Food Intolerance

Food Intolerance

What is Food Intolerance?

Some persons experience an unpleasant physical reaction (nausea, vomiting, bloating) upon ingesting certain foods making them unwell. Adverse reactions to foods are seen commonly and can be broadly divided into two categories, namely food allergy and food intolerance. It is essential to distinguish between food allergies and other food-related illnesses because the treatment is different.

Food intolerance also termed non-IgE (immunoglobulin E) mediated food hypersensitivity or non-allergic food hypersensitivity involves difficulty in digesting certain foods. It should be distinguished from a food allergy which is an immune system mediated reaction.

Facts on Food Intolerance

  • Food intolerance is not the same as food allergy
  • Food intolerance involves difficulty in digesting certain foods
  • Food intolerance is an adverse chemical reaction (not immune-mediated response) to certain foods
  • Food intolerance has been found to be associated with conditions such as irritable bowel syndrome, asthma, and chronic fatigue syndrome
  • Symptoms of food intolerance are related to the amount of the offending agent ingested (unlike in food allergy where unpleasant reactions can be triggered by minute amounts of the food)
  • It does not involve the immune system, i.e., not mediated by IgE (food allergy is an adverse immune reaction)
  • Food intolerance will not cause severe, life-threatening consequences that may be seen in food allergy, for example, anaphylaxis (a very rapid allergic reaction) although they can be troublesome and cause the person to become sick
  • In food intolerance, the time interval between ingesting the offending food and symptoms may sometimes be several hours making it difficult to identify the agent. In food allergy onset of symptoms is rapid
  • Allergy testing will be negative in food intolerance

What are the Common Types of Food Intolerance?

Food intolerance is a non-immune adverse reaction to certain foods. It can be broadly divided into three types.

Functional Intolerance

Functional intolerance is caused by defective function of certain digestive processes in the body. These include any of the following.

Enzyme deficiency

The most common cause is an absence of or deficiency of an enzyme needed to digest certain foods such as lactase deficiency leading to lactose intolerance. Other enzyme defects include isolated or combined disaccharide malabsorption syndrome, affecting the absorption of lactose, sucrose and other disaccharides, either singly or entirely. Also, there can be isolated galactase, maltase or saccharase deficiency.

Diamine oxidase enzyme or histamine N-methyl transferase deficiency can result in histamine intolerance.


Transporter Defects

Defective transporter molecules that transport foods into the intestinal cells from the lumen of the intestine, for example, GLUT (Glucose transporter) 2 and GLUT 5 defects. This results in a buildup of these foods in the large intestine where they are acted upon by bacteria with a resultant gas formation and bloating and diarrheal symptoms.

Structural Intolerance

The intolerance that occurs as a result of structural abnormalities in the digestive tract such as outpouchings or diverticula. Common structural defects include intestinal and colonic diverticula, lymphovascular abnormalities, and post-inflammatory strictures.

Bacterial overgrowth in these outpouchings results in the fermentation of foods with gas production and abdominal symptoms. Other causes of bacterial overgrowth include bowel resection or longterm use of certain medications, eg., omeprazole.

Active Chemicals In Foods

  • Salicylates found in oranges, pears, raisins, peppers and some curries
  • Biogenic amines – These include histamine, serotonin (a naturally occurring neurotransmitter), and tyramine-rich foods such as aged cheese, cured meats, and chocolate
  • Caffeine – found in tea, coffee, and caffeinated beverages
  • Aflatoxin – A toxin found in certain foods such as beans. Cooking the beans eliminates the toxin, but severe reactions might occur following the ingestion of undercooked beans


  • Idiosyncratic intolerances include sulfites, aspartame (an artificial sweetener), coloring and flavoring agents used in packaged foods, monosodium glutamate MSU)
  • Chronic infections of the digestive tract such as giardiasis
  • Carcinoid tumors and neuroendocrine tumors secreting amines can induce severe intolerance reaction to minute amounts of these biogenic amines in some persons
  • Celiac disease can cause symptoms of food intolerance

What are the Common Causes or Sources of Food Intolerance?

