This build-up of gases, such as hydrogen, can lead to symptoms like abdominal pain, bloating, and flatulence. We measure hydrogen because people don’t produce it for any other reason, so it serves as a useful marker when food hasn’t been fully digested.

The scientists at Monash discovered that by eating a diet low in FODMAPs, patients saw significant improvement in their symptoms. An enormous number of people have been able to use this information about the low FODMAP diet to improve their symptoms.

However, the low FODMAP diet can be quite restrictive and isn’t designed for long-term use. Since each of the FODMAPs can have different effects on the body, it is important for people to try to identify which specific FODMAPs are a problem for them. For example, you may be able to handle fructose, but not lactose, or vice versa.

Where does breath testing fit in?

Once a FODMAP reaches the large intestine and is fermented by bacteria, some hydrogen is absorbed into the bloodstream. From there, the blood flows around the body and hydrogen passes into the lungs and is released on the breath.

When doctors want to find out whether their patient is having trouble absorbing a particular FODMAP, they will sometimes order a hydrogen breath test in the hospital. This looks for hydrogen on the breath while the patient digests the FODMAP.

To prepare for these tests, the patient will fast for at least 12 hours. They will then visit the hospital or clinic and breathe into a collection bag. The concentration of hydrogen is measured from the sample to provide a baseline level of hydrogen. Since people don’t naturally produce hydrogen, a person who has been fasting should have very little on their breath.

Once the patient’s base hydrogen level is determined, they drink a glass of water combined with a single FODMAP (e.g. lactose). Breath samples are then collected every 15 minutes, typically for up to three hours.

The doctor uses these test results to see how well the FODMAP was absorbed. For example, if a person digests lactose well, their hydrogen levels will remain low throughout the lactose breath test, as shown below.