Conditions & Investigations

Food intolerance

Food intolerance

Food intolenance is much more common than a food allergy – with food allergies, there is an abnormal response from the body’s immune system to produce antibodies to help ‘fight off’ food. With a food intolerance there are a a variety of difference mechanisms which cause food to affect people.

When people have a food intolerance, the onset of symptoms is usually slow and may occur several hours after eating the offending food. The symptoms can also last several hours. Sometimes people can have an intolerance to several foods and it can be very difficult to decide which foods are responsible.

With allergies, patients are usually unable to have even a small amount of the offending food. With an intolerance however, most people can tolerate a certain amount of food before they get symptoms. 

There are a variety of different symptoms which occur with food intolerance – they include generalized symptoms such as fatigue and joint symptoms and rashes, and gastrointestinal symptoms such as diarrhoea, bloating, flatulence.

Food intolerance can have a number of different causes:

Enzyme defects

Enzymes that are present within the bowel help to breakdown natural substances found in certain foods.  If  these enzymes are missing, or there is not enough of them, then eating the food can cause symptoms because the food cannot be properly dealt with by the body. 

One common intolerance is lactose intolerance – this is when tha body lacks an enzyme called lactase which breaks down the sugar lactose, which is found in milk products, into smaller sugards so that they can be absorbed from the gut. Lactose is too large to be absorbed across the gut wall undigested, and its presence in the gut causes gut spasm, pain, bloating and diarrhoea.


Some foods contain naturally occurring chemicals that have an affect on the body, such as caffeine in coffee, tea, and chocolate, or amines in certain cheeses.  Some people seem to be more affected than others by these natural substances in the food, causing symptoms which would not occur in other people unless they ate far larger quantities of the food.

Histamine in foods

Some foods contain histamine naturally, and others (such as certain fish that are not fresh and have not been stored properly) can develop a build-up of histamine in their flesh as they age. In certain people, this histamine occurring naturally in the food can cause symptoms when the food is eaten; typically, rashes, stomach pains, diarrhoea and vomiting and in some cases symptoms that can mimic anaphylaxis.

Salicylates in foods

Many foods naturally contain salicylates, and our tolerance to this can vary. The vast majority of people can eat salicylate-containing foods with no problems, but other people may suffer symptoms if they eat too many foods, which when combined contain a large amount. These salicylate-intolerant people will get better if they eat a diet of low and moderate salicylate foods and avoid those with the highest levels.

Additives in foods

A wide variety of natural and artificial additives are used in colouring, preserving and processing foods. Some people can suffer symptoms provoked by hypersensitivity to food additives.

How is Food Intolerance recognised? 

Certain features such as the pattern and type of symptoms can help to distinguish food intolerance reactions from those that might be a result of food allergy, or some quite different cause unrelated to food.  By keeping an accurate and detailed diary of both foods eaten and symptoms, it is possible to highlight the foods that may be causing a problem. Even if the culprits are not clear, it gives a useful starting point on which to base exclusion and reintroduction diets.  A dietitian can help you manage these diets, which can ultimately give a clear diagnosis.

Apart from coeliac disease and lactose intolerance there not any reliable and validated tests to identify food intolerance.  Because of this, the main tool used to diagnose food intolerance is an exclusion diet (also called a diagnostic diet).

Elimination followed by reintroduction

Embarking on a diet, which excludes certain foods to help find out what is causing symptoms, is known as an exclusion diet.  Certain foods are excluded for a set period of time to see if the symptoms improve or resolve.  This is followed by a reintroduction phase of the diet so that a clear diagnosis can be obtained.  The initial period of exclusion will usually be for two weeks and up to six weeks depending on the symptoms.  During this time, it is important to replace foods with other foods of similar nutrition.  A dietitian can help with this and supervise the exclusion diet.  Embarking on such diets requires a lot of dedication and planning but the results can be life changing.  Choosing the right time for you and your family is essential – avoiding starting the diet during holidays or major celebrations is recommended as sticking to the diet will become more difficult.

It is essential that during this exclusion phase the diet is adhered to 100% and restarted if any mistakes are made so that the results are as accurate as possible.

Exclusion diets may avoid one food or several foods, or may start with a few foods diet where only select foods are allowed and a list is given to be followed closely for an initial set time period.  These diets can lack essential nutrients, so it is vital they are supervised by a dietitian.

Prolonged elimination builds tolerance

Weeks or months of elimination of the reactive food may well lead to reintroduction of the food without reaction. This is known as tolerance, and its maintenance depends on establishing the threshold of both frequency and quantity for that person - in other words, eating the food occasionally may be tolerated, but reintroducing it in large quantities or on a very regular basis (e.g. every day) might lead to symptoms recurring.  This is purely individual so working this out and not restricting the diet more than is necessary is a major consideration.