Food allergy and food intolerance

Written by Ellen Lees
Ellen Lees
Nutritionist Resource Content Team

Last updated 8th April 2024 | Next update due 8th April 2027

A food allergy is a reaction caused by the immune system. Symptoms can range from mild to severe and may affect the gut, skin, breathing ability or the body’s circulation system. Food intolerance is more common, though can be difficult to diagnose. These are not caused by an immune system reaction. Instead, they are caused by many factors, resulting in unpleasant symptoms which may last for several hours or even days.

On this page, we will explain more about the differences between allergies and sensitivity, food intolerances, and how working with a nutrition professional may be able to help you.

Allergy or sensitivity?

If you have a food allergy, your body's immune system will have an unusual reaction to certain foods leading to negative and sometimes very dangerous physical symptoms. Food intolerances are not caused by an immune system reaction and tend to be more common. According to the NHS, nearly two people out of every 100 in the UK have a food allergy and around one in five adults report having a food intolerance.

There are many different names used to describe the body’s adverse reaction to certain types of food, including food intolerance, food allergy and hypersensitivity. These names can often lead to confusion and misunderstanding of what a food allergy or food intolerance is. This confusion can then cause people to seek information and advice from unreliable sources.

In this video, nutritional therapist, Milvia Pili explains the symptoms to look out for and how working with a nutrition professional can help.

Food allergies

A food allergy is when the immune system reacts to certain foods. While they are often mild, they can be more severe and, in some cases, life-threatening. If the immune system mistakenly treats the food as a threat, it will release chemicals. When the chemicals are released, symptoms develop.

Symptoms of a food allergy include:

  • an itchy throat, mouth or ears, a rash (or hives)
  • vomiting
  • swelling of the face, lips or tongue

Almost any food can cause a reaction and people will respond differently to different foods. Yet there are certain foods recognised for commonly causing allergies, especially in children. Common allergens include milk, eggs, peanuts, fish and shellfish.

IgE food allergy and Non-IgE food allergy

Allergic reactions are made up of two key responses within the immune system. The first is the production of an antibody known as immunoglobulin E (IgE) which circulates within the bloodstream. The second response is the mast cell, a cell which occurs in body tissue but is common in sites of allergic reactions.

Children are more likely to suffer an allergic reaction than adults, especially those under three years old. This is often caused by another type of food allergy, known as a Non-IgE food allergy. This form is also caused by the immune system, but not through an antibody reaction.

Anaphylactic shock

Anaphylaxis or anaphylactic shock is a severe and potentially life-threatening allergic reaction. It can affect many areas of the body. A sufferer may experience breathing difficulties, dizziness and severe swelling of the lips, throat and hands.

Individuals who have a history of anaphylaxis may carry an auto-injector of adrenaline, the anti-allergy medicine used to treat the reaction. This should be injected into the outer thigh muscle and held in place for five to 10 seconds. Even if the individual experiencing a reaction carries an auto-injector, it should always be considered a medical emergency.

If you suspect you or someone else is having a reaction, then you must call an ambulance as quickly as possible. While this can be life-threatening, if treated quickly and effectively, most individuals will make a full recovery and not experience any long-term complications or side effects.


One of the most effective ways to manage a food allergy/food intolerance is to identify and then avoid the food that contains the suspected food allergen or trigger. This can be difficult, as some ingredients (i.e. wheat, nuts and milk) are found in many recipes and shop-bought products. One way to manage what you are eating is to read food labels. For foods sold without packaging such as in a bakery, cafe or pub, allergen information will have to be provided either in writing or verbally.

If you are still unsure, you may benefit from contacting a nutrition professional for support and advice. Before making any drastic changes to your diet, it is important to talk to a medical professional.

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Allergy testing

If you suspect you are suffering from an allergic reaction, you should visit your GP immediately. They will take a full and detailed medical history and, if possible, arrange the appropriate tests. Once the allergens have been identified, your GP will then be able to recommend treatment. If they are unable to carry out allergy testing, they may refer you to an allergy clinic for further assessment.

There are many allergy tests that you may take, depending on your reaction and type of allergy. If you develop symptoms quickly (an IgE food allergy) you may be given a blood test or a skin-prick test. If your symptoms develop over hours or days (non-IgE) you will typically be advised to go on a food-elimination diet.

