What is food intolerance?

Some people experience adverse reactions caused by food. Finding out which foods are causing your symptoms can be straightforward for some people and incredibly tricky for others – dependent on what their symptoms are, how quickly the symptoms appear after eating the food and which food (or foods) is causing a problem.

Food intolerances can take some time to diagnose. Although not life threatening, food intolerance 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, like food allergy.

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 of a food intolerance

There are many possible symptoms that could be caused by having an intolerance to food. Depending on the type of food that is suspected depends on the possible range of symptoms. These symptoms may also be experienced in other conditions in the absence of food being a problem so should always be discussed with a healthcare professional. Symptoms commonly effect the digestive, skin and respiratory systems.

Diagnosing a food intolerance

If you suspect a food may be the cause of the symptoms you or your child has experienced it is important to avoid that food and discuss further with a healthcare professional they should take a detailed patient history and decide if a trial elimination and re-introduction is necessary. There are no tests e.g. a blood test used to diagnose a food intolerance. If a non IgE mediated food allergy is suspected then a trial elimination usually for 2-6 weeks is recommended.

Food intolerances can take some time to diagnose. Although not life threatening, food intolerance can and often does, make the sufferer feel extremely unwell and can have a major impact on working and social life.

Keeping a food and symptoms diary of the food eaten and symptoms experienced can be a useful way to build up patterns and highlight suspected food related reactions and reproducibility. You can find a downloadable version of a food and symptoms diary below.

Food and Symptoms Diary

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Food and Symptoms Diary

Food and symptoms diaries can help yourself and a healthcare professional to determine what foods may be causing you or a loved ones reactions.

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Common food intolerances

There are many different types of food intolerances which more commonly affect people than a food allergy. It is important to understand that not all reactions to food are because of a food allergy. Common types of food intolerances include reacting to lactose, gluten, food additives and chemicals.

Histamine intolerance

Histamine, tyramine and phenyl ethylamine are vasoactive amines (also known as Biogenic Amines), chemicals which occur naturally in certain foods. Foods high in histamine, which may cause histamine intolerance, include wine and cheese.

Symptoms of a histamine intolerance

Most people tolerate the amounts found in a normal diet. However, some people experience symptoms to even normal levels of vasoactive amines, which may be due to a reduced ability to break them down in their digestive systems. Symptoms of histamine intolerance include:

  • Headaches
  • Rashes, flushing
  • Runny or blocked nose
  • Diarrhoea, nausea or vomiting
  • Symptoms may occur 30 minutes or longer after eating and the level of intolerance does vary from person to person.

Treatment of a histamine intolerance

Once food allergy or other causes have been ruled out, the best way to establish if vasoactive amines are causing symptoms of histamine intolerance is to try avoiding them for 2-4 weeks. Symptoms need to be monitored by keeping a food and symptoms diary; then by reintroducing foods gradually, you can see how much can be tolerated and how often.

There are no reliable tests to diagnose Vasoactive Amine sensitivity. True food allergies should be ruled out by an experienced clinician before experimenting with the diet.#

Long COVID and the Low Histamine Diet

The low histamine diet has been suggested as a potential treatment for the management of Long COVID. However, there is limited evidence to support this claim with more research needed, therefore this approach is not currently recommended.

Histamine occurs naturally in many foods and a low histamine diet can potentially be very restrictive, leading to nutritional deficiencies if not supported correctly by a dietitian.


More research is needed to explore the effects of a low histamine diet. The diet is therefore not currently recommended in the management of Long COVID.

Lactose intolerance

This is a common disorder arising from an inability to digest lactose (milk sugar) because of low levels of the enzyme lactase. Lactose is the main sugar in milk and milk products from mammals (e.g. humans, cows, goats). Lactose intolerance is often confused with milk allergy, but it is NOT an allergy.

Symptoms of a lactose intolerance

The symptoms of lactose intolerance are:

  • Diarrhoea
  • Bloating
  • Flatulence (wind)
  • Abdominal pain or discomfort
  • Infantile colic
  • And less commonly, it can cause constipation and nausea.

Treatment of a lactose intolerance

Lactose intolerance is treated by following a low lactose diet. Most children and adults with lactose intolerance can tolerate small amounts of lactose in foods. Lactose may act as a prebiotic – feeding healthy gut bacteria and improving the absorption of minerals such as calcium, so try to include it if you can.

