Peak H₂ + CH₄

This test is easy to misunderstand. Hydrogen is the part that points to fermentation. CO₂ is the part that tells the lab whether the breath sample is good enough to trust.

After you drink lactulose or glucose, microbes that meet the sugar break it down and release hydrogen. The gas moves into your blood, reaches your lungs, and leaves in your breath. Human cells do not make hydrogen this way.

What the peak measures

You breathe into a collection tube before the sugar drink and then every fifteen to twenty minutes for the rest of the test. The peak is the highest hydrogen reading across that timeline, usually checked against the carbon dioxide in the same sample.

Carbon dioxide is not the gut signal. Your cells make it all day. A deep lung sample carries enough CO₂; a shallow or leaky sample is diluted by room air. Labs use that clue to reject bad samples or correct hydrogen readings that look too low because the sample was diluted.

What a high hydrogen pattern can mean

Timing is traditionally used to read the curve. An early hydrogen rise is taken to suggest the sugar met too many bacteria in the small intestine. SIBO means too many bacteria are living there. A later rise is read as the sugar reaching the colon and feeding the microbes that normally live there, which fits carbohydrate malabsorption. This timing rule is not validated, and imaging studies show it is heavily confounded: a fast gut can push sugar into the colon early, so an early rise often reflects transit speed rather than small-intestine overgrowth.

That distinction matters if milk, fruit, or sweeteners set you off, but breath testing usually cannot cleanly separate small-bowel from colonic fermentation, especially with lactulose. The same gas can support very different decisions depending on the sugar used, the timing of the rise, and whether your symptoms happened during the test.

Why methane still matters

This test does not replace methane. Some gut microbes consume hydrogen and make methane. In those people, hydrogen can look low while fermentation is still active.

Methane is tied most strongly to constipation and slower gut movement, though this link is not universally confirmed. If constipation is part of your picture, pair this result with peak methane rather than trying to read hydrogen by itself.

Why one result can fool you

Breath testing is a moving target. Prep, substrate choice, oral bacteria, gut speed, recent antibiotics, laxatives, and probiotics can all change the curve.

• Sample quality: low CO₂ means the sample may be too diluted to trust.
• Mouth bacteria: bacteria on your teeth and tongue can make hydrogen before the sugar reaches your gut, especially at the first sample.
• Transit speed: a fast gut can deliver lactulose to the colon early and mimic small-intestine fermentation; a slow gut can delay the rise.
• Recent drugs and supplements: antibiotics can suppress gas production, and laxatives or motility drugs can shift timing. Probiotic effects are less predictable, so follow the lab's pause instructions.

False positives are the main risk with a positive result that does not match symptoms. In a healthy-adult repeat study, lactulose classified about 60% of volunteers as positive, versus roughly 10% to 15% for glucose. In an imaging study of glucose breath tests, 48% of positive tests were explained by normal colon fermentation rather than small-intestine overgrowth.

Tracking after treatment

A single test is a snapshot. It is most useful when you compare a baseline taken while symptoms are active with a repeat test after treatment or a diet trial, then ask whether hydrogen and symptoms moved together.

Do not read day-to-day changes as progress. If you repeat it, use the same substrate, same lab, and the same prep. Otherwise you may be comparing the protocol more than your gut.

What an unexpected result should make you do

A high hydrogen result without symptoms is weak evidence. A high result with the right symptoms, bloating, gas, diarrhea, or clear food-triggered flares, is more useful.

If the hydrogen pattern fits SIBO, treatment usually means a prescription antibiotic plan and a search for why overgrowth happened. If hydrogen is flat but constipation is prominent, methane is the companion result to check. If hydrogen and methane stay flat and symptoms persist, move the workup toward celiac testing, stool inflammation markers, pancreatic digestion, and gut motility.