What is a food intolerance?

A food intolerance is an adverse reaction to a food or foods and is thought to affect 15-20% of us. The condition generally involves the digestive system, with the amount of food eaten directly influencing the type and severity of symptoms – which may occur a few hours after eating. It’s not the same as a food allergy, as you'll see below.


It’s unclear why some people experience food intolerances, although the most common reason given is a ‘leaky gut’. The condition is common among those with Irritable Bowel Syndrome (IBS) and other similar gut problems, with up to 80% of IBS sufferers reporting consistent problems with certain foods. For others, food intolerance is the result of an inability to break down a food, an example being lactose intolerance which is when the sugar (lactose), naturally found in milk, is not fully digested.

A food intolerance can make you feel quite unwell and, in some cases, have a significant impact on your work and social life. However, it’s important to remember that a food intolerance is not life threatening.

Discover our full range of health guides and learn more about food intolerances, such as how to determine whether you have one.

What’s the difference between a food intolerance and an allergy?

A food allergy occurs when your immune system overreacts to a food. Allergy symptoms happen quickly, in some cases within 10-20 minutes, although a reaction can take up to 2 hours. For those with an immunoglobulin E (IgE)-mediated food allergy, just small amounts of the food can cause a severe and even life-threatening reaction called anaphylaxis.

More like this
Glass of milk

What are the most common food intolerances and their symptoms?

Food triggers vary from person to person but some of the most common include:

Intolerant to lactose

Why am I intolerant?

Lactose is the natural sugar in milk and dairy (from cows, goats and sheep). A deficiency in the digestive enzyme lactase means lactose is not completely broken down.

What might my symptoms be?

• Wind
• Bloating
• Abdominal cramps
• Diarrhoea
• Eczema

Intolerant to gluten

Why am I intolerant?

Gluten is a protein found in certain grains such as wheat, barley and rye. This sensitivity is not associated with Coeliac disease or a wheat allergy but may be due to a compromise in gut health.

What might my symptoms be?

• IBS-like symptoms
• Headache
• Fatigue
• Fibromyalgia-like symptoms
• Skin rash, flushing and itching
• Depression
• Anaemia

Intolerant to histamines

Why am I intolerant?

Certain foods are naturally high in histamine, and these include spinach, avocado and fermented foods. A deficiency in the enzyme(s) that break down histamine leads to a build-up in the body

What might my symptoms be?

• Nasal congestion
• Runny nose
• Sinus problems
• Headaches
• Digestive issues
• Menstrual irregularities

Intolerant to fructose

Why am I intolerant?

Fructose is the natural sugar found in fruit, vegetables, honey and agave syrup. An intolerance may be due to one or more of the following:

• An imbalance in gut bacteria
• An inability to absorb simple carbs such as fructose (along with other carbs known as FODMAPs)
• A hereditary deficiency in a necessary digestive enzyme
• A genetic disposition causing a deficiency in a necessary liver enzyme

What might my symptoms be?

• Bloating
• Reflux
• Abdominal discomfort
• Diarrhoea
• Poor blood sugar control

Intolerant to FODMAPs

Why am I intolerant?

FODMAPs are a group of simple carbs (sugars) that include fructose. For one or more reasons the gut does not digest and absorb these carbs allowing them to pass through the intestine attracting water and being fermented by gut bacteria.

What might my symptoms be?

• Altered bowel habits
• Cramping
• Abdominal pain
• Bloating
• Excess wind

Intolerant to salicylates

Why am I intolerant?

Salicylates are found in cocoa, tea, dried herbs and aspirin, and are also used as a food preservative. They may lead to the over-production of chemicals that initiate inflammation.

What might my symptoms be?

• Sinus problems
• Nasal congestion
• Nasal polyps
• Headache
• Skin itching, rash or hives
• Swelling of the face, hands or feet
• Stomach upset
• Gut inflammation

Intolerant to caffeine

Why am I intolerant?

