Some of the signs and symptoms of a food allergy and a food intolerance can overlap, and as a result, many people often confuse the two. It is important to gain an understanding in regard to the differences between a food allergy and a food intolerance in order to be better prepared to handle them. 

In this article you can find:

  • What is a food allergy?
  • What are the symptoms of a food allergy?
  • What is a food intolerance?
  • What are the symptoms of a food intolerance?
  • How do you diagnose a food allergy or a food intolerance?

What is a Food Allergy?

A food allergy is when the body’s immune system reacts negatively to the ingestion of proteins found in a particular food (Sicherer and Sampson, 2009). The immune system (the body’s defence against infection) mistakenly treats these proteins found in the food as a threat. As a result, the body releases numerous chemicals (such as histamine) in attempt to defend itself against the threat, which causes the symptoms of the allergic reaction (MacGlashan, 2003).

Nearly any food can cause an allergic reaction, but there are eight types of foods that are responsible for the majority of food allergies (HSE, 2021). These foods are:

  • Eggs
  • Milk
  • Soya
  • Wheat
  • Peanuts
  • Crustaceans (crab, lobster, prawns)
  • Tree nuts (walnuts, brazil nuts, almonds, pistachios)
  • Fish

Food allergies are classed into 3 groups, depending on symptoms and when the symptoms occur (NHS, 2021).

  • IgE-mediated food allergy 

IgE-mediated food allergies are triggered by the immune system producing an antibody called immunoglobulin E (IgE). Symptoms occur almost immediately after coming in contact with the allergen.    

  • Non-IgE-mediated food allergy  

Non-IgE-mediated food allergies aren't caused by immunoglobulin E, but by other cells in the immune system. This type of allergy can be difficult to diagnose as symptoms can take up to several days to develop. 

  • Mixed IgE and non-IgE-mediated food allergies 

Some individuals may experience symptoms from both IgE-mediated food allergies and non- IgE-mediated food allergies. This type of allergy is referred to as a mixed IgE-mediated and non-IgE-mediated food allergy.  

What Are the Symptoms of a Food Allergy?

Symptoms of an IgE-mediated food allergy include:

  • Tingling or itching in the mouth or throat
  • A raised itchy red rash, or red and itchy skin
  • Swelling of the face, mouth, throat, or other areas of the body 
  • Difficulty swallowing 
  • Shortness of breath 
  • Feelings of dizziness or light-headedness
  • Nausea or vomiting 
  • Abdominal pain
  • Diarrhea
  • Hay-fever like symptoms (sneezing, watery eyes, itchy eyes)

Symptoms of a non-IgE-mediated food allergy include:

  • Hives (red itchy welts on the surface of the skin)
  • Itchy skin or rash
  • Nasal congestion (rhinitis)
  • Vomiting
  • Diarrhea
  • Abdominal cramps
  • Constipation

While the symptoms of a food allergy are often mild, they can be severe. Severe allergic reactions are referred to as anaphylaxis. Without immediate treatment, anaphylaxis can be life-threatening.

Symptoms of anaphylaxis include:

  • Difficulty breathing
  • Difficulty swallowing or speaking
  • Swelling of the face, eyes, or tongue
  • Pain or tightness of the chest
  • Feelings of dizziness (vertigo), light-headedness or weakness
  • Abdominal cramping or pain
  • Flushing of the face
  • Diarrhea
  • Nausea or vomiting
  • Heart palpitations
  • Wheezing
  • Unconsciousness

What Is a Food Intolerance?

A food intolerance refers to the body’s difficulty in digesting a food which can cause unpleasant physical reactions (Tuck et. al, 2019). This impairment may be due to a lack of digestive enzymes or a sensitivity to certain chemicals. Unlike a food allergy, a food intolerance does not involve the immune system, thus there is no allergic reaction, and it is not life-threatening. Moreover, a food intolerance is only triggered when a substantial amount of the particular food is consumed, in comparison to a food allergy which can be triggered by the presence of a small particle (Sicherer and Sampson, 2009). Some people with a food intolerance find that they can tolerate a certain amount of a food before experiencing any symptoms. However, the onset may occur several hours after consuming the food, and may persist for several hours or days.

