What Is a Food Intolerance?

Learn what a food intolerance is, causes and symptoms, common types of food intolerances, and how food intolerances are completely different from food allergies.

Food intolerances (also called food sensitivities) cause adverse symptoms when someone consumes certain foods. They can have a wide range of causes, but they usually involve the digestive system.

These intolerances are completely different from food allergies, because food intolerances never involve the immune system.

Today, we'll break down everything you need to know about food intolerances.

What is a food intolerance?

When someone has a food intolerance, their body can't properly break down (digest) a certain food. Their food intolerance causes them to experience an adverse reaction, with unpleasant symptoms, when they consume that food. Most commonly, these symptoms involve the digestive system and GI tract.

Although these symptoms can make someone feel very uncomfortable, food intolerances are hardly ever life-threatening.

What can cause a food intolerance?

Food intolerances can have a wide range of causes. Here are some of the most common:

  • Someone's body doesn't make enough of an enzyme needed to digest a food
  • A person's body is sensitive to natural toxins in a food, which remain when a food is not properly cooked
  • A person's body is sensitive to a natural chemical in a food
  • Someone has difficulty digesting a food's protein

And some intolerances, like gluten intolerance, have unknown causes.

Common types of food intolerances

Some of the most common types of food intolerances include:

Lactose intolerance

Someone with a lactose intolerance has a small intestine that doesn't produce enough of the lactase enzyme.

The lactase enzyme is needed to digest lactose, which is the sugar found in cow's milk.

When someone with a lactose intolerance consumes a milk product containing lactose, they experience digestive symptoms because they can't digest the lactose.

Lactose intolerance is most common in adults and older children. Some young children also develop it, though, if they don't carry the genes that enable them to produce lactose, or if they were born prematurely.

Learn more about lactose intolerance, and other milk intolerances, here.

Milk protein intolerance

Like lactose intolerance, milk protein intolerance also causes someone to have trouble digesting cow's milk, and causes digestive symptoms when cow's milk is consumed.

But this time, the person's digestive system struggles to break down the proteins in milk, not the sugars.

This is why some children with milk intolerances still have digestive problems when they have lactose-free cow's milk products. They can't tolerate the milk proteins, which have not been removed from these foods.

Compared to lactose intolerance, cow's milk protein intolerance is more common in the youngest children.

Milk-soy protein intolerance

Milk-soy protein intolerance is similar to milk protein intolerance, as it causes someone to have trouble digesting milk proteins. But their digestive system also can't break down soy proteins. So, they experience GI symptoms when they consume either cow's milk or soy.

Gluten intolerance

We don't know what exactly causes gluten intolerance. But people with this food intolerance have difficulty digesting the gluten (a type of protein) in wheat, barley, rye, and other grains. This causes uncomfortable digestive symptoms when they eat foods containing gluten.

Symptoms of food intolerances

Almost all the symptoms of food intolerances involve the digestive system. And most involve the GI tract.

Common food intolerance symptoms include:

  • Stomach pain
  • Abdominal pain
  • Vomiting
  • Diarrhea
  • Loose or abnormal stools
  • Gas
  • Bloating
  • Stomach or abdominal cramps
  • Indigestion
  • Heartburn
  • Nausea

In babies and young children, intolerances to food proteins can also cause the following symptoms:

  • Colicky crying
  • Irritability after consuming the problem food
  • Blood or mucus in stools
  • Spitting up frequently

Symptoms of a food intolerance typically appear minutes to hours after eating a problem food.

How are food intolerances diagnosed?

There are currently no proven tests that can accurately diagnose food intolerances.

To determine whether your child has an intolerance to a certain food, doctors usually recommend removing the suspected problem food from your child's diet and seeing if they still develop the GI symptoms associated with food intolerances.

If removing the food means the symptoms lessen or go away, the problem food is then added back into your child's diet to see if the symptoms reappear.

If the symptoms resume, it's likely that your child has an intolerance to that food. (But remember: since tests aren't available, there's no way to know with certainty.)

How are food intolerances different from food allergies?

Chart showing the different symptoms and causes to show the difference between a food intolerance and a food allergy

Food intolerances are very different from food allergies.

Systems of the body

The main difference between an allergy and an intolerance is the body systems involved.

  • Food allergies always involve the immune system.
  • Food intolerances never involve the immune system and usually involve just the digestive system.

If someone experiences ONLY GI symptoms after eating a certain food, they likely have a food intolerance and not a food allergy. GI symptoms rarely occur by themselves when someone has an allergic reaction.

Learn more about identifying allergies vs. intolerances from the American Academy of Allergy, Asthma and Immunology (AAAAI):


Food allergy reaction symptoms usually appear seconds to minutes after eating a problem food, and almost always appear within 2 hours.

But food intolerance symptoms usually appear minutes to several hours after eating a problem food.

Types of symptoms

While many symptoms overlap between food allergies and intolerances, only a food allergy will cause hives, itchiness, and swelling. You don't experience these symptoms when you have a food intolerance.


Another difference is how severe reactions can become.

  • Food allergies can sometimes involve reactions in many systems of the body. These reactions can sometimes become severe, or even life-threatening.
  • In contrast, food intolerance reactions usually only involve the digestive system, and are rarely life-threatening.

How much food is needed to cause symptoms?

The majority of people with food intolerances can eat small amounts of the offending food with no issues. Many have to eat a relatively larger amount of the food before they experience any digestive symptoms.

But if someone has a food allergy, they'll consistently experience an allergic reaction every time they eat their problem food. And even the smallest amount of the problem food can trigger an allergic reaction.

Are there ways to lessen symptoms when a food is eaten?

With an intolerance, the offending parts of a food (like lactose or gluten) can often be removed, so the person can eat the food with no issue. For example, someone can drink lactose-free milk or eat gluten-free bread.

Someone can also take pills that aid their digestion and help prevent intolerance symptoms (for instance, someone with a lactose intolerance can take a pill that contains the lactase enzyme they need to digest milk).

With a food allergy, there's no way to break down a food so the person can freely eat meals containing it with no issue. Instead, people with food allergies need to avoid their problem foods completely.

There is an emerging type of therapy (oral immunotherapy, or OIT) that can desensitize someone to a problem food, after they already have an allergy to the food. When the therapy works, the child is less likely to have a severe reaction when exposed to the food.

But this will only protect an allergic child when they're accidentally exposed to a small amount of a food. The therapy doesn't make it so someone with an allergy can choose to eat that food in a meal. These therapies are also still being studied, and are not widely available.

For more on the difference between food intolerance symptoms and food allergy symptoms, please read our complete guide.

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