Caffeine is in coffee, tea, energy drinks and cola. A genetic disposition results in lower levels of the liver enzyme (CYP1A2) that metabolises caffeine.

What might my symptoms be?

• Anxiety
• Rapid heartbeat
• Insomnia

Woman lying down with head and stomach ache

How can I test for a food intolerance?

A food intolerance test typically involves a blood sample that may be obtained via a home finger-prick test or a laboratory blood draw. The blood sample is assessed for the presence of circulating immunoglobulin-G (IgG) food or drink antibodies (which is why this type of test is known as an IgG test). Test providers assert that elevated levels suggest an increased sensitivity to that food or drink.

Why take an IgG food intolerance test?

People often choose this form of test because it is quick, convenient and can be performed in a home setting. Many believe it helps them gain greater insight into which foods may be a problem for them and as such helps them design an appropriate elimination diet.

Are IgG food intolerance tests reliable?

Although the presence of IgG antibodies supports an immune response, this response might not be associated with the symptoms of any food intolerance. This is because IgG antibodies play a normal and important role in helping maintain our tolerance to food – with this in mind the presence of IgG antibodies may simply confirm that you’ve eaten the tested food recently and your body is having a normal response. For this reason, many experts argue that there is no substantiated evidence to support these tests for the diagnosis of a food allergy or a food intolerance. They are still subject to review and are not recommended as a diagnostic tool by the British Dietetics Association, the NHS or NICE.

Food intolerances are complicated because their causes and symptoms are wide and varied, and they are more likely to be initiated by your digestion rather than your immune system. This means there is no reliable diagnostic biomarker to correctly target and identify what might be your problem food.

If you do decide to try a IgG food intolerance test, the results should be viewed in the context of your overall clinical picture, rather than being seen as diagnostic. Furthermore, before conducting the test, you should continue to include the suspect food(s) in your diet, otherwise your results may not be representative.

Google searches for food intolerance testing has increased by a staggering 250% in just 12 months, as an increasing number of people turn to the convenience of ‘at home’ tests in an attempt to throw some light on their digestive problems. This increase in demand is understandable given food intolerances are estimated to affect up to 20% of the population, although it is difficult to arrive at an exact figure because of the varied nature of symptoms and potential causes.

Many believe the increase in food intolerances could be due to our modern Western diet and lifestyles – for example, we eat more ultra-processed foods containing additives and chemicals, we spend more time indoors and most of us have, at least once in our lives, used antibiotics. All of these factors contribute to changes in the balance and diversity of our gut microbiota, which plays an important role in maintaining the health of our gut.

Are IgG food intolerance tests safe for everyone?

Conducting a test that is likely to lead to dietary restriction is inappropriate for some people – these include those with an eating disorder, the elderly as well as children and young people. These groups should only follow a restricted diet under the close supervision of a registered dietitian or nutritionist, otherwise their diet may be inadequate for their needs and detrimental to their growth and development. Other groups who should not restrict their dietary intake, unless advised to do so, include pregnant and breast-feeding women.

Excluding food groups or combinations of different foods may cause nutritional inadequacies, malnutrition and a reduced quality of life. For example, following a gluten-free diet when you have not been diagnosed with Coeliac disease or non-Coeliac gluten sensitivity may influence the health of your gut by changing the balance and diversity of beneficial gut bacteria.

Woman writing a food journal

What could I do instead of taking a food intolerance test?

The NHS suggests the most appropriate way to diagnose a food intolerance is to monitor your symptoms when you eat certain foods. Once you identify the possible food culprit(s) you can restrict them one at a time to see if you have any improvements in your symptoms before carefully re-introducing them. This trial-and-error approach may help narrow down suspect foods and allow you to discover that you can tolerate a certain amount of the food without symptoms.

