Hydrogen Breath Test: What It Is, Why It's Used, and What to Expect

When your stomach bloats after milk, or you feel gassy after eating beans, it might not be just bad digestion—it could be something measurable. The hydrogen breath test, a non-invasive diagnostic tool that measures hydrogen gas in your breath to detect malabsorption of sugars and bacterial overgrowth in the small intestine. Also known as breath hydrogen analysis, it’s one of the most common ways doctors figure out why certain foods make you feel awful. Unlike blood tests or scopes, you don’t need needles or sedation. You just breathe into a tube at intervals, and the machine tells you if your gut is fermenting sugars it shouldn’t be.

This test is especially useful for spotting lactose intolerance, a condition where the body can’t break down lactose, the sugar in dairy, leading to gas, cramps, and diarrhea. But it’s not just about milk. It also catches SIBO, small intestinal bacterial overgrowth, where too many bacteria grow where they shouldn’t, fermenting carbs and causing chronic bloating, pain, and even nutrient loss. These aren’t rare problems—up to 70% of adults have some trouble digesting lactose, and SIBO affects millions who’ve been told they have "irritable bowel syndrome" without knowing why. The hydrogen breath test gives answers where other tests fail.

What happens during the test? You fast for 8–12 hours, then drink a sugar solution—usually lactose, fructose, or glucose. For the next 2–3 hours, you blow into a bag every 15–20 minutes. If your body can’t digest the sugar, bacteria in your gut eat it and release hydrogen. That hydrogen gets absorbed into your blood, then comes out through your lungs. A spike in hydrogen levels? That’s your gut saying, "Hey, something’s off." It’s simple, safe, and surprisingly accurate.

Doctors use this test to guide treatment. If you test positive for lactose intolerance, you might cut back on dairy. If SIBO shows up, antibiotics or a low-FODMAP diet could help. It’s not perfect—some people don’t produce hydrogen, so methane tests are sometimes added—but it’s the best starting point for unexplained gut trouble. And because it’s cheap and non-invasive, it’s often the first step before more expensive scans or endoscopies.

What you’ll find in the posts below are real-world stories and science-backed tips about how digestive issues show up, how tests like the hydrogen breath test fit into the bigger picture, and what to do once you get results. From how diet changes after a positive test, to why some people still feel bad even when results are normal, these articles cut through the noise. You won’t find fluff—just clear, practical info to help you understand your body and talk smarter with your doctor.