DEGENERATIVE AND COGNITIVE DISEASES: Edited by Giovanni Frisoni and Paul G. UnschuldGut microbiota, 1013 new pieces in the Parkinson's disease puzzleScheperjans, Filipa,bAuthor Information aDepartment of Neurology, Helsinki University Hospital bDepartment of Clinical Neurosciences (Neurology), University of Helsinki, Helsinki, Finland Correspondence to Dr Filip Scheperjans, MD, Department of Neurology, Helsinki University Hospital, Haartmaninkatu 4, 00290 Helsinki, Finland. Tel: +358 9 4711; e-mail: email@example.com Current Opinion in Neurology: December 2016 - Volume 29 - Issue 6 - p 773-780 doi: 10.1097/WCO.0000000000000389 Buy Metrics Abstract Purpose of review Gastrointestinal dysfunction is highly prevalent in Parkinson's disease and may precede motor symptoms by more than a decade. It has been proposed that the neurodegenerative cascade may actually be initiated in the gut with subsequent spreading to the brain and that gut microbiota could be involved in this process. This review provides a short introduction into the methodology of microbiome-wide association studies and discusses the recently published first comprehensive assessments of gut microbiota in Parkinson's disease. Recent findings Three case–control studies have studied gut microbiota composition in Parkinson's disease and all found significant differences between Parkinson's disease patients and controls. However, most of the differentially abundant taxa as well as associations of microbiota with clinical variables differed between studies. This may at least in part be explained by methodological differences between studies in terms of selection of participants, analysis pipelines, statistical analysis, and confounder control. Summary Current evidence suggests that there are alterations of gut microbiota in Parkinson's disease, but the exact nature of these changes is not established. Future larger studies should assess gut microbiota in Parkinson's disease covering diverse geographical regions, ethnicities, disease stages, and phenotypes using well-defined selection criteria for patients and controls and standardized methodology. Copyright © 2016 Wolters Kluwer Health, Inc. All rights reserved.