Hydrogen and Methane Breath Testing

hydrogen and methane breath test provides information about the digestion of certain sugars or carbohydrates, such as milk sugar (lactose) or fruit sugar (fructose). This will help determine if you are intolerant to, or can’t digest, certain sugars. One example is lactose (milk sugar) intolerance, a disorder in which people develop symptoms after eating foods such as dairy products or which contain lactose as an additive.

The test is also used for detecting abnormal growth of bacteria within the small bowel (SIBO – Small Intestinal Bacterial Overgrowth). We usually use lactulose (a non-digestible sugar) for this test as it travels through the whole bowel. We also use glucose if we are particularly interested in the upper part of the small bowel. Bacterial overgrowth can cause a variety of symptoms including nausea, diarrhoea, bloating, gas, and abdominal cramps.

Information Sheets

Hydrogen and methane gas excreted from the body is produced by intestinal bacteria. Bacteria, normally in the large intestine, produces hydrogen and methane through fermentation of carbohydrates – such as lactose and fructose which is why we use these substrates for the breath test. Some of the hydrogen and methane produced by bacteria is absorbed by the intestinal lining whereby it enters the vasculature (blood vessels) and is transported to the lungs.

The gases are then exhaled by the lungs as you breathe. We collect samples of your breath every 15 minutes after ingesting the test solution in a foil bag for subsequent analysis. In patients with small intestinal bacterial overgrowth (SIBO), the bacteria begin to ferment sugars / carbohydrates before you have had chance to digest them. This produces an early rise in the breath hydrogen or methane (within the first 60 minutes after ingestion). Because the small bowel is….well…small, this early fermentation can distend the bowel wall and cause symptoms.

For lactose intolerance, the individual has a deficiency in lactase, the enzyme that breaks down lactose. Normally, lactose is broken down in the small intestine by lactase and very little lactose reaches the large intestine where the bacteria break it down to produce hydrogen. In lactose intolerance (lactase deficiency), the ingested lactose is not metabolised in the small intestine and reaches the colon where it is metabolised by colonic bacteria producing a large amount of hydrogen which is measured in the breath sample. This excessive fermentation can cause symptoms and diarrhoea.

For fructose intolerance, the individual does not absorb all of the fructose they ingest and it reaches the colon where it is metabolised by colonic bacteria producing a large amount of hydrogen which is measured in the breath sample. This excessive fermentation can cause bloating symptoms and either constipation or diarrhoea (often alternating between both).

  • The Nurse or Clinical Scientist will verify that you have not had anything to eat or drink after midnight.
  • A breath sample will be collected by having you exhale into the test machine.
  • A glass of water mixed with either lactose, lactulose, or fructose will be given to drink. You should drink this whole amount.
  • Breath samples will be collected approximately every 15 minutes.
  • During the test, you should take notice of your symptoms and inform the technician, if you have your typical symptoms for which the test is being performed.
  • During the test, you should not eat; eat sweets, smoke, sleep, or exercise.
  • When the test is over, generally after two or four hours, you may leave. You may return to your usual diet and activity after the test.
  • The report will be sent to your Doctor within 5-7 days,

It is completely normal that you may experience symptoms that you usually suffer from. It is important to take notice of your symptoms and inform the technician of the changes you feel throughout the procedure.

For four (4) weeks before your test:

You should not take any antibiotics.

For one (1) week before your test:

Do not take any laxatives or stool softeners (for example Colace, Milk of Magnesia, Ex-Lax) or stool bulking agents (for example Metamucil or Citrucel). You should also not undergo any test that requires cleansing of the bowel, such as colonoscopy or barium enema.

The day before your test:

You may consume only the following foods and drinks: plain white bread, plain white rice, plain white potatoes, baked or broiled chicken or fish, water, non-flavoured black coffee or tea. Only salt may be used to flavour your food. Butter or margarine is not permitted. Fizzy drinks such as lemonade or cola are not permitted.


Eating prohibited foods could give false results for the test. Specifically, avoid beans, pasta, fibre cereals and high fibre foods.

The night before the test: have an early dinner of rice and meat.

For 12 hours before your test:

Apart from small amounts of water, you must stop eating and drinking 12 hours before the test. For example, if your test is at 9:00 a.m., you would stop eating and drinking at 9:00 pm the night before (see your appointment letter for precise timings). You may continue to take your usual prescription medicines (only those not on the prohibited list) with water until 12 hours before the test. Take only essential medications the morning of your test. Please bring all prescription medications to your appointment. If in doubt, please call us.

The day of your test:

Apart from small amounts of water, you should not eat or drink anything in the morning. You may take your medications with a small amount of water. If you are diabetic requiring insulin or diabetic pills, ask your GP if you should change your morning dose. Generally, half of your normal long acting insulin is given. Oral hypoglycaemic medications are usually not taken that morning until completion of the test and resumption of eating meals. If in doubt please check with your referring Consultant. The morning of your test, please brush your teeth but be careful not to swallow the toothpaste.


Do not sleep or exercise while the test is being done. Your test may last for two to four hours. Please allow yourself sufficient time to complete your test.

This test is an important part of the investigation and the future management of your stomach condition. The result assists your doctor in planning appropriate treatment for you so it is important for you to keep your appointment.

If you have any queries about the tests, please contact Dr Anthony Hobson on 07850 114 966.


+44 (0) 207 486 7777


22 Upper Wimpole Street,

73-79 King Street,
M2 4NG


Hydrogen Breath Test Patient Information

Download your Hydrogen-Breath Test Patient Information sheet

Contact Us

Please call us now on +44 (0) 207 486 7777.

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