In children, common allergy-provoking foods include cow's milk protein, egg whites, wheat, soya bean, cod fish and peanuts. Contact your GP to discuss any suspected food allergies or intolerances before putting a child on a restricted or elimination diet. Self-diagnosis can lead to malnutrition. 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.
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, 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.
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, 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.
