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A Practical Guide to
        Suspected Food Allergy

Written by Dr Adrian Morris

Part 1
 
§ Mechanism of пїЅtrueпїЅ Food Allergy
 
§ Mechanism of пїЅtrueпїЅ Food Intolerance
 
§ How does Food Allergy manifest?
 
§ The Common Offenders

Part 2
 
§ How to Diagnose Food Allergy
 
§ The Diagnostic Gold Standard
 
§ Provocation Testing
 
§ Food Allergy Prevention
 
§ Which Hypo-allergenic Milk Formula?
 
§ Avoid Certain Foods

Part 3
 
§ Pitfalls in Food Allergy Treatment
 
§ Enlist the help of a Dietitian
 
§ Hidden Sources of Allergen
 
§ Allergen Avoidance
 
§ A Controversial Allergic Condition
 
§ Further Reading

Food allergy is frequently  regarded as a type of пїЅfad topicпїЅ in the domain of fringe medicine.  This may often be due to a lack of objectivity in eliminating personal bias and psychological factors in the diagnosis.  Up to 15% of the population report having had adverse food reactions. The true prevalence of  food allergy is lower and seems to range from 1% to 4% of the general population, about 6% of the paediatric population but does occur in as much as 25% of children with eczema.  Consequently the general public perceive that food-related allergy is under-diagnosed, whilst most doctors feel that it is over diagnosed.  Many individuals have themselves noted that they are prone to develop adverse reactions to a specific food.  They have then dealt with the problem by simply avoiding that food without ever seeking the advice of a doctor.

Adverse reactions to food may be divided into four categories:

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Immune mediated reactions to food involve immediate hypersensitivity (IgE)  and delayed T-cell mediated responses (non-IgE).  These reactions may occur to minute amounts of the offending food in the diet. 

They depend on the bowel surface integrity and the individualпїЅs ability to mount an abnormal immune response.  This is true FOOD ALLERGY and accounts for 20% of adverse reactions to food.

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Non-Immune mediated  reactions involve  malabsorption due to intestinal enzyme deficiencies (lactose intolerance), adverse reactions to naturally occurring chemicals  in food (salicylates, histamine, serotonin and tyramine), as well as reactions to food containing contaminants (bacterial toxins), preservatives (sodium benzoate and sulphites), flavourants (mono sodium glutamate) and colourants (tartrazine).  These reactions may often be dose related and are referred to as FOOD INTOLERANCE.

Ingested toxins such as glycoalkaloids (in potato), cyanogenic glycosides (in beans), bacterial endotoxic contaminants, and free histamine from degradation of the amino acid histidine in scombroid fish results in scrombotoxicity – a pseudo allergic reaction typified by intense itching, tachycardia and flushing. This  reaction is due to the direct toxic effect of the spoiled food and is called FOOD TOXICITY.

Finally, some people are psychologically convinced that certain foods disagree with them, however, when tested fail to react to that food.  This occurs quite commonly and  is referred to as FOOD AVERSION.

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Mechanism of пїЅtrueпїЅ Food Allergy

Allergic sensitization to food will occur in infants if there is a breach in the bowel surface integrity followed by  an adverse immune response to a foreign food protein penetrating the intestinal surface.  Certain foods seem to be more allergenic than others (Table 1).

Fortunately enzymatic degradation of food in the intestinal tract tends to reduce this allergenicity.  Secretory IgA, an antibody, also plays a protective role by combining with the allergens on the intestinal epithelial surfaces, further  reducing allergen penetration.  Mast cell which release allergy mediators such as histamine are present in the gastrointestinal surface and are found in increased numbers in allergic individuals.  If intestinal barriers are weakened  by inflammatory disease or IgA deficiency, allergen sensitization and allergic disease is more likely to occur.

In infancy, reduced digestive capability and increased intestinal surface permeability are factors which enhance allergic sensitization to foods.  As the gastrointestinal surface barrier naturally improves with age , so the incidence of food allergy tends to decrease with age.  Food is most allergenic in the fresh form and cooking  reduces or eliminates this allergenicity.

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Mechanism of пїЅtrueпїЅ Food Intolerance

Additives such as Sulphur dioxide or sulphites affect up to 40% of children with asthma, they are commonly ingested in dried fruit, soft drinks and sprayed onto fresh produce.  [ read more ]   It is the sulphur dioxide gas which is eructed from the stomach that results in throat irritation and wheezing.  Allergic individuals also seem to have low levels of sulphite oxidase, the enzyme which normally metabolises sulphites in the diet. Sensitivity to Tartrazine the yellow food colourant is, despite itпїЅs lay publicity, quite a rare cause of food intolerance, it affects only 0.1% of the population.  This colourant seems to act by causing histamine release via non-immune mechanisms and so exacerbates the symptoms of allergy.  Certain allergic individuals with reduced levels of the histamine degrading enzyme Diamine Oxidase develop sneezing, flushing, runny nose, headaches and wheezing after ingesting foods rich in histamine.  This reaction seems to be quite common and is often confused with food allergy.  Histamine and Tyramine occur naturally in fish, certain cheeses, cured meats, some alcoholic beverages and Marmite.

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How does Food Allergy manifest?

Allergic  reactions to food may manifest in the form of nausea, vomiting, flatulence, abdominal pain, cramping, diarrhoea and with non gastrointestinal illnesses such as rashes, eczema, swelling, nasal allergy, wheeze, angioedema and in some cases, even life threatening anaphylaxis. 
 

Fresh fruit, vegetables and spices may cause local tissue reactions on the lips, in the mouth and in the throat – this is the so called Oral Allergy Syndrome (OAS).  Oral allergy tends to occur more commonly in individuals who are allergic to pollen. Some food allergic people may also develop flushing, shortness of breath, hives and fainting following strenuous exercise.  This condition is known as food related Exercise-induced Anaphylaxis.  It can occur for up to 24 hours after ingesting an offending food such as shellfish, celery and wheat.  Evidence now suggests that Coeliac Disease may be due to delayed T cell-mediated allergy to dietary gliadin found in wheat, rye barley and oats.  Preservatives and other additives tend to predominantly induce reactions outside the intestine such as rashes, wheezing and exacerbate eczema.

There is still considerable controversy as to whether  the hyper-activity syndrome and migraine are allergic in nature – so far scientific evidence is in conflict, but so-called Myalgic Encephalomyelitis (M.E.) and the Chronic Systemic Candidiasis  syndrome do not appear to have any food allergy basis.

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The Common Offenders

Only when the adverse reaction occurs immediately after eating the causative food may the diagnosis of  food allergy be easy to make.  Otherwise with delayed reactions to food, such as when cell mediated and immune complex mediated mechanisms are involved,  the offending food is very difficult to isolate.

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Infants tend more commonly to develop allergies to Hen egg white, Cows milk protein, Wheat, Peanuts, Bony Fish, Citrus and even Soya protein, whilst adults tend to be allergic to foods

 such as Bony Fish, Shellfish, Peanuts, Tree-nuts, Tomatoes and Chocolates. Although the above foods are more commonly implicated in food allergy, almost any food can be a potential allergen