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Diagnostics > Breath Tests

Hydrogen breath tests: All you need to know

Dr. med. André Sommer

Dr. med. André Sommer

The hydrogen breath test (also called the H2 breath test) is often used to detect carbohydrate intolerance. It can be used to diagnose a lactose- (milk sugar), fructose- or sorbitol-intolerance, as well as a bacterial overgrowth in the small intestine. At the same time, it can also calculate the time it takes for food to go from the mouth to the large intestine. This article explains exactly how the test works and what should be considered when performing it.

When is a hydrogen breath test performed?

The hydrogen breath test is performed when a carbohydrate intolerance or malabsorption is suspected. This means either that corresponding food components in the intestine are not digested sufficiently due to an enzyme deficiency or that there are problems with uptake in the bloodstream. Instead, the undigested sugar compounds get into the large intestine, where they are broken down by bacteria with the development of gases. This is why symptoms such as flatulence and abdominal pain are often present. Additional liquid attracted by the sugar can also increase the risk of diarrhea. Other symptoms like fatigue, headache, dizziness and nausea are possible as a result.

Symptoms usually develop 15 minutes to 2 hours after the respective food has been consumed. Lactose intolerance mainly involves milk products such as cow's milk, yogurt, some cheeses, and butter. Fructose is primarily found in fruit and certain vegetables, but is also often added to foods as a sweetener. Sorbitol is a sweetener that can often be found in low-calorie “light” products as a sugar substitute.

What other diseases are breath tests used for?

With some diseases there is an improper colonization of the small intestine by bacteria (small intestinal bacterial overgrowth, SIBO). Bacteria are usually found primarily in the large intestine. If, for example, they can already be detected in the small intestine as a result of a bowel disorder, this can lead to flatulence and diarrhea. The hydrogen breath test can be used to prove colonization of the small intestine and is less invasive and cheaper than a small intestine examination.

As a result of medication and hormonal fluctuations, the gut motility and thus the transportation time of the food pulp can be reduced or accelerated. Diarrhea or constipation can result. To confirm this suspicion, a hydrogen breath test can help.

How does hydrogen get into our breath?

Most carbohydrates such as milk sugar or fructose are split in the intestine by specialized enzymes and then absorbed into the bloodstream through the intestinal wall.

If the gut has poorly functioning enzymes, either as a congenital disorder (primary enzyme deficiency) or because, for example, the intestinal wall is damaged through inflammation (secondary enzyme deficiency, e.g. in celiac disease), carbohydrates can no longer be digested completely. They then enter the large intestine, where they are used by the bacteria located there. But because bacteria choose different chemical pathways than the human intestine, this produces gaseous substances, in particular, hydrogen (H2).

What’s the idea behind the hydrogen breath test?

Hydrogen is absorbed into the blood through the intestinal wall and reaches the lungs within a few minutes. In the lungs, the hydrogen is then released into the alveoli and (together with carbon dioxide) is exhaled. The exhaled air can then be analyzed for hydrogen content.

The fewer functioning enzymes in the intestine, the more carbohydrates get into the large intestine and are processed there with the production of hydrogen—and the more hydrogen is ultimately exhaled through the lungs. Thus, the hydrogen content in the exhaled air is an indirect measurement for the amount of working enzymes.

What happens during the test?

In a hydrogen breath test, a certain type of sugar is administered under controlled conditions (depending on the presumption of intolerance), and the hydrogen content in the exhaled air is measured at specified times. If the hydrogen content does not rise at all, intolerance can be ruled out. If it increases after 90 to 120 minutes, there is probably a food intolerance. If there is an increase in hydrogen after around 30 to 60 minutes, it can be concluded that either hydrogen-producing bacteria are already in the small intestine (an indication of small intestinal bacterial overgrowth) or the passage of the intestine is very strongly accelerated, and the sugars have already reached the large intestine.

How is a hydrogen breath test performed?

The hydrogen breath test is a non-invasive test. Blood is therefore not drawn, and surgery or anesthesia are not necessary. The test can be carried out as a straightforward outpatient procedure in a gastroenterology practice. For the entire test, 2-3 hours are needed.

If there is actually an intolerance, the test can cause symptoms such as bloating and diarrhea. For this reason, if there is reasonable suspicion of food intolerance, enough free time should be planned after the test. After an inconspicuous test, vehicles and machines can be operated without restriction. The hydrogen test can also be done with children if they are able to follow the breathing instructions.

How does a breath test start?

At the beginning of the hydrogen breath test, the basic value of the hydrogen content in the exhaled air is measured. To do this, the patient blows into a tube that is connected to a measuring instrument. You hold your breath for 20 seconds before exhaling. The patient then drinks a glass of water on an empty stomach in which a measured amount of sugar has been dissolved.

To prove lactose intolerance, 25 grams of milk sugar, for example, are dissolved, as is fructose. Should a sorbitol intolerance need to be tested, 5-8 grams of sweetener are sufficient.

How long does a hydrogen test last?

