Subjects will present to the GI Motility clinic after a 12 hour fast. After a baseline breath sample and ingestion of 10g-lactulose syrup (Inalco Spa, Milano, Italy, packaged by xactdose Inc., South Beloit, IL), end-alveolar breath samples will be taken every 15 minutes for a three hours. Breath samples will be analyzed using a model SC Quintron gas chromatograph (Quintron Instrument Company, Milwaukee, WI) which determines breath hydrogen and methane concentration in parts per million (ppm). Subjects will not be allowed to smoke or exercise within 2 hours prior to performing the test (64). Methane exchanges quickly across the alveolar membrane, so hyperventilation can falsely lower the concentration in breath. Subjects will also be required to sit for the duration of the breath test such that exertion will not increase alveolar clearance of this gas. If modest hyperventilation occurs, the concentration of carbon dioxide in the breath sample will be used to correct for quality of alveolar sampling. Note that in the clinical study breath test results will be blinded to the investigator and patient to avoid influencing the interpretation of transit and serotonin levels.