Food allergies

Common food allergies

In children, common allergy-provoking foods include cow's milk protein, egg white from hens, wheat, soya bean, cod fish and peanuts.

In adults, nuts including Brazil, almond, hazelnut, peanut and walnut are common allergens. Seafood such as fish, mussels, crab, prawn, shrimp and squid may also cause allergic reactions.

Localised oral allergies may occur in young adults in association with silver birch tree pollen allergy. They get an itchy mouth and throat on eating certain fresh fruit (apple, cherry, peach and nectarine), raw vegetables (carrot, celery and potato) and nuts.

Symptoms of food allergies

Typically, an immediate food allergic reaction will involve the immune system. Within minutes, traces of the offending food in the diet can trigger generalised rashes, itching, diarrhoea, vomiting, swelling of the lips and soft tissues, breathing difficulties and even shock.

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.

Delayed reactions to food may also occur, which can aggravate eczema in infants. Coeliac disease is a delayed immune reaction to the gluten part of wheat.

This damages the intestinal lining, resulting in abdominal bloating, discomfort, diarrhoea or constipation. It also decreases absorption of essential foods from the intestine resulting in anaemia, lethargy and nutritional deficiencies. These changes may be subtle and can easily be missed.

Food intolerance

Food intolerance reactions are of slower onset than allergic reactions, don't involve the immune system and aren't usually life threatening. They're often called pseudo-allergic reactions.

Lactose intolerance, for example, is the inability to digest the cow's milk sugar lactose, caused by deficiency of the sugar-digesting enzyme lactase in the intestine.

This is common in people of southern European or African descent and results in smelly diarrhoea, pain and bloating after drinking cow's milk or taking in dairy products. Lactose intolerance doesn't cause rashes, weight gain or lethargy.

Natural histamine may be absorbed too rapidly from food in the diet and effectively lead to a histamine 'rush' with headaches, palpitations and flushing that mimics an allergy.

Then there are adverse reactions to chemical preservatives and additives in food, such as sulphites, sodium benzoate, salicylate, monosodium glutamate (MSG), caffeine and tartrazine.

These reactions are usually dose-related, with small amounts of the food being tolerated but larger amounts leading to reactions such as urticaria, flushing, abdominal pain, vomiting and diarrhoea.

Food toxicity and aversion

Natural poisons occur in some foods, such as mushrooms and potatoes. Bacteria in putrefying meat and fish can cause toxic food poisoning.

These reactions occur in all people who consume the toxic foodstuff and don't involve any 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 effects.

Their reaction is psychological and it can be difficult to convince them that they're not allergic to a particular food.

Diagnosing food allergy and intolerance

Food allergy can be diagnosed by means of skin-prick tests to various foods or by a RAST (radioallergosorbent test) on a blood sample. Skin testing with fresh food extracts is more accurate.

The gold standard in food allergy testing is the double-blind placebo-controlled food challenge (DBPCFC) under careful supervision in a hospital, but it is time consuming and costly.

If no food can be identified, but an allergic reaction is strongly suspected, an elimination diet lasting two to four weeks should be instituted. This involves eating only a limited number of foods that are unlikely to cause allergies, such as lamb, rice, pears and sweet potato.

Once the allergic symptoms settle, foods are slowly reintroduced one at a time to identify the offending substance. This should only be done under the supervision of a dietician, as children can end up in a state of malnutrition on a prolonged restriction diet.

Food intolerances to pseudo-allergens are difficult to diagnose as there are no reliable blood or skin tests available.

Preventing food allergies

For high-risk families (those with severely allergic parents or siblings), it's recommended pregnant women avoid cigarette smoking and prepare to breastfeed exclusively.

Exclusive breastfeeding seems to reduce the incidence of allergies, especially allergic infantile eczema.

Although in the past doctors have advocated that breastfeeding mothers avoid allergenic foods such as cow’s milk, hen's eggs and nuts, as traces may appear in breast milk, recent studies indicate it makes little or no difference to allergies what the mother consumes in her diet while pregnant or breastfeeding.

Expert allergists and consultant dieticians have pointed out that avoiding all potentially allergy-provoking foods after weaning is more likely to cause malnutrition and less likely to have any long-term benefit for preventing allergies.

There's good evidence that exclusive breastfeeding for the first four to six months has some allergy-protection effects, but avoiding potentially allergy-provoking foods such as cow's milk, hen's egg, wheat, soy, fish and nuts in the infant's diet beyond this period offers no benefit to the allergy-prone child.

Infant dietary advice has been a controversial area of allergy and, despite previous recommendations to avoid cow's milk and eggs in the first year and peanuts or nuts for up to three years, the current evidence indicates this practice will have no beneficial effect in preventing allergy.

A healthy, nutritious diet is more important for a growing child and avoiding certain basic foods offers no benefit to the allergy-prone child unless he or she has a diagnosed food allergy. But it's prudent to slowly introduce new foods one at a time into a baby's diet and if any adverse reaction is noted (such as rashes, swelling or vomiting), immediately discuss this with your GP or practice dietician who may then refer your child for appropriate food-allergy tests.

Once a food allergy has been confirmed, the most effective preventive treatment is complete avoidance of that food. If the food can't be avoided, oral sodium cromoglicate may be taken continuously, but it is expensive and only moderately effective in preventing adverse food reactions.

Sodium cromoglicate is very safe and can be bought without prescription.

Dietary diary

Before visiting your GP or an allergy clinic, it's important to keep a detailed diary of all foods consumed and symptoms over a two-week period. This should list all meals, snacks, drinks, medication and supplements taken. Any allergy symptoms should be recorded with a note of time and intensity.

The allergy clinic nurse or dietician will go through the diary with you, looking for a pattern of reactivity and causal relationship of symptoms to foods and drinks.

This diary and your personal allergy history are important in directing allergy tests to the correct culprits. Allergy testing without a good personal allergy history is usually unhelpful and often leads to an incorrect food-allergy diagnosis.

Simple elimination and restriction diets

In suspected food allergy, with the aid of your detailed food diary and symptoms, you'll often be able to isolate a particular food as the cause of your allergy. It's then recommended you eliminate all sources of that foodstuff for two weeks to confirm diagnoses.

If your assumption is correct, elimination of that food should lead to full symptom relief and reintroduction of that food should bring the symptoms back. If that doesn't occur, you have implicated the wrong food and need to consider other possible culprits.

If cow's milk is eliminated from the diet, calcium needs to be supplemented in growing children. Glucose (as in sugar) doesn't provoke allergies and shouldn't therefore be eliminated from the diet.

Sometimes children and adults may have typical food allergy symptoms attributed to meals, but despite keeping a thorough food diary remain unable to identify any culprit foods. A short, two-week 'hypo-allergenic' or restriction diet is then recommended. This contains only foods that are unlikely to cause allergies.

If this diet is continued beyond two weeks, calcium, vitamins and essential oils need to be supplemented under the supervision of a qualified dietician. Such diets will lead to malnutrition in infants and small children and should only be done under medical supervision.

A typical restriction diet includes: