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Food allergy or intolerance?

Many different terms are used to describe adverse reactions to food, including food hypersensitivity, food intolerance, food allergy and many other medical and non-medical terms. This is a source for confusion and has promoted a growing enterprise for inappropriately trained people to flog many alternative tests professed to diagnose allergies and intolerances.  So let me try and help you sort out a bit of the confusion.

Food Allergy

Food allergy is a reaction caused by our immune system’s reaction to a food, causing stressful and often severe symptoms. The immune system usually makes specific IgE antibodies to ‘fend off’ the allergens existing in these foods. Consequently histamines and other naturally occurring chemicals in the body are released, which subsequently cause inflammation. Tests called skin prick tests or specific IgE blood test can only confirm this type of allergy, when they are associated with clinical symptoms.

Symptoms caused by an allergic reaction to food can range from skin reactions, which include itching and rashes (urticaria); swelling (angioedema); gut symptoms, vomiting, stomach pain, diarrhoea; respiratory symptoms such as blocked or runny nose, coughing and wheezing. These symptoms usually develop quickly.

Severe cases (anaphylaxis) can be life threatening, necessitating immediate medical attention with adrenaline injection and admission to A & E. Symptoms may include swelling of the lips, tongue or face, throat constriction, breathing difficulties and quick pulse and heart rate. Loss of consciousness can occur in extreme cases. Typically symptoms start within a few minutes of consuming or coming in to contact with the offending food, although they may be delayed by one to two hours. Those at risk of anaphylaxis should carry or have an adrenaline device available at all times. The GP can prescribe this where appropriate.

There is another type of food allergy, known as Non-IgE food allergy, which is a true allergic reaction, as it is also caused by the immune system but is not caused by a specific antibody reaction. This happens most commonly in infants, although it occasionally carry on. The symptoms are usually delayed, from hours to two to three days, but can also cause severe symptoms and inflammation to the gut, accompanied with pain, vomiting, diarrhoea or constipation. It is frequently associated with moderate to severe dermatitis, sometimes to respiratory symptoms and is hardly ever diagnosed as a food allergy.

Luckily food allergy is quite rare, affecting approximately 2% of the adult UK population and up to 8% of children. If you suspect that you or your child may be suffering from a food allergy, speak to your GP who will be able to help you or may refer you for specialist advice to a hospital allergy clinic.

The foods that most commonly cause allergic reactions are peanuts, tree nuts (such as almonds, Brazil nuts, and walnuts), eggs, milk, fish, shellfish sesame, soya, wheat and in particular gluten. You should be aware that any food can cause an allergic reaction and some foods are more likely to cause reactions in certain ethnic minorities.

Key aspects of food allergy

  • Food allergy involves the body’s immune system and is a reaction to a specific food or foods

  • Symptoms can be mild or severe and can involve the skin, gut, breathing or the entire body’s circulation system

  • Some GPs have the skills to diagnose and manage foods allergies. Difficult cases or where multiple or severe food allergy exists, should be referred to allergy specialist service in a hospital

  • Patients with food allergy should be referred to a nutritionist or dietitian to help with the everyday management of their food allergy, as many lack adequate nutrition when avoiding whole food groups

  • IgE mediated food allergies are easier to diagnose and if the offending foods are totally excluded it is possible to remain completely symptom-free, although totally avoiding allergens can be very a challenge

  • Non-IgE mediated food allergy requires specialist diagnosis and is difficult to manage without the initial input from a nutritionist or dietitian

  • Reactions can frequently occur to very small quantities of foods so complete exclusion is critical

  • Some people can tolerate a well-cooked version of the food they are allergic to but will react to the food in its part-cooked or raw state. E.g. egg in a cake is often tolerated but the same person will react to boiled or scrambled eggs, and mayonnaise. Experimentation must be under the guidance of an allergist or specialist allergy nutritionist or dietitian who will subscribe an elimination diet

Food intolerance

Food intolerance is not as clear cut and the cause can be quite challenging to diagnose. Although not life threatening, it can and often does, make the sufferer feel extremely unwell and can have a major impact on working and life in general. Enduring symptoms can also affect the person psychologically as they might feel that they will never get better.

Food intolerance reactions do not involve IgE antibodies or the immune system. The mechanisms for most types of food intolerance are unclear. Reactions are usually delayed, taking place between several hours and sometimes up to several days after eating the offending food. The symptoms caused by these reactions are usually gut symptoms, such as bloating, diarrhoea, constipation and IBS, skin problems such as eczema and pain in the joints.

Symptoms can affect different people in different ways but may last for hours or days depending on the symptoms, and since it is possible to be intolerant to several different types of foods at the same time it becomes very hard to determine whether food intolerance is the cause of chronic illness and which foods may be the culprit. Many people with food intolerance have a number of symptoms. Sometimes the symptoms are ambiguous and not always easily diagnosed. People may complain of non-specific problems such as brain fog, lethargy, headaches or feeling bloated. These are frequently additional to more major problems relating to bowels and skin.

Food intolerance can be caused by diverse factors, such as lifestyles with erratic food intake and poor nutritional intake or high intakes of refined foods, poor intake of dietary fibre or high fat diets.

Often, gastro-intestinal infection can trigger ongoing symptoms such as pain or loose stools after consuming certain types of foods, for example lactose in milk. The length of time this might take varies but after eliminating the problem foods, usually for some months, they can usually be slowly reintroduced over time. Some people lack the enzymes which are required to digest foods. For example in lactose intolerance, the enzyme ‘lactase’ is not produced in large enough quantity in the gut to break down the lactose (milk sugars) in animal milk. This is common in some parts of the world but uncommon in Caucasian Europeans. Others may react to the chemicals that are created naturally in foods such as caffeine, salicylates and histamine in foods like strawberries, chocolate and aged cheese. An additional cause of food intolerance is additives in foods, such as sulphites, which are added to processed foods in order to give them a longer shelf life. They are also found in fruit drinks and wine. A reaction to a food that has ‘gone off’ such as salmonella poisoning is toxic reaction to a food, which will typically affect anyone consuming it.

Key aspects of food intolerance

  • Typically reactions are delayed and symptoms may take several hours, or even several days to show up

  • Multiple symptoms can take place and be many and varied, from migraine to bloating, diarrhoea, lethargy, joint pain and a general feeling of poor health

  • Chemicals in foods such as caffeine, salicylates, MSG and naturally occurring chemicals like histamine, can also cause food intolerance responses

  • Reactions can happen after ingesting small amounts of an offending food but are commonly triggered by larger amounts – some people report they can tolerate a food if eaten once a week or in small daily portions but any more than this causes symptoms to appear.

Getting a diagnosed

  • Before starting an elimination diet you should consult your GP who may refer you to a nutritionist or a dietitian

  • To help with the diagnosis be sure to keep a food and symptoms diary

  • Apart from lactose intolerance, which can be diagnosed by a GP, there is nothing that diagnoses food intolerance with 100% certainty but this test is as good as it gets:

  • Keep a diary of all suspected food associated reactions, with written records, dated photographs with labels of suspected packaged foods, will help to pin-point which foods may be causing your symptoms. Sometimes the food triggers will be obvious whilst other reactions are extremely difficult to identify, when you will be advised to seek the help of a nutritionist or a dietitian.


Not all allergies and intolerances are food related. Some can be quite bizarre in nature.

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