Background: Rhamnogalacturonan-I (RG-I) is a pectic polysaccharide with emerging prebiotic and immunomodulatory potential. This randomised, double-blind, placebo-controlled trial (ID: NCT06081972) evaluated the effects of carrot-derived RG-I (cRG-I) supplementation, compared to placebo (maltodextrin), on gut microbiota composition and immune cell activation in healthy adults. Methods: A total of 54 participants (18–70 years old) were randomised in a double-blind manner to receive either 500 mg/day of cRG-I or placebo for four weeks. Pre-screening ensured balanced randomisation based on habitual fibre intake and faecal Bifidobacterium counts. Questionnaires assessed potential gut health and well-being effects, while in vitro and ex vivo models were used to evaluate effects on intestinal permeability. Results: cRG-I was well tolerated with excellent compliance. Faecal Bifidobacterium counts increased significantly, peaking at week 3. Isobutyric acid levels rose, though no other SCFAs differed. Immunologically, cRG-I enhanced the percentage of circulating myeloid dendritic cells expressing activation markers (CD86, HLA-DR) on. Stool consistency improved slightly. Preclinical models further showed that cRG-I and its fermentation products protected intestinal barrier integrity under stress. Conclusion: These results support cRG-I as a safe, low-dose dietary intervention capable of beneficially modulating gut microbiota, immune responses, and barrier function in healthy adults within a short supplementation period.

Low-dose carrot rhamnogalacturonan-I (cRG-I) has shown consistent modulatory effects on the gut microbiota and immune function in humans. In this study we investigated its effects on the microbial composition and metabolite production of the gut microbiota of small (5–10 kg), medium-sized (10–27 kg), and large (27–45 kg) dogs, using inulin and xanthan as comparators. Fecal samples from six dogs of each size group were evaluated. Overall microbiome composition, assessed using metagenomic sequencing, was shown to be driven mostly by dog size and not treatment. There was a clear segregation in the metabolic profile of the gut microbiota of small dogs versus medium-sized and large dogs. The fermentation of cRG-I specifically increased the levels of acetate/propionate-producing Phocaeicola vulgatus. cRG-I and inulin were fermented by all donors, while xanthan fermentation was donor-dependent. cRG-I and inulin increased acetate and propionate levels. The responses of the gut microbiota of different sized dogs to cRG-I were generally consistent across donors, and interindividual differences were reduced. This, together with the significant increase in P. vulgatus during fermentation in both this study and an earlier human ex vivo study, suggests that this abundant and prevalent commensal species has a core capacity to selectively utilize cRG-I.