Food intolerance or food allergy?

Written by: Professor Stuart Bloom
Edited by: Cal Murphy

Food intolerances and allergies can have a serious impact on a person’s life, forcing them to plan their meals around avoiding certain foods. What are food intolerances and how are they different from allergies? Expert gastroenterologist Dr Stuart Bloom is here to explain.

What are the most common food intolerances?

Food intolerance has been reported with a very wide variety of foods. While some food intolerances can be very serious and on occasion life-threatening, most are much less serious. The three most common food intolerances that have been described are to dairy, egg and wheat.


What are the causes?

This is an area of active research, and is still not perfectly understood. Some cases may be due to fermentation of food by colonic bacteria, but other causes involve the effects of food on bowel motility or intestinal irritability and spasm.


What are the symptoms of a food intolerance?

While the human gut is incredibly complex and well-adapted, it only has a relatively few ways of producing symptoms. These include nausea, vomiting or abdominal distension if the upper gut is involved and diarrhoea, abdominal bloating and, less commonly, constipation if the lower gut is involved.


Could my symptoms be due to something else?

It’s important to get an expert medical assessment if symptoms become very intrusive or distressing, and especially if there are any so-called “alarm” symptoms. These include:

  • Unexpected weight loss
  • Fever
  • Vomiting blood
  • Blood in the stool
  • Sudden change of bowel habit that persists for longer than a couple of weeks.

Any of these symptoms should prompt an urgent consultation with your doctor.


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

This is an active area of study because of media interest and because of the number of patients on particular diets, such as a gluten-free diet.

Genuine food allergy is a serious condition, and usually involves an immediate hypersensitivity reaction. This can involve a general itchy sensation, swelling of the lips, eyes and tongue, and asthma-like symptoms due to bronchoconstriction (spasm of the airways). It’s usually in response to particular food components, e.g. shellfish, nuts, etc. or particular ingredients e.g. sesame seeds. Immediate symptoms can include lip-swelling and tingling, shortness of breath, and swelling of the mouth or tongue, but can usually be treated with antihistamines and injection of adrenaline via a syringe e.g. epipen.

Not all immune-mediated food intolerances are immediate. Coeliac disease is an immune-mediated intolerance to a particular part of one of the proteins in wheat, called gluten. The disease involves inflammation of the intestine and requires testing via blood tests and a camera test of the bowel (endoscopy). The incidence of coeliac disease is about 1/300 of the population. Many more people put themselves on a so-called gluten-free diet (usually this will be low gluten because it is quite difficult to keep to a genuine gluten-free diet) because they feel better avoiding wheat products.

Often, we don’t know the underlying basis for food intolerance, and have no specific test for it (skin patch testing is of very little value). However avoiding some specific foods does make some patients feel better.


What happens during a food intolerance test?

Specific tests for food intolerance include measuring immunoglobulin levels which can be abnormal in cases of immune hypersensitivity. Coeliac disease can be investigated by looking for a specific antibody to tissue transgutaminase. Skin prick testing is not useful for food intolerance testing. Testing for food allergies is very specialised and requires a visit to a clinical immunologist.

Hydrogen breath tests can be helpful. These are simple, rapid and non-invasive outpatient tests which measure the concentration of hydrogen in expired air after a variety of dietetic challenges. We commonly use one of three test substances:

  • Lactose can be used to test for lactose intolerance, which is the commonest reason for dairy intolerance
  • Lactulose or glucose can be used to test for small intestinal bacterial overgrowth (SIBO), which is not really a food intolerance, but can cause similar symptoms of distension and diarrhoea.
  • Fructose can be used to test for intolerance of fructose and related sugars called fructans, and these are targeted in patients who are put on a low FODMAP diet.


How is food intolerance managed?

The first step is expert medical assessment to make sure there is no underlying medical condition, e.g. inflammation or structural problem with the digestive tract. Once a positive diagnosis has been made, dietary exclusion is usually the next step, although some physicians will use medications to reduce histamine release.

Involvement of a dietician can be very helpful to identify foods which trigger symptoms. Recently, it’s become clear that many food intolerances involve complex carbohydrates, which can be managed with a low FODMAP diet. FODMAP stands for fructose, oligo-, di- and monosaccharides and polyols. Reducing these can be effective in about 70% of cases. Overall, a structured diet is usually effective at reducing symptoms.

By Professor Stuart Bloom

Professor Stuart Bloom is a gastroenterologist based in London. He is an expert in inflammatory bowel disease, as well as the management of irritable bowel syndrome and food intolerance. He leads the inflammatory bowel disease clinic at University College London Hospitals, where he has worked as a consultant since 1996.

Professor Bloom is the Senior Author of the current British Society of Gastroenterology (BSG) guidelines for managing Colitis and Crohn’s disease, published in 2010. He was chair of the UK clinical research network in Gastroenterology from 2008-2013. He is currently chair of the UK IBD registry.

Professor Stuart Bloom is also an accredited bowel cancer screening colonoscopist, with a low rate of complications during colonoscopies. He has been recognised for his expertise and contributions to medicine in his field (The Leslie Parrott Prize from the National Association of Crohn’s and Colitis (1994), the President's medal from the British Society of Gastroenterology (2013)).

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