Food intolerance or non-immune mediated or non-allergic food hypersensitivity is due to difficulty in digesting certain foods. It should be distinguished from food allergy which is an immune system mediated reaction.

FOOD INTOLERANCE AND ALLERGY
• Food intolerance and food allergy are two separate conditions
• Food intolerance is due to difficulty in digesting certain foods; food allergy is immune system mediated
• Food intolerance involves an adverse chemical reaction (non-immune) to certain foods.
• Food intolerance can be associated with conditions such as chronic fatigue syndrome, irritable bowel syndrome, asthma
• Symptoms of food intolerance are related to the amount of the offending agent (in contrast to food allergy where unpleasant reactions can be induced by minute amounts of the food).
• Although troublesome and causing sickness, food intolerance will not cause serious life threatening consequences that may be seen in food allergy eg anaphylaxis
• In food intolerance, 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

FOOD INTOLERANCE TYPES
It can be broadly divided into three types

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

• Enzyme deficiency
The most common reason is a lack or deficiency of a specific enzyme needed to digest certain foods eg lactase deficiency leading to lactose intolerance. Others include galactase, maltase or saccharase deficiency.

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

Structural Intolerance
Food intolerance caused by structural abnormalities in the digestive tract such as diverticulosis, bowel surgery and narrowing of bowel due to inflammation. Bacterial overgrowth that occurs in these conditions causes fermentation of ingested foods with gas production and abdominal symptoms.

Intolerance due to chemicals in food
• Caffeine – tea, coffee and caffeinated beverages
• Aflatoxin – A toxin found in certain foods such as beans. Severe reaction may occur upon eating undercooked beans
• Salicylates – oranges, pears, raisins, peppers and some curries
• Biogenic amines – tyramine rich foods such as cured meats, aged cheese, and chocolate can cause reactions

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

• Dairy and dairy products
• Non-celiac gluten sensitivity (a protein found in wheat)
• Celiac disease
• Giardiasis infection
• Caffeine – coffee, tea, certain beverages
• Salicylates – fruits, vegetables, teas, coffee, spices, nuts and honey, medication
• Amines –The most common amine implicated in food intolerance is histamine usually occurring in fermented food, dried fruits, vinegar, citrus fruits, old cheese, cured meats, sour buttermilk, and alcoholic drinks such as beer and wine.
• 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 may cause digestive problems, particularly in persons with irritable bowel syndrome (IBS).
• Sulfites – grapes and aged cheese, preservatives in foods, drinks and certain medicines
• Fructose – honey, high fructose corn syrup, certain fruits and vegetables

Food allergy is commonly caused by eggs, milk, peanuts, soya and wheat

CLINICAL FEATURES
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
• Cough
• Running nose
• Hives – skin rash or reaction
• Tiredness

Food allergy also causes similar symptoms but life threatening food allergy called anaphylaxis can occur sometimes with rapid thready pulse, fall in blood pressure, severe narrowing of airways, dizziness and loss of consciousness

DIAGNOSIS
It is important to differentiate between food intolerance and food allergy as the treatment is different. Since symptoms often overlap, diagnosis may not be always easy

Maintain a food diary noting down the list of foods are eaten, nature of symptoms and when they appear

A detailed and careful history is important for making the correct diagnosis. Points to note that are suggestive of intolerance are

• Symptoms occurring more slowly, often several hours after ingestion
• Patient feels sick but symptoms are never life threatening
• Symptoms often associated with ingesting 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 in food intolerance
In skin testing the skin is pricked with a tiny probe coated with fresh food or food extract, usually done on the forearm or upper back. Skin tests are done by an allergy specialist

Serum IgE levels can be done without an allergic specialist although a specialist should interpret the results and advise management

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

Miscellaneous tests
• Fecal test for giardiasis infection
• Imaging studies to rule out structural defects
• Rarely intestinal biopsy to rule out underlying bowel pathology eg celiac disease or inflammatory bowel disease

TREATMENT
• Identify and eliminate offending food from the diet
• Check ingredients on the pack before buying
• Try reintroducing the food in small amounts and see how much you can safely tolerate
• Take supplements to prevent deficiencies, eg calcium and protein supplementation in lactose intolerant children
• Mild food allergy can be treated with anti-histamines but severe food allergy is potentially life threatening and requires urgent hospitalization and treatment of anaphylaxis