This study will test the influence of lactulose of the human gut microbiota. Healthy volunteers will ingest 50g of lactulose and donate stool samples 1 day before as well as 1-2 days and 14 days after the test. Using sequencing and metabolomics techniques the investigators will identify changes in microbiota composition upon lactulose exposure.

This is a clinical study for investigating the influence of lactulose on the composition and metabolic activities of the intestinal microbiota. We will recruit 40 healthy volunteers. Subjects will be randomized and receive either 50g lactose (intervention group) or 50g sucrose (control group).

Subjects will donate fecal samples 24h before and after (1-2d, 14d) lactulose ingestion. After ingestion H2-in expiratory air and symptoms will be followed for 3 hours.

Frozen samples will be analyzed for their microbiota composition and key metabolites.

Inclusion Criteria:

• Subjects free of relevant abdominal complaints (=healthy)
• Written informed consent
• Working at ETH Zurich/University of Zurich, experience in handling of -80°C freezers on biosafety level 2

Exclusion Criteria:

• Previous history of gastrointestinal disease or surgery (excludes appendectomy, hernia repair and anorectal disorders)
• Known diabetes mellitus, scleroderma, neurological impairment or other major current disease
• Subjects unable to stop medication that alters gut flora: proton pump inhibitors, laxatives and antibiotics at least 4 weeks before study entry.
• Pregnancy beyond week 12 (no pregnancy test will be performed)
• Involvement in any other clinical trial during the course of this trial, nor within a period of 14 days prior to its beginning or 14 days after its completion