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AbstractBackground The beneficial effects of a plant-based diet on gut microbiota diversity are well documented, however, its impact on clinical bowel health and defecation patterns are less well understood. Vegetarian diets have been associated with a higher bowel movement (BM) frequency as well as softer stools in cross-sectional studies. The effects of the de-novo adoption of a vegan diet on bowel health, however, have never been investigated in a randomized-controlled trial.Materials and Methods The present study examined bowel health and defecation patterns in relation to diet and nutrient intake in a young and healthy sample of n = 65 physically-active German university students who were randomly assigned to either a vegan or a meat-rich diet for eight weeks. Bowel health assessment included the Bristol Stool Form Scale (BSFS), the Gastrointestinal Quality of Life Index (GIQLI) and the Cleveland Clinic Fecal Incontinence Score (CCFIS). Nutrient intake was assessed using weighed food diaries. The study was prospectively registered at the German Clinical Trial Register (DRKS00031541).Results Weekly BM frequency slightly increased in vegans, whereas it remained unaltered in participants assigned to a meat-rich diet. Fiber intake increased significantly in vegans (34.89 (18.46) g/d) whereas it decreased in those assigned to the meat-rich group (22.79 (12.5) g/d). No significant intergroup differences in BSFS and CCFIS patterns were observed. Adoption of a vegan diet neither resulted in a transient increase in abdominal discomfort nor in a decreased gastrointestinal quality of life, which was comparable across the diet groups.Conclusions The short-term de-novo adoption of a vegan diet did not negatively affect markers of bowel health in this study.