Goat, sheep and other animal milks are not suitable for lactose intolerance, as lactose is the main sugar in all mammalian milks.

Gluten intolerance / Gluten sensitivity

Gluten intolerance is a comparatively newly recognised condition, although there is still a lot of controversy as to whether or not it exists and whether it is caused by gluten or another protein found in wheat. It is unclear if it is an intolerance or whether the immune system is involved and it is also unclear if it is lifelong or whether it is a temporary condition.

Intolerance due to gut fermentation

Many people will experience IBS-type symptoms such as bloating, wind, gurgling and abdominal pain after eating wheat and this can be due to the fermentable carbohydrate known as ‘fructans’ which are found in wheat, barley and rye. The immune system is not involved and instead the symptoms are caused by bacteria in the large intestine fermenting the poorly absorbed fructans. Patients who suffer with IBS or gut fermentation should be referred to a specialist dietitian who can guide them through the ‘Low FODMAP’ Diet which removes all
fermentable foods from the diet including the fructans.

Symptoms of a gluten intolerance

Patients commonly report a mixture of symptoms in response to eating wheat which include:

  • Abdominal pain
  • Altered bowel habit
  • Bloating
  • Nausea
  • Reflux

However gluten intolerance is also associated with symptoms outside the gut such as:

  • Foggy mind
  • Joint pains
  • Fatigue
  • Depression
  • Headaches
  • Anxiety
  • General lack of well being

Treatment of a gluten intolerance

Presently there are no tests for gluten intolerance and diagnosis is made by excluding coeliac disease and wheat allergy and using wheat elimination to see if symptoms resolve followed by wheat reintroduction to determine if symptoms reappear. Patients should be referred to a specialist dietitian who can guide them through the appropriate dietary regimen.

Unfortunately tests are unhelpful for this type of reaction so diagnosis is made using a combination of symptoms history and a trial of exclusion and reintroduction.

Coeliac disease

Coeliac disease is a condition where your immune system attacks your own tissues when you eat gluten. This damages your small intestine so you are unable to take in nutrients.

Coeliac disease is caused by an adverse reaction to gluten, which is a dietary protein found in 3 types of cereal:

  • wheat
  • barley
  • rye

Gluten is found in any food that contains those cereals, including:

  • pasta
  • cakes
  • breakfast cereals
  • most types of bread
  • certain types of sauces
  • some ready meals

In addition, most beers are made from barley.

Symptoms of coeliac disease

Eating foods that contain gluten can trigger a range of gut symptoms, such as:

  • diarrhoea, which may smell particularly unpleasant
  • stomach aches
  • bloating and farting (flatulence)
  • indigestion
  • constipation

Coeliac disease can also cause more general symptoms, including:

  • tiredness (fatigue) as a result of not getting enough nutrients from food (malnutrition)
  • unintentional weight loss
  • an itchy rash
  • problems getting pregnant (infertility)
  • disorders that affect co-ordination, balance and speech (ataxia)

Children with coeliac disease may not grow at the expected rate and may have delayed puberty.

Treatment for coeliac disease

There’s no cure for coeliac disease, but following a gluten-free diet should help control symptoms and prevent the long-term complications of the condition.

Even if you have mild symptoms, changing your diet is still recommended because continuing to eat gluten can lead to serious complications. This may also be the case if tests show that you have some degree of coeliac disease even if you do not have noticeable symptoms.

It’s important to ensure that your gluten-free diet is healthy and balanced. An increase in the range of available gluten-free foods in recent years has made it possible to eat both a healthy and varied gluten-free diet.

Specialist advice

A dietitian (specialist in food and nutrition) is a useful resource for those who are concerned over a suspected food intolerance or need dietary advice on food avoidance and replacement. It is important not to unnecessarily exclude several foods as this can result in the diet being restricted and lacking important nutrients. If after taking a dietary history a food or food additive is identified, a dietitian will be able to guide on an elimination and reintroduction diet if this is deemed necessary. To access a dietitian through the NHS speak to your GP.  

Elimination diets involve removing the suspected food from the diet for a set time to see if this helps improve any symptoms. Reading food labels is an important part of ensuring that the food is avoided as some foods are not always an obvious ingredient. More information can be found in the reading a food label fact sheet. During an elimination diet, usually 4 weeks is long enough to know whether removing the suspected food from the diet has helped or not. After this time re-introducing the food is recommended to confirm if the food triggers symptoms again and is how a diagnosis of a food intolerance is made. 

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