Goal and principle

Goal and principle

Lactose is present in milk and many dairy products. If a person experiences nausea, abdominal cramps, bloating, excessive flatulence or diarrhea after consuming milk and/or milk products, that person may be lactose intolerant.
Normally, lactose is broken down by the enzyme lactase in the small intestine. Patients with lactose intolerance have a deficiency of this enzyme, meaning that the lactose is not broken down in the small intestine, but rather passes further on into the large intestine. Hydrogen gas (H2) is made by gut bacteria during the breakdown of carbohydrates (in this case, the milk sugar or lactose). This hydrogen gas is absorbed by the blood, then passes through the lungs before being exhaled. The more hydrogen gas that is found in the exhaled air, the more evident lactose malabsorption is.

Lactose intolerance points to the discomforts and symptoms experienced when ingesting milk and milk products. This is usually due to lactose malabsorption, but not always. Sometimes, lactose malabsorption does not cause symptoms.

This study is perfectly safe, including for pregnant women and children.

Test procedure

Test procedure
  • At the start of the examination, you will be required to exhale through a mouthpiece into a measuring device to determine the basic hydrogen level of your breath before the ingestion of lactose.
  • You then take the test substance: 25g of lactose dissolved in 2 glasses of tepid water. You will not be administered any radioactive products. The tests do not carry any other risks.
  • You will then need to inhale through the mouthpiece and into the measuring device every 15 minutes.
  • The test takes 3 hours. During the test, you must stay sitting quietly. You may not walk around, nor eat or drink. Bring something to read, paperwork, etc.
  • If you are lactose intolerant, you quite possibly could experience abdominal cramps, nausea, etc. during the test. This is normal, since you were administered a fairly high dose of lactose.
  • At the end of the test, you will be given the result.

How to prepare for the test

How to prepare for the test

For a successful test and to avoid inaccurate results, please observe the following measures:

• Starting 4 weeks before the test: no antibiotics may be taken and no colonoscopy may be performed

• Starting 1 week before the test: no reflux medication and no acid blockers may be used

• Starting 3 days before the test: no laxatives may be used, certainly no lactulose

• The day before the test:

- Avoid milk and fruit juice

- Avoid foods rich in fibre

• Last meal: light meal, with not a lot of fibre and with no dairy.

• 12 hours before the test:

- continue FASTING (no eating nor drinking fluids)

- no smoking nor chewing gum

• The day of the test:

- No vitamins, laxatives or antibiotics.

- Continue taking your normal medication (anticoagulants, blood pressure and cholesterol medications) just as you do regularly every day.

- Brush your teeth thoroughly before the test with clean water (no toothpaste). Do not use any denture adhesive the day of the test.

- Drink a glass of warm water in the morning to facilitate peristalsis and to flush out intestinal bacteria.

If you have diabetes, please consult your physician.

Centres and specialist areas

Centres and specialist areas
Obesity Centre
Digestive Centre

Latest publication date: 23/11/2022
Supervising author: Dr Vanderstraeten Erik