Food allergies trigger 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.
Here are some key points about food intolerance. More detail and supporting information is in the main article.
- Symptoms of food intolerance tend to take longer to appear than symptoms of allergies
- The symptoms are varied and can include, migraine, cough, and stomachache
- Some food intolerance is caused by the lack of a particular enzyme
Gluten is one of the most common causes of food intolerance.
It can be difficult to determine whether the patient has a food intolerance or an allergy because the signs and symptoms often overlap.
According to James Li, M.D., Ph.D., a Mayo Clinic allergy specialist, when it is an allergy, even small amounts result in symptoms, as may be the case with peanuts. Whereas, with food intolerance, tiny amounts will usually have no effect.
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 arrive.
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 the culprits can take a long time.
According to the Australian NSW Food Authority, the following are the most common symptoms of food intolerance:
- Runny nose
- Feeling under the weather
- Stomach ache
- Irritable bowel
There can be many causes of food intolerance, and we will take a look at each of these in turn.
1) Absence of an enzyme
Enzymes are needed to digest foods fully. 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. If lactose 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, found that fructose intolerance is common in children with recurrent or functional abdominal pain.
Nearly all foods require an enzyme for proper digestion. According to the British Allergy Foundation, enzyme deficiencies are a common cause of food intolerance.
2) 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.
3) Food poisoning - toxins
Some foods have naturally-occurring chemicals that can have a toxic effect on humans, causing diarrhea, nausea, and vomiting.
Undercooked beans have aflatoxins 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.
4) 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 to 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).
5) 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.
The chemicals are found in many foods and most people can consume salicylate-containing foods without any adverse effects. However, some people suffer symptoms after eating large amounts. Salicylate intolerant individuals should avoid foods that contain high levels.
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.
Processed foods with flavor additives are usually high in salicylates as well.
Some common types of food intolerance are:
- histamine, present in mushrooms, pickles, and cured food
- additives such as artificial sweetners, coloring, or other flavorings
Some people experience a reaction after eating bread, but this does not necessarily indicate a gluten intolerance. Anyone who suspects they may have a gluten intolerance should see a doctor before giving up gluten, as cereals can be an important source of various nutrients.
Food additives and intolerance
Processed meat can contain nitrates that are the source of some people's food intolerances.
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 percent of people.
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 United States 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 study found that about 7 percent of people have an intolerance to wine.
- Some colorings - especially carmine (red) and annatto (yellow).
It is not easy to determine whether somebody has a food intolerance or allergy because the signs and symptoms often overlap. Certain patterns in the symptoms can help a doctor 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.
Skin prick tests are not 100 percent reliable.
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 to blame. 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 2 weeks and 2 months. If during this period the adverse reactions resolve, it becomes more likely that 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. However, skin prick tests are not 100 percent reliable.
- Blood test - this measures levels of IgE (immunoglobulin E) antibodies. These tests are not 100 percent 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.
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 supplements that may help digestion.
Some people find that if they stay off the specific 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.