A food allergy is different from food intolerance, although some people may not always know how these vary. A food allergy involves an immune system response by the body, while a food intolerance does not.

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A person with a food intolerance cannot digest a substance in certain foods properly, often because they have an enzyme deficiency. A food allergy does not relate to an enzyme deficiency.

The table below shows features associated with either a food allergy or an intolerance.

OnsetSymptoms appear later.Symptoms appear soon after eating the food, usually within 2 hours, but sometimes after 4–6 hours.
AmountThe person can eat a very small quantity of the food with no adverse reaction.The person cannot tolerate even small amounts of the allergen or food they are allergic to. Exposure to even a tiny amount of the food will produce a severe reaction.
ExposureA reaction will occur only if the person eats the specific food.A reaction may occur if the person eats a food that someone prepares for them in an environment that contains the allergen.
EffectReactions can be severe and extremely unpleasant but are rarely life-threatening.Severe and potentially life-threatening reactions that may include anaphylaxis (anaphylactic shock).
Common trigger foods
  • Beans
  • Cabbage
  • Citrus fruit
  • Grains that contain gluten
  • Milk, or lactose
  • Processed meats
  • Eggs
  • Fish
  • Groundnuts, or peanuts
  • Milk
  • Tree nuts (Brazil nuts, walnuts, almonds, and hazelnuts)
  • Soybeans
  • Shellfish
  • Wheat

An immune response or an enzyme deficiency?

Food allergies and intolerances are unwanted reactions to food that some people experience, but they are not the same and happen for different reasons.

Immune response: When a person has a food allergy, their body’s immune system responds incorrectly to a substance known as an allergen.

An allergen is not necessarily a harmful substance. Doctors call them allergens because they trigger an immune system response in certain people. However, allergens do not cause an adverse effect in most people.

According to the United States Food and Drug Administration (FDA), the following foods are most likely to cause a reaction:

  • milk
  • eggs
  • peanuts (groundnuts)
  • Brazil nuts, walnuts, and hazelnuts (tree nuts)
  • fish
  • shellfish
  • wheat
  • soybeans

Enzyme deficiency: A food intolerance usually means that a person has an enzyme deficiency.

Enzymes are substances in the body that enable people to digest food. If a person has an enzyme deficiency, their body cannot digest certain foods properly. The problematic food depends on the enzyme that is lacking.

Food intolerance may also result from:

  • certain chemicals in foods
  • food poisoning due to the presence of toxins
  • the natural occurrence of histamine in some foods
  • the presence of salicylates that occurs in many foods
  • specific food additives

Food allergies and food intolerance have different symptoms.

Allergy symptoms

According to the FDA, an allergic reaction to food can cause:

  • hives
  • itchiness
  • swelling of the face, lips, and tongue
  • swelling of the throat and airways, leading to breathing difficulties
  • dizziness, lightheadedness, and fainting
  • gastrointestinal symptoms, including vomiting and diarrhea
  • a sudden drop in blood pressure
  • a fast, irregular pulse
  • a loss of consciousness

If swelling occurs in the airways, this can make it difficult for a person to breathe. If the airway closes, this can become fatal.

An allergic reaction can happen suddenly. If a person shows symptoms of an allergic reaction, they need medical help immediately.

Intolerance symptoms

The main symptoms of a food intolerance are:

  • intestinal gas
  • abdominal pain
  • diarrhea

Other symptoms may occur, but the core symptoms relate to a person’s gut.

Some symptoms of food allergy and food intolerance can be similar. This makes an accurate diagnosis more difficult.

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If a person reacts to a food, the treatment will depend on whether they have an allergy or an intolerance.


A person with an allergy may have a mild reaction on one occasion and a severe reaction on another occasion. About 20 percent of people with an allergy have a severe reaction.

Anaphylaxis can range in severity from mild to life-threatening. It can happen quickly and is a medical emergency.

The symptoms of anaphylaxis include:

  • skin reactions
  • swelling
  • difficulty breathing
  • a sudden drop in blood pressure

Anyone who knows they have an allergy and may have a severe reaction should carry an injector, such as an EpiPen, that delivers epinephrine, or adrenaline. The should carry two doses in case one is not sufficient.


A food intolerance will not usually need urgent treatment. The best treatment is a long-term plan to manage the problem.

Managing an intolerance usually starts with an exclusion diet. This is when a person avoids a food that may be causing the problem for some time, usually 2 to 6 weeks.

The individual may benefit from keeping a food diary to record whether their symptoms improve. After this, they reintroduce the food and note any new reactions.

People may need to repeat this monitoring with different food items to pinpoint which food is causing the problem.

Sometimes, people can reintroduce a food without any reaction occurring. It may be that they develop a tolerance, or that a small amount of the food does not cause a problem any longer.

Food intolerance varies widely between individuals, and so each case is likely to have its own characteristics. Sometimes, an underlying problem makes the intolerance worse and needs appropriate treatment.

If a person has a food allergy or an intolerance, they may have to avoid the food that appears to cause it. They may also have to check the ingredients of food products carefully, whether buying for home consumption or eating out.

In summary, prevention means:

  • A person with a known allergy must avoid the trigger food. Not doing so could be dangerous. The person also should carry an epinephrine autoinjector for use in case of an emergency.
  • A person with an intolerance can avoid discomfort by avoiding the trigger food, but they will not face a life-threatening situation.

Since 2004, the Food Allergy Labeling and Consumer Protection Act (FALCPA) states that all packaged foods that manufacturers produce in the U.S. must carry information in simple, clear language about the eight most common allergens.

These widespread allergens are milk, eggs, wheat, soy, peanuts, tree nuts, fish, and crustacean shellfish.

There has been concern recently that the cases of allergy and intolerance are rising.

The National Institute of Allergy and Infectious Diseases (NIAID), part of the National Institutes of Health (NIH) report that around 5 percent of children and 4 percent of adults have a food allergy.

In 2016, the Centers for Disease Control and Prevention (CDC) estimated that 6.2 percent of the population had experienced a food allergy in the previous 12 months.

However, in 2017, a study of electronic health records of more than 2.7 million people in the U.S. suggests that the percentage of the population with either an allergy or intolerance is lower than this, at 3.6 percent overall.

People may not always be aware of the difference between an allergy and an intolerance, and they may not always report an allergy or intolerance to their doctor. For these reasons, it can be difficult to establish exact figures.