Some common food intolerances include:

  • Dairy
  • Gluten
  • Caffeine
  • Amines (present in high amounts in aged, overcooked or processed meats, and overly ripe fruits)
  • FODMAPs (fermentable oligosaccharides, disaccharides, monosaccharides and polyols short-chain carbohydrates)
  • Preservatives (sulphites, salicylates)
  • Fructose
  • Artificial sweeteners (aspartame)
  • Flavour enhancers (MSG)
  • Food colourings
  • Toxins (alcohol)
  • Histamine (present in high amounts in fermented dairy products, fermented vegetables, pickled vegetables, cured or fermented meats, kombucha)

What Are the Symptoms of a Food Intolerance?

Symptoms of a food intolerance include:

  • Abdominal pain
  • Excess gas
  • Bloating
  • Diarrhea
  • Headaches
  • Runny nose
  • Malaise (feeling under the weather)

If a food intolerance is left untreated it can also result in:

  • Weight loss
  • Lethargy
  • Anemia
  • Migraines
  • Psychological symptoms (confusion, depression)

How Do You Diagnose A Food Allergy and Food Intolerance?

Your doctor can diagnose a food allergy or a food intolerance. If you are experiencing symptoms of a food allergy or food intolerance, the doctor may perform and exam and analyse your medical history (sometimes symptoms of other disorders or conditions are similar to symptoms of a food intolerance or a food allergy, such as irritable bowel syndrome, inflammatory bowel disease, coeliac disease, stress, and anxiety). They may also ask a range of questions at first, such as, what are the type of symptoms you experience, how long does takes for your symptoms to develop, whether you eat the food raw or cooked, or whether you were eating/generally eat the food at home or elsewhere.

If your doctor believes it is an allergy causing your symptoms, they may then begin a series of allergy tests. This includes:

  • Skin tests (this test involves applying a small amount of the suspected allergen to the skin and watching for a reaction)
  • Blood tests (blood tests checks the blood for antibodies (a protein the body produces to fight harmful substances) against a possible allergen
  • Exclusion diets and food diaries (this form of testing involves eliminating the suspected allergen from the diet for several weeks and watching for symptoms when the food is again)

A food intolerance can be more difficult to diagnose, particularly when a person has several food intolerances. There are many companies that produce food intolerance tests, however they are not based on scientific evidence and are not recommended by Irish Nutrition & Dietetic Institute (INDI, 2021). The best means of determining whether or not you have a food intolerance is to follow an exclusion diet (also known as an elimination or diagnostic diet) and take note of symptoms in a food diary. Taking note of which foods you eat, the symptoms that appear, and their timings can you and your doctor determine which foods are causing adverse reactions.

Some people discover that if they avoid a certain food for a period of time, they have no reaction when they consume it again. This is known as tolerance. Maintaining tolerance is often a matter of determining how long to refrain from consuming the food and how much to consume when it is reintroduced. Your doctor can also help you with this.



  1. ie. 2021. Symptoms of a food allergy - [online] Available at: <,-food/symptoms-of-a-food-allergy.html> [Accessed 1 May 2021].
  2. ie. 2021. Food Intolerance Testing - INDI. [online] Available at: <> [Accessed 1 May 2021].
  3. MacGlashan, D., 2003. Histamine. Journal of Allergy and Clinical Immunology, 112(4), pp.S53-S59.
  4. uk. 2021. Food intolerance. [online] Available at: <> [Accessed 1 May 2021].
  5. Sicherer, S. and Sampson, H., 2010. Food allergy. Journal of Allergy and Clinical Immunology, 125(2), pp.S116-S125.
  6. Tuck, C., Biesiekierski, J., Schmid-Grendelmeier, P. and Pohl, D., 2019. Food Intolerances. Nutrients, 11(7), p.1684.
  7. Yu, W., Freeland, D. and Nadeau, K., 2016. Food allergy: immune mechanisms, diagnosis and immunotherapy. Nature Reviews Immunology, 16(12), pp.751-765.