Food intolerance, also known as non-IgE mediated food hypersensitivity or non-allergic food hypersensitivity, refers to difficulty in digesting certain foods. It is important to note that food intolerance is different from food allergy.
Food allergy triggers the immune system, while food intolerance does not. Some people suffer digestive problems after eating certain foods even though their immune system has not reacted - there is no histamine response.
Foods most commonly associated with food intolerance include dairy products, grains that contain gluten, and foods that cause intestinal gas buildup, such as beans and cabbage.
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What are the signs and symptoms of food intolerance?
It can be difficult to determine whether the patient has food intolerance or allergy, because often signs and symptoms overlap. When it is an allergy even small amounts result in symptoms, as may be the case with peanuts, according to James Li, M.D., Ph.D., Mayo Clinic allergy specialist.1
With food intolerance tiny amounts will usually have no effect. Doctors can also test for Immunoglobin E (IgE) antibodies.
A symptom is something the sufferer feels and describes, such as pain or discomfort, while a sign is something others can detect, such as a rash.
The symptoms of food intolerance generally take longer to emerge, compared to food allergies. Onset typically occurs several hours after ingesting the offending food or compound and may persist for several hours or days. In some cases symptoms may take 48 hours to emerge.
Some people are intolerant to several groups of foods, making it harder for doctors to determine whether it might be a chronic illness or food intolerance. Identifying which foods are culprits can take a long time.
According to the Australian NSW Food Authority2, the following are the most common symptoms of food intolerance:
Lactose in milk products is a common food intolerance
- Runny nose
- Feeling under the weather
- Stomach ache
- Irritable bowel
What are the causes of food intolerance?
Absence of an enzyme
Enzymes are needed to fully digest foods. If some of these enzymes are missing or insufficient, proper digestion may be undermined.
People who are lactose intolerant do not have enough lactase, an enzyme that breaks down milk sugar (lactose) into smaller molecules that the body can break down further and absorb through the intestine. Lactose cannot be absorbed through the gut wall into the bloodstream. If it remains in the digestive tract it can cause spasm, stomachache, bloating, diarrhea and gas.
People with an allergy to milk protein have similar symptoms to those with lactose intolerance; that is why lactose intolerant individuals are commonly misdiagnosed as allergic.
Researchers from Mary Bridge Children's Hospital and Health Center in Tacoma, WA, USA, found that fructose intolerance is common in children with recurrent or functional abdominal pain.3
Nearly all foods require an enzyme for proper digestion. According to the British Allergy Foundation4, enzyme deficiencies are common causes of food intolerance.
Chemical causes of food intolerance
Certain chemicals in foods and drinks can cause intolerance, including amines in some cheeses, and caffeine in coffee, tea and chocolates. Some people are more susceptible to these chemicals than others.
Food poisoning - toxins can cause food intolerance
Some foods have naturally-occurring chemicals that can have a toxic effect on humans, causing diarrhea, nausea and vomiting. Undercooked beans have aflotoxins that can cause extremely unpleasant digestive problems. Fully cooked beans do not have the toxin. Hence, people may wonder why they react to beans after one meal, and not after another.
Natural occurrence of histamine in some foods
Some foods, such as fish that has not been stored properly, can have an accumulation of histamine as they "rot". A number of people are particularly sensitive this naturally-occurring histamine and develop skin rashes, abdominal cramps, diarrhea, vomiting and nausea. Often, the symptoms are similar to anaphylaxis (a strong allergic reaction).
Salicylates are present in many foods
Salicylate intolerance, also known as salicylate sensitivity occurs when somebody reacts to normal amounts of ingested salicylate. Salicylates are derivatives of salicylic acid, which occurs naturally in plants as a defense mechanism against harmful bacteria, fungi, insects and diseases.
Citrus fruits have high levels of salicylates
Salicylates are found in many foods. Most of us can consume salicylate-containing foods without any adverse effects, but a number of people suffer symptoms after eating large amounts. Salicylate intolerant individuals should avoid foods that are high in salicylates.
