The large intestine normally contains several pounds of bacteria, but the small intestine should contain relatively few bacteria. When bacteria are present in the small intestine in significant numbers, digestive symptoms such as gas, bloating, abdominal discomfort, and diarrhea may occur.
Some anaerobic bacteria—bacteria that thrive in low-oxygen environments—produce hydrogen gas when they ferment certain carbohydrates. If excessive numbers of these bacteria are present in the small intestine, hydrogen production increases.
This principle is used in the hydrogen breath test. During the test, the patient fasts for approximately 12 hours and provides a baseline breath sample. The patient then consumes a measured dose of a specific sugar, such as lactulose or glucose. Breath samples are collected at regular intervals, typically every 15 minutes, for three to five hours.
The test is interpreted based on how much hydrogen appears in the breath and how quickly it appears. Early rises in hydrogen suggest fermentation occurring in the small intestine rather than the large intestine, which may indicate small intestinal bacterial overgrowth (SIBO).
The hydrogen breath test is not perfect. Some bacteria do not produce hydrogen, and certain individuals predominantly produce methane instead. In addition, test results can sometimes be misinterpreted due to variations in intestinal transit time. Despite these limitations, breath testing can be a useful, noninvasive tool for identifying abnormal bacterial activity in the small intestine when interpreted in the proper clinical context.