Food intolerance can be due to various foods. Some of the common food intolerances include the following

  • Dairy and dairy products - Lactose (a sugar found in milk) intolerance (due to lactase enzyme deficiency which is needed to digest lactose). Common symptoms include gassy stomach and bloating, abdominal pain and diarrhea
  • Gluten – General term for proteins found in wheat, barley, rye, and triticale. Common symptoms of intolerance include bloating and abdominal discomfort and headaches
  • Caffeine – A bitter chemical found in certain beverages such as tea, coffee, soda, and energy drinks. Common symptoms include fast heartbeat, anxiety, and sleeplessness
  • Salicylates – A natural chemical found in certain foods such as tea, coffee, honey, fruits, vegetables, spice, and nuts. It is also added as a food preservative and is a component of certain medications, eg., aspirin. Common symptoms include diarrhea, stuffy nose, and hives
  • Amines – These are chemicals found in several foods and are produced on storage due to bacterial action and fermentation. Histamine is the most common amine implicated in food intolerance. Foods rich in histamine include fermented food, vinegar, dried fruits, citrus fruits, cured meats, aged cheese, sour buttermilk, and fermented alcoholic drinks such as beer and wine. Common symptoms include itching, hives and stomach cramps
  • FODMAPs - Stands for (fermentable oligo-, di-, mono-saccharides and polyols). They are short-chain carbohydrates found in several foods such as apples, milk, honey, beans, bread, and beer. They are known to cause digestive problems, especially in persons with irritable bowel syndrome (IBS)
  • Sulfites – These are chemicals found naturally occurring in certain foods such as grapes and aged cheese. They are used as preservatives in foods, drinks and certain medicines. Common symptoms include wheezing, stuffy nose and fall in blood pressure
  • Fructose – It is a simple sugar occurring in honey, high fructose corn syrup, certain fruits, and vegetables. Fructose intolerance is due to inability to digest this sugar and can cause diarrhea, gas and bloating

Although food intolerance is not a serious condition, it can nevertheless negatively impact the quality of one’s life, and it is important to identify the food or the chemical and eliminate it from the diet.

Causes of Food Intolerance

What are the Signs and Symptoms of Food Intolerance?

According to the Australian NSW Food Authority, the most common symptoms of food intolerance include the following

  • Bloating
  • Stomach pain
  • Diarrhea
  • Vomiting
  • Headache
  • Migraines
  • Wheezing
  • A cough
  • Running nose
  • Hives – skin rash or reaction
  • Tiredness
Signs and Symptoms of Food Intolerance

How do you Diagnose Food Intolerance?

It is not easy to diagnose food intolerance as symptoms of food allergy and food intolerance often overlap. It is important to distinguish between food intolerance and food allergy as the management is different.

Patients are advised to maintain a food diary and write down the list of foods eaten, type of symptoms and when they appeared. The food diary helps the doctor or dietitian determine which foods might be causing the adverse reactions and advise on steps to tackle the problem.

A detailed and careful history is important and may aid in the correct diagnosis. Points to note in the history to rule out possible food allergy include -

  • Symptoms occur more slowly, often several hours after ingestion
  • Symptoms are never severe or life-threatening, although can affect quality of life
  • Can happen with several foods
  • Symptoms more likely to occur if considerable amounts of the food is ingested
  • Allergy testing is negative

Food Allergy Testing:

Blood and skin tests may be done to rule out food allergy. These tests will be negative for food intolerance.

In skin testing, the skin is pricked with a tiny probe coated with food extract or fresh food. The testing is usually done on the forearm or upper back after cleaning the surface with alcohol. Skin tests are performed by an allergy specialist.

Skin Prick Tests Help Detect Food Specific IgE

Blood tests (to measure IgE antibody levels) are widely available and can be done without an allergic specialist although the specialist is needed to interpret the results and advise management.

Apart from lactose intolerance and celiac disease, there are no diagnostic tests to diagnose food intolerance reliably.

  • Lactose intolerance can be diagnosed by a lactose-tolerance test, lactose breath test or stool pH test.
  • Celiac disease is diagnosed by blood tests to check for antigliadin antibodies, endomysial antibodies or anti-tissue transglutaminase antibodies.

Miscellaneous tests:

  • Stool test for giardiasis
  • Structural abnormalities of the bowel may be diagnosed by imaging studies
  • Intestinal biopsy may rarely be necessary to rule out underlying pathologies such as inflammatory bowel disease or celiac disease

How do you Treat Food Intolerance?

Intolerance to foods cannot be cured. The only way to treat food intolerance is by the following measures

  • Identify and eliminate the food from the diet
  • Reintroduce the food in small quantities gradually and check how much you can safely tolerate
  • Check food labels for ingredients before buying
  • Supplements may be needed to prevent deficiencies. For example, lactose-intolerant children may need calcium and protein supplementation

Latest Publications and Research on Food Intolerance