Food Intolerance

By Dr. Liji Thomas, MD

Food intolerance may be described as an adverse non-immunologic chemical reaction in the body to a non-toxic component of food. This occurs in the absence of any structural abnormality of the digestive tract. It is not caused by an antigen-antibody reaction, due to the immune system producing an abnormal reaction to a group of molecules in the food.

It may produce similar symptoms to mild food allergy. However, it does not ever cause the severe reactions characteristic of food anaphylaxis, which is the most serious form of food allergy. It usually has prominent gastrointestinal symptoms, because it is caused by the body’s inability to digest or to absorb some parts of the food ingested. This is due many times to specific enzymatic deficiencies.

A common example is lactose intolerance. Here the problem is not an immune reaction to milk protein or any other milk component. Instead, it is due to the absence of the enzyme lactase, which is necessary to hydrolyze lactose, the prominent sugar present in milk. The resulting accumulation of lactose in the gut increases the osmotically active content of the large intestine, and promotes the fermentation and breakdown of lactose by the intestinal flora. The buildup of fermentation products in the intestines leads to bloating, abdominal cramps and diarrhea.

In a few cases, food intolerance may be due to the presence of certain amines in the food, which can cause effects on the vascular system, such as histamine.


Food intolerance may be signaled by the occurrence of migraines, rashes with raised red wheals and pruritus, or bowel symptoms. It has also been linked to other conditions, such as irritable bowel syndrome, asthma or chronic fatigue.

The symptoms of food intolerance may not always occur immediately after eating or drinking the offending food. In many cases they are found to occur up to 12 – 24 hours afterwards. They are also dose-related, so that it may not be easy to link the symptoms to the foods that caused them. In other words, there is a threshold amount, which needs to be ingested for the food to produce symptoms of intolerance. Thus, a normal serving of food is often eaten before the signs of intolerance set in.

A more complete list of symptoms of food intolerance would contain:

  • Migraines or headaches
  • Diarrhea
  • Tremor or a feeling of nervousness
  • Sweating
  • Palpitations
  • Increase in the rate of respiration
  • Burning sensations of the skin
  • Constriction of the chest or the face
  • Wheezing
  • Itching raised red skin rashes or other symptoms of allergy


The causes of food intolerance vary between people. In most cases people are sensitive to one or a few foods, but not to all. The reason for intolerance may include:

  • An enzyme deficiency leading to indigestion or malabsorption
  • The presence of pharmacologically active substances in food
  • The inborn errors of metabolism

The most common foods that produce chemical intolerance are:

  • Milk and milk products, such as yogurt or cheese
  • Chocolate
  • Eggs, and especially egg albumin
  • Flavor enhancers, such as monosodium glutamate (MSG)
  • Strawberries, citrus fruits and tomatoes
  • Red wine
  • Food additives
  • Certain amines, such as histamine

Diagnosis and management of food intolerance

Food intolerance is nonspecific by definition. The diagnosis is based upon the observed reaction when suspected foods are excluded or deliberately introduced into the diet.

The safest and easiest way to prevent the symptoms of food intolerance is to avoid exposure to the food. However, in some cases, it is possible to induce tolerance under medical supervision. This may be done by stopping the food for some time, and later reintroducing it in small doses.


Reviewed by Jonas Wilson, Ing. Med.

Further Reading

Last Updated: Sep 26, 2016

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