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What is the difference between a food allergy and a food intolerance?

What is the difference between a food allergy and a food intolerance?Photo from Unsplash

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What is the difference between a food allergy and a food intolerance?

Your immune system has evolved to effectively protect your body from damaging invaders such as bacteria and viruses. Allergy occurs when your body’s immune system goes into overdrive and recognises a harmless substance (allergen), such as food or pollen, as harmful and reacts to it. This reaction can range from uncomfortable to dangerous.

In the majority of patients with food allergies, there is an initial overproduction of IgE antibodies to a harmless substance and when this substance is encountered again by the body, the IgE antibodies lead to activation of cells called mast cells. Mast cell activity can cause unpleasant symptoms, such as itching, sneezing and wheezing.  It can also sometimes lead to dangerous effects, such as difficulty breathing, throat swelling and low blood pressure, leading to anaphylactic shock and death. Much more rarely, allergies can be caused by other cells of the immune system and not IgE antibodies and we will discuss this later.

Food intolerances, on the other hand, do not involve the body’s immune system but instead are an issue with digestion. The commonest food intolerance is to lactose.

How can I tell if I have a food allergy or a food intolerance?

‘I know that something isn’t agreeing with me. I get so much tummy pain and bloating, made worse by certain meals. I must have an allergy. I think I need to be referred for allergy tests.’ We regularly have patients presenting similarly.

Most allergies to foods cause quick reactions, usually within one hour of eating. They will also occur every time you eat the allergen. The commonest food allergens are peanuts, tree nuts, shellfish, eggs, cow’s milk and wheat. [i] If an allergy is suspected, we would usually recommend a referral to an allergist who will take a detailed history of the event(s) and likely run blood tests for IgE antibodies to the food and any related foods. They may also carry out skin prick tests. If allergy is confirmed, you will normally be told to avoid that food and traces of it. You will typically be prescribed an epipen.

Food intolerances, on the other hand, usually cause undesirable symptoms up to hours after eating. Some patients can relate several unpleasant symptoms to one food type eg headache, nausea, bloating and stool changes. Other patients feel that multiple foods trigger their symptoms. If you cannot tell which food is causing your symptoms, it probably means you do not have an allergy and it is unlikely that one diagnostic test will point towards the underlying cause. We often recommend keeping a food diary and trialling an elimination diet to see if symptoms resolve when the food is not eaten over a period of time.  It may mean you have irritable bowel syndrome and it is worth discussing your symptoms with a GP.

Can I be tested for lactose intolerance?

We are often asked about lactose intolerance. In this case, the body lacks enough lactase enzyme to digest lactose. Symptoms may include tummy pain, bloating, excess gas, nausea or bowel changes. Lactose is present in dairy foods, such as butter, yoghurt and cheese as well as in some processed foods. If suspected, we will often recommend an elimination diet of lactose-containing foods to see if this improves your symptoms. Some labs offer a hydrogen breath test where the amount of hydrogen in your breath is measured after drinking a lactose-containing drink.  There are also breath tests for methane. These tests are not always reliable.

What about cow’s milk protein allergy?

Cow’s milk protein allergy falls into the much rarer category of non Ig-E mediated delayed food allergies. Cow’s milk protein allergy is said to affect 1% of babies. It can cause symptoms of reflux, vomiting, diarrhoea, constipation, inconsolable crying and difficulty feeding. Symptoms typically occur several hours or even days after ingesting cow’s milk. It can also be associated with persistent eczema or nasal congestion. It can be hard to diagnose as many healthy newborns have some of the symptoms above as their guts mature. If you are concerned that your baby might have cow’s milk protein allergy after reading the above, please get in touch with one of us.

I think I have an allergy to wheat or gluten. Can I be tested for it?

Some people have an Ig-E mediated allergy to wheat whereby they have typical allergic symptoms (eg rash/mouth swelling/itching) within an hour of ingesting wheat; and this can be dangerous, as above. Many more people have digestive issues, such as tummy pain, bloating and diarrhoea, associated with eating gluten.  Coeliac disease is a non Ig-E mediated autoimmune disorder whereby the body’s immune system reacts to the gluten protein in food, causing inflammation and destruction of the small intestine lining. There are blood tests which can confirm a diagnosis of coeliac disease, which is important as further consumption of gluten could cause lasting damage. Many people with digestive issues related to eating gluten or wheat will test negative for coeliac disease. These people may have gluten sensitivity or an intolerance to fructans in food (found in wheat, barley and rye)[ii]. There are some blood tests which can point towards your likelihood of having gluten insensitivity but the best way to tell if you have this is by eliminating gluten and seeing if your symptoms improve.

What does it mean if I develop an uncomfortable feeling in my mouth after eating certain foods?

Some people will find that they develop an itchy mouth or uncomfortable throat after eating certain raw fruits, vegetables and nuts. They will usually not have other symptoms, which indicate a more serious food allergy. This condition is known as oral allergy syndrome or pollen food syndrome. It usually occurs in hayfever sufferers (who are allergic to pollen) and the immune system is confused between similar proteins in these foods to proteins found in some pollens. Treatment with an anti-histamine is usually effective.

Why do I have multiple allergies?

Those with one allergic disorder are more prone to having others. For example, some people who have eczema then go on to develop asthma and hayfever and even a food allergy. We also know that a predilection for allergies runs in families. It is thought that a genetic variant leads to the production of IgE antibodies to harmless substances (eg peanuts/pollen). However, very interestingly, we know that environment also plays a role. One study showed that South-East Asian children born in  Australia are 1400 percent more likely to develop a food allergy compared to South-East children born in Singapore[iii].Another study, called the LEAP study, showed that early consumption of peanuts in infancy leads to a lower risk of developing peanut allergy[iv]. It is also believed that a lack of bacterial gut diversity can lead to more allergy as gut bacteria are important for signalling to immune cells not to react to harmless substances[v].

If you suspect you have a food allergy or intolerance, it’s better to seek medical guidance rather than self-diagnosing. Good management may improve your quality of life and help prevent potentially serious health issues. Each person’s reaction to food can be unique, and personalised medical advice is the best course of action.

To book an appointment, please use our online booking site here or give us a call on 0207 245 9333.



[iii] A.E. Clarke et al (2021), ‘Demographic characteristics associated with food allergy in a Nationwide Canadian Study’ Allergy, Asthma and Clinical Immunology Official Journal of The Canadian Society of Allergy and Clinical Immunology, 17 (1)

[iv] LEAP | A clinical trial investigating how to best prevent Peanut Allergy (

[v] ‘Understanding Allergy’, by Dr Sophia Farooque 2022

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