How a Food Intolerance Is Diagnosed

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A food intolerance, or food sensitivity, is not the same thing as a food allergy. Instead, a food intolerance means that you have trouble digesting a particular food. For instance, a food that you are sensitive to may cause abdominal pain, gas, bloating, or diarrhea. In other words, a food intolerance impacts your digestive system. Meanwhile, a food allergy impacts your immune system.

For instance, true food allergies involve a specific process in which your immune system produces antibodies known as immunoglobulin E (IgE) in response to certain foods it mistakenly believes are harmful to your body. The release of IgE triggers a cascade of cellular responses, including the release of histamines that cause the symptoms we recognize as an allergy.

But there are times when IgE antibodies have nothing to do with your adverse food reaction. Instead, you may have a food intolerance that results in many of the same symptoms. To diagnose a food intolerance, your doctor first has to eliminate the possibility that you have a food allergy.

Self-Checks/At-Home Testing

Diagnosing a food intolerance (also known as food hypersensitivity) can be tricky since the mechanism behind an intolerance can vary. Your doctor or allergist may need to perform a variety of investigations to get to the bottom of your case. This usually involves the exclusions of the possible causes, one by one, until the actual culprit is revealed.

But she needs your help. Your doctor will likely ask you to take several steps at home that will help determine if you have a food intolerance. These steps including keep a food diary and participate in an elimination diet, both of which can provide helpful diagnostic information.

Keeping a Food Diary

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If your doctor is unsure whether food is the cause of your symptoms, he or she may recommend that you keep a food diary. A food diary can also be used to monitor dietary patterns to better pinpoint the cause of your symptoms.

The diary should be a complete record of not only the foods you've eaten over a given period of time (usually a week), but also an accurate record of the timing and characteristics of any symptoms you may have experienced.

Other doctors may go so far as to ask you to record any activities you many done prior to your symptoms to assess whether they may have contributed. In some cases, stress and physical exertion can play as much a role in your symptoms as the foods you eat.

A food diary is often used in combination with other diagnostic tools or as a first step in opening an investigation.

To make a food diary work, you need to provide as much detail as possible and not exclude facts that you either deem irrelevant or would rather not share (like the amount you drink or smoke).

Elimination Diets

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An elimination diet works by the process elimination. By keeping track of your symptoms as you eliminate suspected foods from your diet, you can often pinpoint the cause of your intolerance, such as dairy or gluten.

While different doctors will conduct an elimination diet in different ways, the basics are similar: to exclude the foods you suspect are causing your symptoms and then to reintroduce them after several days or weeks to see if symptoms reappear.

By taking the time to identify your trigger foods, you can often gain better control of your chronic or recurrent symptoms. It is also a way to hone the ideal diet if you are struggling with gastroesophageal reflux disease (GERD) or irritable bowel syndrome (IBS).

Once you've stabilized your diet, you can slowly increase the number of foods you eat and eliminate any that sets off an attack. As a diagnostic tool, elimination diets can be used to investigate a wide variety of food intolerance symptoms.

Labs and Tests

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Most diagnostic blood tests for food intolerances are controversial because they don't so much pinpoint the cause of a disorder but identify characteristics in the blood that may or may not support the diagnosis.

However, there are two conditions where blood tests should be one of the first steps toward establishing a diagnosis: celiac disease and lactose intolerance.

The celiac blood test is designed to detect autoantibodies released by the immune system in people with celiac disease. While useful, the test is prone to false-positive and false-negative results, and other tests are usually required to ensure a definitive diagnosis.

By contrast, the lactose intolerance test requires you to drink a liquid containing lactose. After two hours, your doctor will take a blood sample to measure how much glucose is in your blood. If your level does not rise, it means that your body is not digesting lactose and is intolerant.


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Endoscopy is a non-surgical examination used to diagnose some gastrointestinal issues, including celiac disease. Using an endoscope, a flexible tube with a light and camera, your doctor can view images of your small intestine on a digital monitor and take a sample of tissue (biopsy) for evaluation in the lab.

For this test, the endoscope is lowered into the stomach, usually through your esophagus. When checking for celiac disease, gastroenterologists will look for patterns of damage in the intestine, namely the flattening of finger-like villi lining the intestinal wall (known as villous atrophy).

According to research from the Mayo Clinic, the prevalence of celiac disease in the United States is 0.7 percent, or roughly one out of every 141 people.

Endoscopy is not usually used for other types of food intolerance unless you are experiencing digestive tract bleeding, chronic constipation or diarrhea, or severe persistent stomach pain.

Hydrogen Breath Test

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hydrogen breath test (HBT) is a diagnostic tool used to detect bacterial overgrowth in the intestine as well as the malabsorption of carbohydrates such as lactose and fructose. The HBT measures the amount of hydrogen in your breath. Hydrogen is a byproduct of lactose consumption in people with lactose intolerance.

The HBT is simple, non-invasive, and performed after an 8- to 12-hour fast. After baseline results are taken, you will be given an oral solution to drink containing lactose. After one hour, you will be retested. A sharp rise in hydrogen is strongly suggestive of lactose intolerance.

With that being said, around one-third of people with lactose intolerance will not emit excess hydrogen but rather another gas known as methane. As such, the HBT may return a false negative result in people with the characteristics and symptoms of lactose intolerance.

According to research, as many as 36 percent of Americans have some level of lactose malabsorption using the HBT. While many will manifest with mild to severe lactose intolerance symptoms, others will have no symptoms at all.

A Word from Verywell

It can be tricky for your doctor to determine what is causing your symptoms of food intolerance. Symptoms often appear hours after you've eaten, so trying to pin cause to effect is not always easy.

Diagnosing a food intolerance is ultimately a process of trial and error, and there is no pretending that the process is easy. Elimination diets can be hard to implement, especially if you eat out frequently, and food diaries don't always capture everything you eat.

Even so, your best bet is to follow your doctor's advice diligently, even if it means giving up fast food for lunch or taking a few minutes after lunch to write down everything you've just eaten. If you are patient and remain diligent, you are more likely to find the cause of your problem.

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