…he was doing a breath hydrogen test. If you know the amount of hydrogen someone is exhaling orally, it's a simple matter to extrapolate the amount they're exhaling rectally. This is because a fixed percentage of hydrogen produced in the colon is absorbed into the blood and, and when it reaches the lungs, exhaled. The breath hydrogen test has given flatus researchers a simple, consistent measure of gas production that does not require the subject to fart into a balloon.
Mary Roach, Gulp: Adventures on the Alimentary Canal
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