Food allergy is almost always the result of sensitization of body cells by a food protein which has been absorbed into the blood intact. Food protein is normally digested to its component amino acids which are then transferred to the blood, but very minute quantities of food protein can sometimes escape this digestion and traverse the intestinal wall. This undesirable event can occur at any age but is most likely during infancy, which is why foods known to produce allergy (shellfish, egg white, cows’ milk, wheat, nuts, fish) should not be fed to infants, especially those with a family history of allergy. If it is necessary to feed these allergy-producing foods to infants the foods should be thoroughly cooked. This changes the proteins sufficiently to make them less likely to cause allergy. Many tissues of the body can become sensitized, particularly the lungs, the skin and the eyes and the allergic reactions may be acute or chronic, immediate or delayed for several hours. Once tissues have become sensitized to a food the change may be life-long or it may slowly disappear over the years if that food is avoided. Tissues can become sensitized to more than one food so that several may have to be omitted from the diet. Among the common illnesses caused by allergy to food are skin rashes, blisters, asthma, conjunctivitis, abdominal pain, diarrhoea and headache. All these conditions, of course, have other more common causes. True food allergies are uncommon and difficult to diagnose. Breast milk may contain intact proteins from the mother’s diet and a susceptible infant can become sensitized to these proteins. In a family with allergy it is advisable for the mother during lactation to heat thoroughly any foods to which her family are sensitive.
Why some children develop a food allergy and some do not is not known, although family histories of allergic conditions strongly suggest that there is an inherited trait. Where both parents have allergic reactions there is a more than 50 per cent chance that their children will also develop one. The commonest foods to cause allergy in young children are eggs and cows’ milk, but this may be because these two foods are fed often and may be insufficiently heated. When thoroughly heated, egg white and cows’ milk may be fed without harm. Allergic children often develop eczema and in many of them cows’ milk may cause bleeding into the intestine. This intestinal blood loss may occasionally produce anaemia. In some extreme cases, especially in allergy to nuts, a bronchial allergic response may be so great as to cause death by suffocation. Children under the age of three years from a family with hay fever, asthma or eczema should not be given foods containing peanuts.
Treatment of allergy
If the allergic response is immediate there is usually little difficulty in deciding which food caused the reaction, which may come on within a few minutes of eating it. In some cases, however, the allergic response may be delayed for several hours and an accurate diagnosis is then more difficult. The only satisfactory treatment is to avoid totally the offending item of food on a permanent basis. This can be very hard to do. Persons allergic to nuts need to be particularly cautious because ground-up nuts are often used as fillers in many processed foods (there should be a warning on the label). Desensitization can be tried but it is very rarely satisfactory and can be hazardous. Antihistamine drugs are sometimes of value, especially for skin events, while drugs which dilate the bronchi are valuable in asthmatic attacks. Other treatments under medical supervision are available.
This is a condition in which a particular food causes one or more undesirable effects which are not mediated by the immune system. These reactions are not true allergies and they are far more common than true allergies. For example, some foods may cause hyperacidity with acid regurgitation, an exceptional sense of fullness, generalized abdominal discomfort, nausea and diarrhoea. Intolerance to lactose (milk sugar) occurs frequently in non-white adults and is brought about by a lack of a sufficient amount of the enzyme lactase in the small intestine, so that intact lactose enters the large intestine where micro-organisms produce excess gas and lactic acid from the sugar, causing abdominal cramps and diarrhoea. Another example is intolerance to fat in people who cannot adequately digest it, resulting in excess colonie gas being formed and substances which induce diarrhoea. A few people react excessively to caffeine in tea and coffee and may get palpitations and a sense of anxiety.