Salicylates are present in most plant-sourced foods, including the majority of fruits and vegetables, spices, herbs, tea and flavor additives. Mint-flavoring, tomato sauce, berries, and citrus fruits have particularly high levels of salicylates. Processed foods with flavor additives are usually high in salicylates as well.
Food additives are common causes of food intolerance
Food additive intolerance has been a steadily-growing problem over the last thirty years, because more and more foods contain additives. Even so, food additive intolerance is not estimated to affect more than 1% of people.
Food additives are used to enhance flavors, make foods look more appealing, and to increase their shelf life.
Examples of food additives include:
- Artificial colorings
- Artificial flavorings
- Flavor enhancers
Of the thousands of additives used in the food industry, a relatively small number are thought to cause problems.
The following food additives are known to cause adverse reactions in people:
- Nitrates - known to cause itching and skin rashes. Processed meats are generally high in nitrates and nitrites.
- MSG (monosodium glutamate) - used as a flavor enhancer. Known to cause headaches.
- Sulfites - used as a food preserver or enhancer. Commonly used in wines. In the USA and European Union wines bottled after 1987 and 2005 respectively must state on their labels if they contain sulfites at more than 10 parts per million. A German study5 found that about 7% of people have an intolerance to wine.
- Some colorings - especially carmine (red) and annatto (yellow).
How is food intolerance diagnosed or confirmed?
It is not easy to initially determine whether somebody has a food intolerance or allergy, because the signs and symptoms often overlap. Certain patterns in the symptoms can help a health care professional distinguish between the two. In the vast majority of cases, food intolerance symptoms take much longer to appear than food allergies.
Patients are advised to keep a diary and write down which foods are eaten, what the symptoms were like, and when they appeared. The data in the diary can help a dietician or doctor identify which foods are causing adverse reactions and what steps to take.
Doctors commonly recommend a skin prick test to examine reactions to substances.
Apart from lactose intolerance and celiac disease, there is no accurate, reliable and validated test to identify food intolerance. The best diagnostic tool is an exclusion diet, also known as an elimination or diagnostic diet.
Intolerance to regularly-eaten foods may result in adverse reactions running into each other. When this occurs, it is difficult to identify which foods are the culprits. There is a higher risk that a chronic condition or disease is erroneously diagnosed. Exclusion diets are extremely useful in isolating the culprit foods.
In a typical exclusion diet the suspected food is removed from the diet for a set period, usually between two weeks to a couple of months. If during this period the adverse reactions resolve, it becomes more likely the culprit has been found. This can be further confirmed if it is then reintroduced and symptoms return.
The doctor may recommend a skin test and/or a blood test to rule out a food allergy:
- Skin prick test - this determines the patient's reaction to a specific food. A small quantity of the suspected food is placed on the patient's back or forearm. The skin is pricked with a needle, allowing some of its substance to penetrate below the skin surface. Allergic people will react with a raised bump. Skin prick tests are not 100% reliable.
- Blood test - this measures levels of IgE (immunoglobulin E) antibodies. These tests are not 100% reliable either. The presence of IgE antibodies may be a part of the normal human response and indicate tolerance, rather than an adverse reaction, according to a study published in CMAJ.6
In food intolerance, unlike food allergies, there are several signs and symptoms which occur simultaneously, they may be general and non-specific, making diagnosis challenging. Dieticians and doctors emphasize that a detailed food and symptom diary is important.
The best current treatment for food intolerance is to either avoid certain foods or eat them less often and in smaller amounts, as well as taking some supplements that may help digestion.
Tolerance can improve
Some people find that if they stay off the culprit food for a while, they have no reaction when eating it again - this is known as tolerance. Maintaining tolerance is often a question of knowing how long to abstain, and how much of it to eat when it is being reintroduced. As each person reacts differently, the only way to determine this is by trial-and-error.
What is the difference between food allergy and food intolerance?
If you would like more information about food allergies, please see our article, "Common Food Allergies"
Written by Christian Nordqvist