In food allergy, an abnormal immune system response results in the body making antibodies to ‘fight off’ a food.  However, some people suffer symptoms after eating certain foods even when they are not producing antibodies against them.  A variety of different mechanisms can cause foods to affect people in this way; commonly these are known as food intolerance.

Food intolerance is much more common than food allergy.  The onset of symptoms is usually slower and may be delayed by many hours after eating the offending food. Because of this it can be difficult to determine whether food intolerance is the cause of chronic illness, and which foods may be responsible.  The symptoms caused by these reactions are numerous but have been associated with gut symptoms such as bloating, diarrhoea, constipation, and IBS, and skin problems such as eczema. Other symptoms are vague and not easily diagnosed.  People may complain of non-specific problems such as brain fog, lethargy, headaches, or feeling bloated.

Food intolerance can have a number of different causes:

Enzyme defects

Enzymes are required to help with the breakdown of natural substances found in certain foods.  If these enzymes are missing, or in short supply, then eating the food can cause symptoms because part of the content of the food cannot be properly dealt with by the body.

In lactose intolerance, for example, the body lacks the enzyme (lactase) that breaks lactose (milk sugar) down into smaller sugars ready for absorption 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, diarrhoea and ‘failure to thrive’. Incidentally, these same symptoms can occur in milk allergy, when the body has made antibodies to milk protein, which causes an immune reaction when you drink milk. Hence, you cannot always distinguish allergy from intolerance by symptoms alone without expert help. Most foods require some enzyme activity in their digestion, and enzyme deficiencies can be an important factor in food intolerance.


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.


A number of foods contain naturally occurring substances that can exert a toxic effect causing symptoms of vomiting and diarrhoea.  In cases such as kidney beans or chick peas that are undercooked, there are aflotoxins, which cause these symptoms.  If they are cooked fully the toxins are not present.  This can give a very confusing message to someone who seems to tolerate a food sometimes but not others.

Histamine in foods

Some foods contain histamine naturally, and others (such as fish that are not fresh) can develop a build-up of histamine 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, manusea 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 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.

By keeping a diary of all suspected food related reactions, this will help to identify which type of food is causing your symptoms.  Sometimes the food triggers will be obvious whilst other reactions are very difficult to identify.

The time relationship between eating the food and getting symptoms depends on many factors.  If the food is only eaten very occasionally, symptom onset after digestion ranges from almost immediate to several hours. However, this is different when the food is being consumed regularly, when each ‘reaction’ will run into the next, leading to the development of chronic, almost continual, symptoms with no clear immediate reaction each time the food is eaten.  When this happens it can be more difficult to identify culprit foods.  This highlights the need for attention to detail and the need to record everything fully in the food and symptom diary.

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.

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.

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.

In some people, underlying conditions can either cause symptoms or make food intolerances worse.  In these cases, treatment for the underlying condition should be a priority in order to allow symptoms to improve. Excluding other possible diagnoses first is imperative.