The natural history of food allergy in adults is slightly different from that in children, who tend to rapidly outgrown their allergies. As a result food allergy is more common in children but most will outgrow it. Up to 6% of children suffer with true food allergy, while only 1-2% of adults have true IgE mediated food allergy.

If foods are completely avoided, up to 30% of adults will become clinically non-reactive to an offending food over a 2-year elimination period. They will however remain atopic - that is maintain a positive Skin Prick Test or retain specific IgE antibodies to the food, but have no reaction if they eat the food. Certain foods are highly allergenic and allergy to them is unlikely to be outgrown - these are usually foods with heat resistant allergens. Peanuts, Nuts, Shellfish and Fish fall into this category. Food allergy also varies from country to country depending on local eating habits - the more a food is consumed in a country - the higher the incidence of allergy to it. In the USA we see plenty of Peanut allergy, In Scandinavia we see predominantly fish allergy, in Eastern Europe it's Poppy Seed allergy, Sesame allergy in the Middle East, while in Japan, Rice allergy is a significant problem.

Food Allergy has always been a bit of a Cinderella Science often the topic of Media Fads and Fringe Medicine. It is falsely perceived to be the domain of controversial alternative medical practitioners and as a result is generally frowned upon by mainstream academic medicine. However, there can be some very serious and life threatening consequences of food allergy. Consider the dire results of Fish and Peanut Anaphylaxis.

At our Allergy Clinics at the Guildford Nuffield Hospital and at the London Medical Centre, 90% of our referrals are for suspected food allergy. Patients are very aware of the symptoms of food allergy, which can be quite alarming. The prospect of a life-threatening anaphylactic reaction is always a major public concern. Patients may present with urticaria that they attribute to food allergy. An increasingly common food allergic presentation are oral symptoms such as itching and swelling of the mouth and throat obstruction with respiratory difficulties.

Successful testing for allergy can be very difficult without a clear history implicating a particular food. The tests we have available are often not 100% specific. We see both false negative and false positive skin and blood tests and then fresh food challenge testing may be necessary.

The subject becomes less confusing if we use a simple classification for all Adverse Reactions to Food This Food Hypersensitivity includes all reproducible food related reactions and may be both Allergic and Non-allergic. I however divide Adverse Reactions to Food into four distinct groups.

Firstly, classical True Food allergy, as in Peanut anaphylaxis - where we have an immediate catastrophic IgE immune mediated reaction. This will require emergency adrenaline injections and medical resuscitation.

Next is the group of Food Intolerance which are not immune mediated but may be due to enzyme deficiencies, and other mechanisms that mimic true allergic reactions.

Biological contaminants or poison present in the food causes Food Toxicity, which then cause recognisable toxic reactions.

Finally, there is Food Aversion where the patient is convinced that they are allergic to a food, but when challenged with the food, fail to have any reaction.

True Food Allergy is an Immediate IgE Mediated Allergic Hypersensitivity reaction, which involves antibodies that cause tissue Mast Cells to release histamine, resulting in tissue inflammation and swelling. A small protein particle called an allergen is responsible for triggering the antibody response. The antibody called Immunoglobulin E (IgE) accounts for 90% of True Food Allergic reactions. A reaction can occur to minute traces of the offending allergen and in exquisitely sensitive individuals, even airborne food allergen can trigger anaphylaxis - as in fish and peanut allergy.

Increasingly we see Delayed T-cell mediated food allergic reactions - these take over 24 hours to evolve and are typically seen with Contact Dermatitis in adults and Coeliac Disease in children. We refer to these as Non-IgE Allergic Hypersensitivity reactions

Classical IgE Mediated food allergy presents with immediate Urticarial rashes, tissue angioedema of the face and neck and even anaphylaxis with shock and circulatory collapse. In adults, the foods commonly implicated in anaphylaxis are Peanuts, Tree nuts, Shellfish, Fish and Egg.

Another interesting immediate food allergic phenomenon is that of the Oral Allergy Syndrome. Here the allergic reaction is localised to the mouth and throat and is triggered by allergy to Fruit and Vegetables such as Apple, Peach, Celery, Tomato and Cherry.

In adults we also see exacerbations of eczema and rhinitis in food allergy but rarely isolated asthma due to food allergy. Other manifestations involving the lower GI tract are vomiting and diarrhoea. Food-induced Oesophagitis and Enteropathy is more common in children. Frequently patients think they are allergic to one food but may end being allergic to 3 or more foods after evaluation.

Food Intolerance (or Non-Allergic Hypersensitivity) is an adverse reaction that is not immune mediated and generally doesn’t lead to anaphylaxis as no specific IgE response is generated. Reactions are dose-dependant - smaller amounts of the offending food are tolerated, but at a certain dietary threshold a clinical response will occur.

This may be due to an enzyme deficiency as in Lactose Intolerance. Here the enzyme Lactase is depleted resulting in cramps, flatulence and frothy diarrhoea after drinking cows milk. It is an inherited trait and affects up to 10% of the population, beginning in teenage years and gets worse with advancing age. Lactose Intolerance is commoner in Afro-Americans, Hispanics, Asians and Mediterranean populations. Lactose intolerance is reliably diagnosed with a "hydrogen breath" test or reducing sugars measured in the diarrhoeal stool. Intolerance to Sucrose in table sugar and fruit can lead to similar symptoms if there is a deficiency of the intestinal enzyme which breaks sucrose down to absorbable fructose and glucose.

Food additives such as colourants and preservatives may trigger pseudo-allergic reactions. These are not IgE mediated, but possibly involve direct Mast Cell and Basophil Histamine Release. Histamine may even occur naturally in foods and when absorbed in the GI tract, the histamine content triggers an allergic-like reaction. A similar reaction can also be seen with naturally occurring dietary salicylate in aspirin sensitive people. Other chemicals found in food can cause ill effects such as caffeine induced palpitations. Capsaicin in chilli’s induces a typical "hot curry" burning oral sensation. While alcohol induced flushing and nasal congestion due to local vasodilatation.