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Allergy or Intolerance?

Many different names are used to describe adverse reactions to foods, including food hypersensitivity, food intolerance, food allergy and many other medical and non-medical terms. This causes confusion and has promoted a growing enterprise for inappropriately trained people to sell many alternative tests purported to diagnose allergies and intolerances.

Food Allergy

Food allergy is a reaction caused by the immune system’s reaction to a food, causing distressing and often severe symptoms. The immune system usually makes specific IgE antibodies to ‘fight off’ the allergens found in these foods. This results in the release of histamine and other naturally occurring chemicals in the body, which subsequently cause inflammation. Tests called skin prick tests or specific IgE blood test can only confirm this type of allergy, when they correlate 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, tummy pain, diarrhoea; respiratory symptoms such as blocked or runny nose, coughing and wheezing. These symptoms usually develop rapidly.

The most severe cases (anaphylaxis) can be life threatening, requiring 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 rapid pulse and heart rate. Loss of consciousness can occur in extreme cases. Normally symptoms start within a few minutes of eating 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 if it is necessary.

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 occurs most commonly in infants, although it occasionally persists. The symptoms are usually delayed, from hours to two to three days, but can also cause severe symptoms and inflammation to the gut, with pain, vomiting, diarrhoea or constipation. It is frequently associated with moderate to severe eczema, sometimes to respiratory symptoms and is rarely diagnosed as a food allergy.

Thankfully food allergy is quite rare, affecting approximately 2% of the adult UK population and up to 8% of children. Should you suspect that you or your child may be suffering from a food allergy, you should 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, brazils, walnuts), eggs, milk, fish, shellfish sesame, soya, wheat. You should be aware that any food can cause an allergic reaction and some foods are more likely to cause reactions in certain ethnic groups.

Key points on 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 whole 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 an NHS allergy specialist service in a hospital as recommended by NICE 2011 (National Institute of Clinical Excellence)
  • Patients with food allergy should be referred to a registered dietitian to help with the practical management of their food allergy, as many lack adequate nutrition when omitting whole food groups
  • IgE mediated food allergies are easier to diagnose and if the culprit foods are totally excluded it is possible to remain completely free from any symptoms, although totally avoiding allergens can be very difficult
  • Non-IgE mediated food allergy requires specialist diagnosis and is difficult to manage without the initial input from a registered dietitian
  • Reactions can often occur to trace amounts of foods so complete exclusion is essential
  • Some people can tolerate a well-cooked version of the food 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 and scrambled eggs, and mayonnaise. Experimentation must be under the guidance of an allergist or specialist allergy Dietitian.

Food intolerance

Food intolerance is not so clear cut and the cause can take some time 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 social life. Ongoing symptoms can also affect the person psychologically as they feel 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, occurring 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 joint pain.

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

Food intolerance can be caused by different factors, such as lifestyles with erratic food intake and poor nutritional intake or high intakes of refined foods, poor intakes of dietary fibre or high fat diets. Commonly, gastro-intestinal infection can trigger ongoing symptoms such as pain or loose stools after consuming certain foods, for example lactose in milk. The duration of this varies but after excluding the problem foods, usually for some months, they can usually be slowly reintroduced in time. Some people lack the enzymes needed to digest foods. For example in lactose intolerance, the enzyme ‘lactase’ is not produced in the gut in large enough amounts to break down the lactose (milk sugars) in mammalian milks. This is common in some parts of the world but uncommon in white Europeans. Others react to the chemicals that are produced naturally in foods such as caffeine, salicylates and histamine in foods like strawberries, chocolate and ripe cheese. Another cause of food intolerance is additives in foods, such as sulphites, which are added to processed foods 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 usually affect anyone consuming it.

Key points on food intolerance

  • Usually reactions are delayed and symptoms may take several hours, or even several days to appear
  • Multiple symptoms can occur 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, monosodium glutamate and naturally occurring chemicals like histamine, can also cause food intolerance reactions
  • Reactions can occur after ingesting small amounts of a culprit food but are usually 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.

Getting a diagnosis

  • Before commencing on an elimination diet you should consult your GP who may refer you to 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 are NO tests that are scientifically proven to diagnose food intolerances
  • Keep a diary of all suspected food related reactions, with written records, dated photographs with labels of suspected packaged foods, will help to identify which foods may be causing your symptoms. Sometimes the food triggers will be obvious whilst other reactions are very difficult to identify, when you will need the help of a registered dietitian.




Last update: October 2015                           Next review date: October 2018
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