What is a food allergy?

A food allergy is an immune system response. It is caused when the body mistakes an ingredient in food — usually a protein — as harmful and creates a defense system (antibodies) to fight it. An allergic reaction occurs when the antibodies are battling an "invading" food protein. The most common food allergies are shellfish, nuts, fish, eggs, peanuts, and milk.

What is food intolerance?

Food intolerance is a digestive system response rather than an immune system response. It occurs when something in food irritates a person’s digestive system or when a person is unable to properly digest, or break down, the food. Intolerance to lactose, which is found in milk and other dairy products, is the most common food intolerance.

How common are food allergies and intolerances?

Food allergies affect about 1 percent of adults and 7 percent of children, although some children outgrow their allergies. Food intolerances are much more common. In fact, nearly everyone at one time has had an unpleasant reaction to something they ate. Some people have specific food intolerances. Lactose intolerance, the most common specific food intolerance, affects about 10 percent of Americans.

How can you tell the difference between an allergy and intolerance to food?

Food allergies can be triggered by even a small amount of the food and occur every time the food is consumed. People with food allergies are generally advised to avoid the offending foods completely. On the other hand, food intolerances often are dose related; people with food intolerance may not have symptoms unless they eat a large portion of the food or eat the food frequently. For example, a person with lactose intolerance may be able to drink milk in coffee or a single glass of milk, but becomes sick if he or she drinks several glasses of milk. Food allergies and intolerances also are different from food poisoning, which generally results from spoiled or tainted food and affects more than one person eating the food. Your health care provider can help determine if you have an allergy or intolerance, and establish a plan to help control symptoms.

What are the symptoms of a food allergy?

Symptoms of a food allergy can range from mild to severe, and the amount of food necessary to trigger a reaction varies from person to person. Symptoms of a food allergy may include:

Anaphylaxis is a very serious and potentially fatal allergic reaction that involves a sudden drop in blood pressure, loss of consciousness and body system failure.

What are the symptoms of food intolerance?

Symptoms of food intolerance include:

What causes food allergies and intolerances?

Food allergies arise from sensitivity to chemical compounds (proteins) in food, even compounds that are found naturally in food. Food allergies are more common in people whose family members have allergies, suggesting a genetic — or hereditary — factor may be involved with the development of food allergies.

Food allergies develop after you are exposed to a food protein that your body thinks is harmful. The first time you eat the food containing the protein, your immune system responds by creating specific disease-fighting antibodies (called immunoglobulin E or IgE). When you eat the food again, it triggers the release of IgE antibodies and other chemicals, including histamine, in an effort to expel the protein "invader" from your body. Histamine is a powerful chemical that can affect the respiratory system, gastrointestinal tract, skin or cardiovascular system.

The allergy symptoms you have depend on where in the body the histamine is released. If it is released in the ears, nose and throat, you may have an itchy nose and mouth, or trouble breathing or swallowing. If histamine is released in the skin, you may develop hives or a rash. If histamine is released in the gastrointestinal tract, you likely will develop stomach pains, cramps or diarrhea. Many people experience a combination of symptoms as the food is eaten and digested.

There are many factors that may contribute to food intolerance. In some cases — as with lactose intolerance — the person lacks the chemicals, called enzymes, necessary to properly digest certain proteins found in food. Also common are intolerances to some chemical ingredients added to food to provide color, enhance taste and protect against the growth of bacteria. These ingredients include various dyes and monosodium glutamate (MSG), a flavor enhancer.

Substances called sulfites, which may occur naturally — as in red wines — or may be added to prevent the growth of mold, also are a source of intolerance for some people. The Food and Drug Administration has banned the use of spray-on sulfates to preserve fruits and vegetables, but sulfates are still found naturally in some foods. Salicylates are a group of plant chemicals found naturally in many fruits, vegetables, nuts, coffee, juices, beer and wine. Aspirin also is a compound of the salicylate family. Foods containing salicylates may trigger symptoms in people who are sensitive to aspirin. Of course, any food consumed in excessive quantities can cause digestive symptoms.