Food elimination

If you are suspected to be suffering a non-IgE allergic reaction, you may be put on a food elimination diet. This will involve the suspected food being removed from your diet for two to six weeks before the food slowly being reintroduced. If you notice a recovery during the elimination, only for symptoms to return when reintroducing the food, you may have an allergy or intolerance.

Please do not attempt a food-elimination diet without discussing the changes with your GP or a nutrition professional.

Once the trigger food is identified, you should eliminate it from your diet. However, before starting any elimination diet, it is important to consult a registered nutritional therapist who will explain how to replace the trigger food from your diet. They will design a personalised, balanced diet plan for you, as your new diet should provide a full range of essential nutrients.

- Read Simona Novakovic BSc (Hons) Nutritional Therapy MSc Medical Sciences MBANT CNHC's article on Do I have food intolerences?

Food intolerance

An intolerance is not easy to diagnose despite being more common than a food allergy. While not life-threatening, symptoms can leave a person feeling unwell and interfere with daily life.

Food intolerance reactions do not involve the immune system so the causes of many food intolerances are unclear. Symptoms are often delayed, occurring hours later and sometimes lasting several days. The symptoms associated with food intolerance are usually gut-related, such as diarrhoea, bloating, constipation and skin problems such as eczema. It is common for people with food intolerance to experience multiple symptoms, but not all of these are recognisable. People may experience non-specific symptoms, such as headaches, brain fog and lethargy.

As it is possible to be intolerant to more than one food, it can be difficult to determine whether food intolerance is the cause of chronic illness and which foods are responsible.

What causes food intolerances?

Food intolerance can be caused by several factors. Certain lifestyles can affect how the body reacts to a specific food. If you suspect your lifestyle or eating habits may be a factor, you may benefit from seeking professional advice. A nutrition professional will be able to work with you to recognise the foods that may be causing symptoms and offer suggestions as to how you can avoid them.

Types of intolerance

While any type of food can potentially cause an adverse reaction in the body, there are some foods that more commonly cause a reaction than others. Individuals may experience lactose intolerance, gluten intolerance and wheat intolerance. Others may experience an intolerance to naturally occurring food compounds, such as caffeine.

Coeliac disease is an autoimmune disease which causes the individual to be intolerant to the protein, gluten. The severity of symptoms and side effects vary for each individual, but common symptoms include abdominal pain, diarrhoea and bloating.

According to Coeliac UK, the condition affects one in 100 people in the UK, though they estimate nearly half a million people are undiagnosed. In some cases, symptoms can be very mild. This often leads to the condition going undiagnosed or being misdiagnosed as a food intolerance or another digestive disorder.

How can a nutrition professional help with food intolerances and food allergies?

If you suspect you or someone you know may have a food allergy, a food intolerance, coeliac disease or gluten sensitivity, then it is important to seek a diagnosis from your doctor.

A qualified nutrition professional can help you understand the foods that are causing your symptoms and assist you in the successful implementation of an exclusion and reintroduction diet. They will advise you on how to avoid the confirmed/suspected allergen, to prevent symptoms while maintaining a healthy, balanced diet that meets your nutritional requirements.

Following any form of exclusion/elimination diet can be very dangerous if not done under the supervision of a qualified nutrition professional. This is due to the risk of missing out on vital nutrients, especially when major food groups are excluded. For example, excluding wheat and dairy needlessly can lead to nutrient deficiencies in the long term.

What can I expect when visiting a nutrition professional?

You may be asked to complete a food and symptom diary before your first appointment. This will give the dietitian/nutritionist a better picture of what you eat and which foods are likely to be causing symptoms.

During your first appointment, the nutrition professional will gather as much information as possible about you. This may include questions about your medical history and more detailed questions about your diet. From this, a personalised meal plan/diet with supporting written information will be provided for you.

For people with undiagnosed food intolerances, it is recommended that you return for a follow-up appointment four to six weeks after the first appointment. You can review your progress and make necessary adjustments to your diet, depending on whether your symptoms have resolved or not.

For people with a diagnosed food allergy, it may be that you only require one or two follow-up appointments to ensure that the correct foods are being avoided and that the diet is well-balanced. Further face-to-face appointments or contact via email can usually be arranged as required.

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