Effects of D-Glucosamine on the Growth of Human Gut-Dominant Microbiota <i>in vitro</i> and Bowel Movements in Healthy Individuals

D-Glucosamine hydrochloride (GlcN), a monomer produced by the hydrolysis of chitosan, is a dietary supplement used worldwide to mitigate cartilage degeneration. Previous reports have shown that some dietary glucosamine migrates to the colon. However, the effect of glucosamine alone on colonic microbiota and bowel movements remains poorly understood. In this study, we evaluated the effect of glucosamine on the growth of 46 dominant human colonic bacterial species and 24 other important bacteria <i>in vitro</i>. Among the 70 gut bacterial species tested, the growth of 57 (81 %) was significantly enhanced by 0.5 g/L GlcN, with the most prominent growth activity (> 5-fold) observed in <i>Anaerotruncus colihominis</i>, <i>Pseudoflavonifractor capillosus</i>, and <i>Roseburia hominis</i>. These results indicate that a wide range of the tested gut bacteria can utilize GlcN, similar to the effect of conventional dietary fiber in improving bowel function. Next, we conducted an open-label, single-arm trial involving 29 healthy individuals to determine the effects of 1,500 mg GlcN/day, a commonly used dose. Stool color significantly changed during the 2 weeks of GlcN intake from brown to ocher (<i>p</i> < 0.01), suggesting enhanced colonic fermentation. The stool odor and the sensation of incomplete evacuation improved significantly (<i>p</i> < 0.05). Numerical measurements of bowel movements revealed significant increases in stool volume, defecation frequency, and the number of days of defecation during GlcN intake (<i>p</i> < 0.001). Thus, dietary glucosamine may stimulate gut microbiota growth in the colon and promote bowel movements. This study was registered with the University Hospital Medical Information Network (regd. no. UMIN000056757).