Purpose: The effect of prebiotics intake after administration of a synbiotics mixture (a probiotic, Bifidobacterium longum, and a prebiotic, xylooligosaccharide containing sugar [XOS]) on human intestinal microflora and defecation characteristics was investigated in a randomized controlled trial. Methods: Twenty-five healthy young volunteers (11 males and 14 females) were randomly assigned to 2 groups (BL2XO2 and BL2XO6). The synbiotics mixture was orally administered to both groups for 2 weeks, and the prebiotics were subsequently administered to the BL2XO6 group for 4 additional weeks. The daily dose of the synbiotics mixture comprised 10 10 colony-forming unit of Bifidobacterium longum and 10 g of XOS, and during the prebiotics period, the daily dose of prebiotics comprised only 10 g of XOS. The fecal pH, microflora, and defecation characteristics were analyzed at baseline and at weeks 1, 2, 4, and 6. Results: The counts of B. longum and Bifidobacterium spp. in the BL2XO6 group exhibited a steady, increasing trend during the synbiotics and prebiotics periods, whereas those of the BL2XO2 group exhibited considerable variation in each week of the study period. Although there was no significant difference, the counts of fecal Bifidobacterium in the BL2XO6 group tended to be higher than those of the BL2XO2 group at week 6. The growth of Lactobacillus

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