A person with a food intolerance has difficulty digesting certain foods. It is important to note that a food intolerance is different from a food allergy.

Food intolerances are common. According to some estimates, they may affect up to 20% of the population.

Food intolerances are more common in those with digestive system disorders, such as irritable bowel syndrome (IBS). According to the IBS network, most people with IBS have food intolerances.

While a food allergy results from an immune response to a specific food, food intolerances usually involve the digestive system, not the immune system.

This article examines the causes, types, and symptoms of food intolerance, as well as the diagnostic process and how people can manage them.

A plastic bottle of milk with a blue cap has been dropped on the floor, and milk is spilling everywhereShare on Pinterest
Paul Burns/Getty Images

A person with a food intolerance will often experience discomfort soon after eating certain foods. The symptoms are varied and usually involve the digestive system.

Common symptoms of food intolerance include:

  • bloating
  • excess gas
  • stomach pain
  • diarrhea
  • migraine
  • headaches
  • a runny nose
  • malaise, which is a general feeling of being under the weather

In people with a food intolerance, the amount of that food they eat determines the severity of their symptoms.

The symptoms of food intolerances can take a while to emerge. The onset may occur several hours after ingesting a food, and the symptoms may persist for several hours or days.

It can be difficult to determine whether someone has a food intolerance or an allergy because the signs and symptoms of these conditions overlap.

Food intolerances arise if the body is unable to digest a certain food. This impairment may be due to a lack of digestive enzymes or a sensitivity to certain chemicals.

Foods commonly associated with food intolerance include:

  • milk
  • gluten
  • food colorings and preservatives
  • sulfites
  • other compounds, such as caffeine and fructose

The following sections look at the various causes and types of food intolerances.

Lactose intolerance

The body uses digestive enzymes to break down foods. If a person lacks certain enzymes, they may be less able to digest certain foods.

Lactose is a sugar that occurs in milk. People with lactose intolerance do not have enough lactase, an enzyme that breaks lactose into smaller molecules that the body can easily absorb through the intestine.

If lactose remains in the digestive tract, it can cause:

  • spasms
  • stomachache
  • bloating
  • diarrhea
  • gas

Fructose intolerance

Fructose is a sugar present in fruit, some vegetables, and honey. Although rare, fructose intolerance can also be due to the lack of an enzyme known as hereditary fructose intolerance.

Fructose malabsorption, in which the body is missing a protein that allows it to absorb the sugar from the intestine, is much more common.

In these individuals, the fructose in foods ferments in the gut, leading to gas, fullness, bloating, cramps, and diarrhea.

Gluten intolerance

Gluten is a protein that occurs in some cereals, including wheat, barley, and rye. A person with gluten intolerance experiences discomfort, such as pain, bloating, or nausea, after eating foods that contain gluten.

Gluten intolerance is also associated with nondigestive symptoms, such as:

  • brain fog
  • headaches
  • joint pain
  • fatigue
  • depression
  • anxiety
  • a general lack of well-being

Gluten intolerance differs from celiac disease — an autoimmune system response to gluten — and wheat allergy, an allergic response to wheat. However, the symptoms of these conditions may be similar.

Gluten intolerance is also known as nonceliac gluten sensitivity.

The symptoms of celiac disease and gluten intolerance typically improve when a person eliminates gluten from the diet but returns when they reintroduce it.

Salicylate intolerance

Salicylates are compounds that occur in many plant foods, including fruits, vegetables, herbs, and spices. They are also common in products containing artificial flavorings and preservatives, such as toothpaste, chewing gum, and candies.

Most people can tolerate moderate amounts of salicylates in their diet, but some have a reduced tolerance.

Symptoms can include:

  • hives
  • rashes
  • stomach pain
  • diarrhea
  • fatigue
  • a runny nose
  • wheezing

Food poisoning

Some foods have naturally occurring chemicals that have a toxic effect on humans, causing diarrhea, nausea, rashes, and vomiting.

For example, undercooked beans contain aflatoxins that can cause extremely unpleasant digestive problems. Fully cooked beans do not have the toxin. As cooking time can affect the amount of toxins, people may react to beans after one meal but not after another.

Similarly, ingesting certain types of spoiled fish can lead to scombroid fish poisoning. This toxic reaction occurs from eating fish high in histamine due to improper storage or processing. It can mimic a severe allergic reaction.

Many people are concerned that they may have an intolerance to food additives. However, according to the Asthma and Allergy Foundation of America, most studies show that few additives cause problems and affect relatively few people.

Food producers often use additives to enhance flavors, make foods look more appealing, and increase their shelf life. Examples of food additives include:

  • antioxidants
  • artificial colorings
  • artificial flavorings
  • emulsifiers
  • flavor enhancers
  • preservatives
  • sweeteners

Of the thousands of additives that the food industry uses, experts believe that only a relatively small number cause problems. The following food additives can cause adverse reactions in some people:

  • Nitrates: These preservatives are common in processed meats. Symptoms of nitrate intolerance can include headaches and hives.
  • Monosodium glutamate (MSG): This flavor enhancer can cause headaches, chest tightness, nausea, and diarrhea in those with an intolerance.
  • Sulfites: Common sources of these preservatives include wine, dried fruits, fresh shrimp, and some jams and jellies. People with a sulfite intolerance may experience chest tightness, hives, diarrhea, and sometimes, anaphylaxis.

Food intolerances and allergies can have similar symptoms, making diagnosis challenging. Diagnosis is also complicated when a person has several food intolerances.

Food intolerances can mimic the symptoms of chronic digestive conditions, such as IBS. However, certain patterns in the symptoms can help a doctor distinguish between the two.

People can keep a food diary to document which foods they eat, the symptoms that appear, and their timings. This information can help people and their healthcare professionals determine which foods are causing adverse reactions.

Apart from lactose intolerance and celiac disease, there are no accurate, reliable, and validated tests to identify food intolerances. The best diagnostic tool is an exclusion diet, also known as an elimination or diagnostic diet.

The doctor may recommend a skin prick test or blood test to rule out a food allergy.

For the skin prick test, a healthcare professional places a small amount of food onto a person’s back or forearm and pokes the skin with a needle. A skin reaction indicates the presence of an allergy.

A blood test can measure levels of immunoglobulin E (IgE) antibodies. High levels can indicate an allergy.

Some people find that if they stay off the specific food for a while, they do not react when eating it again. This is known as tolerance. Maintaining tolerance is often a question of working out how long to abstain from eating the food and how much to eat when reintroducing it. A healthcare professional can help.

These conditions have some symptoms in common. However, food intolerance symptoms tend to take longer to appear than food allergy symptoms.

While a food allergy results from an immune system reaction to a specific food, a food intolerance involves the digestive system rather than the immune system.

With a food intolerance, the amount of food the person eats relates to the severity of their symptoms. On the other hand, with a food allergy, even a small amount of food can trigger a severe allergic reaction.

Learn more about food intolerance vs. food allergy.

Food intolerances arise when the body cannot properly digest certain foods. Lactose intolerance and gluten intolerance are common types.

There is currently no cure for food intolerances. The best way to avoid symptoms of a food intolerance is to avoid certain foods or eat them less often and in smaller amounts. People can also take supplements to aid digestion.