Hydrogen breath tests help to diagnose either intolerance to sugars or small intestinal bacterial overgrowth (SIBO).

The test measures how the amount of hydrogen present in your breath changes after you consume a sugar solution. There’s usually very little hydrogen in your breath. Having a higher level of it usually indicates a problem, either from sugar tolerance or bacterial growth in your small intestine.

Your doctor will perform a hydrogen breath test if they suspect that you have an intolerance to a specific sugar or small intestinal bacterial overgrowth (SIBO).

Sugar intolerance

Sugar intolerance means you have trouble digesting a specific type of sugar. For example, some people can’t tolerate lactose, a sugar found in milk or other dairy products.

Lactose is normally broken down in the small intestine by an enzyme called lactase. People who are lactose intolerant can’t make this enzyme. As a result, the lactose moves into their large intestine, where it’s broken down by bacteria instead. This process makes hydrogen, which will show up during a hydrogen breath test.

You can also have intolerance to other sugars, such as fructose.

Small intestinal bacterial overgrowth

SIBO refers to having an unusual amount of bacteria in your small intestine. This can cause many symptoms, including bloating, diarrhea, and malabsorption.

If you have SIBO, the bacteria in your small intestine will break down the sugar solution given during the hydrogen breath test. This results in hydrogen, which a hydrogen breath test will pick up.

Your doctor will ask you to do several things to prepare for your hydrogen breath test.

Four weeks before your test


One to two weeks before your test

Avoid taking:

The day before your test

Only eat and drink the following:

  • plain white bread or rice
  • plain white potatoes
  • baked or broiled plain chicken or fish
  • water
  • unflavored coffee or tea


  • sweet drinks, such as soda
  • foods with a high fiber content, such as beans, cereal, or pasta
  • butter and margarine

You should also avoid smoking or being around secondhand smoke. Inhaling smoke can interfere with your test results.

The day of your test

Avoid eating or drinking anything, including water, in the 8 to 12 hours prior to your test. Your doctor will confirm with you when you should stop eating and drinking.

You may continue to take any usual prescription medications with a small amount of water. Just make sure you tell your doctor about any prescription medications you take, especially if you have diabetes. You may need to adjust your insulin dosage before the test.

The day of your test, you should also avoid:

  • smoking or inhaling secondhand smoke
  • chewing gum
  • using mouthwash or breath mints
  • exercising

To perform a hydrogen breath test, your doctor will start by having you gently blow into a bag to get an initial breath sample.

Next, they’ll have you drink solution containing different types of sugar. You’ll then breath into a bag every 15 to 20 minutes as your body digests the solution. After each breath, your doctor will use a syringe to empty the bag.

While hydrogen breath tests are fairly simple to do, they can take two to three hours, so you may want to bring a book to read in between breaths.

The amount of hydrogen in your breath is measured in parts per million (ppm).

Your doctor will look at how the amount of hydrogen in your breath changes after you drink the sugar solution. If the amount of hydrogen in your breath increases by more than 20 ppm after drinking the solution, you may have a sugar intolerance or SIBO, depending on your symptoms.

A hydrogen breath test is a fairly simple, noninvasive way to check for sugar intolerance or SIBO. However, there are certain guidelines you need to follow in the month leading up to the test. Make sure your doctor goes over exactly what you need to do to prepare so your results are accurate.