This ‘elimination diet’ is best performed using the 3 ‘R’ approach:

1. Record – without altering your usual diet, record in a diary or on a smartphone tracker what, when and how you were eating, along with any symptoms you experience and when those symptoms occur and how long they last. Be thorough and include condiments, treats, drinks and snacks – this is best done in real time so you don’t forget things. Do this over a seven to 14-day period and assess your findings to decide which food or drinks, if any, may be causing you a problem. If at the end of this step you think the culprit maybe wheat or gluten, speak to your GP in case they feel it appropriate to test you for Coeliac disease before proceeding to step two. Similarly, if you suspect a problem with dairy (lactose) or fructose, your GP may suggest a hydrogen breath test.

2. Restrict – once you have identified the suspect food(s), familiarise yourself with alternatives that you can include in your diet and then restrict the suspect food while monitoring and recording how you feel. Do you still get symptoms? If step 1 identified several foods, you should refer to a dietician for guidance to avoid the risk of nutritional deficiencies. Step 2 may take some time, for example three to six months, so don’t be tempted to rush this stage because it is important that you address one food at a time rather than risk compromising your diet.

3. Re-introduce – this step helps confirm your suspicions and may help you understand your tolerance threshold. On day one, re-introduce the suspect food, but only a third of your normal portion – you can increase the amount gradually overtime. If you do get symptoms, wait until you are symptom-free before you try the next test food.

An elimination diet should be used for as short a time as possible, and be followed by re-introduction of the food to establish your own tolerance levels, if any. Performing the food challenge in this way helps to ensure a balanced, varied diet and minimises any detrimental impact on levels of your beneficial gut bacteria.

If you believe your symptoms are the result of a classic allergy rather than a food intolerance, speak to your GP for referral to an NHS-approved diagnostic IgE testing facility, which also offers professional dietetic support.

Phone and breakfast

Do personalised nutrition apps test for food intolerance?

The industry in personalised nutrition technology has grown rapidly over the last decade. Generally speaking, these apps do not test for food intolerance or allergy but may, depending on which you choose, help you better understand how your body responds to certain foods and potentially help improve how you manage your symptoms.

There’s evidence that gut imbalances may precede food sensitivities, including allergies, so – given this close association – the use of these apps may shed light on issues with your unique gut microbiome and potentially help you modify it.

Will my food intolerance go away?

This will depend on why you are experiencing symptoms – for example, if you have been diagnosed with a lactose intolerance this is unlikely to change, but you can take steps to successfully manage your symptoms, such as replacing dairy foods with lactose-free products or fortified plant-based dairy alternatives.

Other food intolerances, that are not caused by an enzyme deficiency or a genetic disposition, may improve over time. The best way to assess this is to conduct an elimination diet, under the supervision of a registered dietitian or nutritionist, then reintroduce small amounts of the food while carefully monitoring your reactions.

If you suspect you have a food allergy or intolerance, are pregnant, breastfeeding or in one of the groups who should not restrict their diet unless under professional supervision, you should consult with your doctor before making any dietary adjustments.

Do you have a question about food intolerances? Ask our team in the comments below.

Want more like this? Try...

What is personalised nutrition?
Our top gluten-free recipes
All our dairy-free recipes
Top 20 low-histamine foods
Low-FODMAP recipes
Top 10 probiotic foods to support your gut health

This article was published in 25 August 2023 by Registered Nutritionist, Kerry Torrens.

Kerry Torrens BSc (Hons) PgCert MBANT is a registered nutritionist with a postgraduate diploma in Personalised Nutrition & Nutritional Therapy. She is a member of the British Association for Nutrition and Lifestyle Medicine (BANT) and a member of the Guild of Food Writers. Over the past 15 years she has been a contributing author to a number of nutritional and cookery publications including Good Food. Find her on Instagram at @kerry_torrens_nutrition_


All health content on goodfood.com is provided for general information only, and should not be treated as a substitute for the medical advice of your own doctor or any other health care professional. If you have any concerns about your general health, you should contact your local health care provider. See our website terms and conditions for more information.

Comments, questions and tips

Choose the type of message you'd like to post

Choose the type of message you'd like to post