How Food Intolerance Is Diagnosed

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A food intolerance—also known as food sensitivity—is an adverse reaction to a certain food or ingredient in your food. It can also be caused by a reduction in digestive enzymes, the substances that help you digest your food. However, most occur for unknown reasons. A person with a food intolerance may experience abdominal pain, nausea bloating, or diarrhea after eating certain foods.

It's important to know that a food allergy and food intolerance are not the same. While both can cause unpleasant side effects, a food tolerance concerns your body’s ability to digest certain foods, while a food allergy can be life-threatening.

One way to diagnose food intolerance is through an elimination diet. This involves removing certain foods from your diet for a period of time before reintroducing them to learn which foods make symptoms worse.

This article will discuss the different methods to test for a food intolerance and how to know if you have one.

Woman with food intolerance and stomach pain

Tharakorn Arunothai / Getty Images

Self-Checks and At-Home Testing

Your healthcare provider will likely ask you to closely monitor your symptoms at home to keep track of which foods are causing unpleasant effects. They may ask you to keep a food diary or try an elimination diet.

Keeping a Food Diary

If you are having trouble determining which foods are causing side effects, you may be asked to keep a food diary.

A food diary can help you identify reactions to certain foods or ingredients. Your food diary should be very detailed and include when you eat, when symptoms occur, and the characteristics of any symptoms. You may also be asked to jot down other contributing factors such as stress or physical activity.

After around one week of tracking your meals in your food diary, closely review the diary with your healthcare provider to see if there are any noticeable patterns or potential food triggers.

Elimination Diet

Once you have identified any possible food triggers, you may be asked to start an elimination diet. The gold standard to diagnose both food allergies and intolerances is through oral food challenges, which is part of the elimination diet.

During an oral food challenge (also called a feeding test), you eat a food slowly and increase the amount little by little. This is done under medical supervision, where your healthcare provider can assess how the food affects you and determine if it is a true food allergy.

However, an elimination diet is often tedious and can be challenging to stick with for the duration.

Safety Tip

Since an elimination diet can lead to nutrient deficiencies, you should start an elimination diet under the supervision of a dietitian or other qualified healthcare provider.

The first step in the elimination diet is to create a list of foods to avoid. This list is usually compiled from your food diary. You'll need to remove or eliminate these foods from your diet for a set amount of time.

If you notice improvements in your symptoms after you have eliminated foods, one of those foods could have been causing your digestive problems. However, it may be tricky to figure out which food was the cause. This is where the challenging phase comes in. You'll need to reintroduce each food separately after several weeks at a time to see if symptoms reappear. This process will repeat until all foods are reintroduced.

Once you've narrowed it down to a certain food that your body doesn't digest well, you can remove it from your diet to prevent unpleasant symptoms.

What About At-Home Sensitivity Tests?

At-home food sensitivity kits should not be used, as they have not been medically validated to diagnose any condition. Food elimination diets and/or food challenges are the only way to diagnose food sensitivities.

Labs and Tests

Unfortunately, there is no single test to diagnose food sensitivities.

However, there are specific tests to help diagnose lactose intolerance. Lactose tolerance tests check to see if your intestines can break down lactose, the natural sugar found in milk. If you cannot break down lactose, you are lactose intolerant.

Hydrogen Breath Test for Lactose Intolerance

During a hydrogen breath test, you'll be required to breathe into a balloon-type container to give a sample of your breath. Next, you will drink a beverage containing lactose. You'll have samples of your breath taken at different times. If hydrogen levels are high in your breath, your body is having difficulty breaking down and digesting lactose.

Blood Glucose Test

Glucose is the primary sugar present in all humans' blood, providing energy for your body to use. However, some people are not able to naturally regulate their blood glucose levels, indicating an issue. This is a hallmark trait of diabetes and is common with some food intolerances. Therefore, in some circumstances, your provider may order a test to check the level of glucose in your blood after drinking a lactose-containing beverage.

If your glucose rises more than 20 milligrams per deciliter (mg/dL), the test is considered to have abnormal results and you may be required to take a glucose tolerance test to ensure you are able to properly absorb glucose.

Imaging

Generally, imaging is not used for diagnosing food intolerances. However, your healthcare provider may recommend endoscopy (a procedure used to examine the body’s internal organs by inserting a tiny camera into the body) if you are experiencing nausea and vomiting, pain, bleeding, or unexplained weight loss.

Differential Diagnosis

Some of the symptoms you may experience when you have a food intolerance are very similar to those of a food allergy. While a food intolerance may cause digestive discomfort and pain, a food allergy can be life-threatening.

A food intolerance occurs when you lack proper enzymes in your digestive system to break down certain foods. Intolerance may also occur due to a sensitivity to additives in foods or naturally occurring chemicals in food.

In contrast, a food allergy involves a reaction within your immune system that can be life-threatening. When you eat a food that contains an allergen, your immune system overreacts and produces antibodies (IgE).

Summary

A food intolerance can cause uncomfortable symptoms, making it difficult to enjoy meals. If you believe you have a food intolerance, you may consider starting a food diary and bringing your results to a visit with your healthcare provider.

From there, your healthcare provider may recommend that you try an elimination diet and slowly reintroduce trigger foods back in. It's important to follow this diet under close supervision of a dietitian or your healthcare provider to avoid any nutrient deficiencies.

A Word From Verywell

Diagnosing a food intolerance can be a long, tedious, journey that requires ample time and effort. It's important to give yourself grace and have a support team to help you as you work through an elimination diet.

A knowledgeable registered dietitian can help you plan your meals and support you during this process. To find a registered dietitian near you, visit The Academy of Nutrition and Dietetics and search under "Find a Nutrition Expert."

9 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. American Academy of Allergy Asthma and Immunology. Food intolerance versus food allergy.

  2. Medline Plus. Food allergy testing.

  3. American Academy of Allergy Asthma and Immunology. The myth of IgG food panel testing.

  4. University of Wisconsin-Madison. The elimination diet.

  5. Harvard Health Publishing. Keep a food diary to track allergies or intolerances.

  6. U.S. Department of Veterans Affairs. Elimination diet.

  7. Medline Plus. Lactose tolerance tests.

  8. Medline Plus. Lactose tolerance tests.

  9. National Institute of Diabetes and Digestive and Kidney Diseases. Upper GI endoscopy.

By Lindsey DeSoto, RD, LD
Lindsey Desoto is a registered dietitian with experience working with clients to improve their diet for health-related reasons. She enjoys staying up to date on the latest research and translating nutrition science into practical eating advice to help others live healthier lives.