After drinking, the patient can take a seat in the waiting room. After 30, 60, 90 and 120 minutes, the hydrogen content in the exhaled air is checked. During this time, the doctor should be informed about symptoms such as abdominal pain or diarrhea.

Parallel to the breath test, further investigations may be carried out. For example, the blood sugar can be measured using a small blood sample from the fingertip. If there is no intolerance, the sugar solutions are absorbed into the blood and the blood sugar level increases. An evaluation can usually be carried out directly after the tests. The test is finished after 2 hours, and the patient can leave the practice on their own.

How do you measure bacterial overgrowth?

To test bacterial overgrowth in the small intestine, glucose is dissolved in the liquid to be consumed. Glucose is usually digested in the first part of the small intestine and doesn’t reach the large intestine. If hydrogen-producing bacteria are already in this part of the intestine, this can be demonstrated in the breath test.

When do I have to drink lactulose?

Lactulose is a fiber that is not absorbed in the intestine at all, but is only broken down by bacteria in the colon. Depending on when hydrogen can be detected in the exhaled air, you know that the lactulose has reached the large intestine. If there are no other indications that the intestinal passage is too fast (e.g. diarrhea), early exhalation of hydrogen may also indicate bacterial colonization of the latter sections of the small intestine.

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Can multiple intolerances be measured with each breath test?

No, only one type of sugar is tested per breath test. If you measured several incompatibilities at the same time, the results would be inaccurate. It is therefore worthwhile to take a good nutritional history beforehand to find out which nutritional components trigger the symptoms. If this is not possible, the tests must be carried out separately on different days.

How is a hydrogen breath test prepared?

The patient must have fasted

In order not to falsify the results of the breath test, the test must be performed on an empty stomach. This means that nothing should be eaten 12–14 hours before the test. The last meal should be a low carbohydrate, small portion meal without flatulence-inducing or heavy side dishes. Drinking is still allowed after this meal, but only uncarbonated water. It’s important that the fasting requirement also includes neither smoking nor chewing gum as both could lead to incorrect results.

Brushing teeth

While there used to be a recommendation not to brush your teeth on the day of the test, this has now been revised. Dental care should be carried out as normal, but without the use of mouthwash and denture adhesive cream.


Most medications can be taken with clear water in the morning of the day. Vitamin tablets should be avoided on the test day, and laxatives should be stopped three days before the test.

No chewing gum

Even during the two-hour test, you must not smoke, eat, drink, or chew gum. If in doubt, the procedure should be clarified in advance with the doctor performing the procedure.

How is a hydrogen breath test evaluated?

The proportion of hydrogen in the exhaled air is measured in ppm (parts per million, particles per million particles). Usually the value is below 10 ppm. If the value increases to 10 to 20 ppm during the test, the test duration should be extended to obtain a clear result. Values above 20 ppm are considered positive, in which case there is a carbohydrate intolerance.

What things can affect a hydrogen breath test?

In addition to previous food intake, smoking or chewing gum, other factors can falsify the test result. In the four weeks before the test, no antibiotics should be taken, otherwise the intestinal flora may be weakened and may not be able to produce hydrogen. Also a colonoscopy should not be performed in the month before the test, as this too can affect the bacteria in the colon.

Some patients develop typical symptoms during the breath test but do not show an increase in hydrogen. This may be because the hydrogen is used directly by other bacteria in the intestine and therefore does not get into the blood. These patients are called non-producers. A diagnosis can then only be made by measuring blood sugar and having a clear medical history. The symptoms should improve if the suspected food component is dispensed with.

What does it mean if a hydrogen breath test is positive?

If the breath test is positive, it most likely means that there is an intolerance to the administered sugar, and the patient should not consume the sugar concerned. Around 90 to 96 people in 100 people are correctly diagnosed.

In certain cases, such as lactose intolerance, the missing enzyme can be taken as a tablet directly before consuming a milk product (in this case, the so-called lactase) instead of avoiding the sugar. Since the remaining enzyme activity differs from person to person, it is also necessary to individually test how many tablets are needed for which amount of lactose.

Is a hydrogen breath test useful for IBS?

Hydrogen breath tests can provide no definitive evidence of irritable bowel syndrome (IBS). However, they can help exclude other possible explanations for IBS symptoms (including bloating, diarrhea, abdominal pain) and thus help the diagnosis. A breath test should be considered, especially if symptoms occur only after certain foods.

What does the research say about breath tests for IBS?

Some patterns of irritable bowel patients who take the hydrogen breath test have been shown in studies but are not yet suitable for diagnosis. Since bacterial overgrowth in the small intestine is associated with IBS, a breath test can be useful in this regard. A lactulose breath test can indicate whether the intestinal passage is too fast, as is the case with many irritable bowel patients.

Studies have also shown a difference in breath tests between irritable bowel patients who were on the recommended low-FODMAP diet and those who were on a regular diet. This suggests that such a diet could reduce general gas production in the intestine and thus lead to less bloating and pain.

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Dr. med. André Sommer

Dr. med. André Sommer

I’m André, a medical doctor from Berlin. Together with a team of medical doctors, nutritionists and data scientists we empower people to understand digestive issues with our app Cara Care.

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