy mouth and throat
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.
What are the causes?Lactose intolerance, for example, is caused by the lack of a specific digestive enzyme lactase. 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? 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 reactions 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's concealed or hidden they consume it with no ill effects. Their reaction is psychological, and it's often difficult to convince people that they're not allergic to a particular food.
Diagnosing food allergy
Food allergy can be diagnosed by means of skin-prick tests to various foods or by RAST (radioallergosorbent test). 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, an elimination diet lasting between two and four weeks 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 restricted diet.
Food intolerance is very difficult to diagnose as there are no reliable blood or skin tests available
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 breastfeed the child exclusively and continue to avoid allergenic foods.
If the mother's unable to breastfeed, then a hypoallergenic formula-milk should be considered.
Goat's milk is often used as a substitute in cow's milk allergy, but it contains similar proteins to cow's milk and so is also likely to cause allergic reactions. Try rice milk or soya instead.
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 read carefully 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 preventive treatment is complete avoidance. If the food cannot be completely avoided, oral sodium cromoglycate may help to prevent adverse reactions.