Oral administration of a low dose of lactulose increases the abundance of genus Bifidobacterium in the large intestine; however, the details of the daily variation in Bifidobacterium have not been researched. To observe how the intestinal microbiota, including Bifidobacterium, change, especially immediately after the initiation of ingestion, we conducted a randomised, placebo-controlled, double-blind crossover study of ingestion of 4 g lactulose/day for 2 weeks in 36 healthy Japanese (including males and females). The primary outcome was the percentage of Bifidobacterium in the faecal bacteria. In the lactulose-treatment group, the percentage of Bifidobacterium was already significantly higher 2 days after starting lactulose ingestion than in the placebo group (20.5 ± 1.2% vs. 17.1 ± 1.2%, p = 0.021). Significant differences were maintained, gradually widening, until the end of the 2-week intervention period. There were significant increases in the percentage and the number of Bifidobacterium with ingestion of 4 g lactulose/day for 2 weeks, but no significant changes in the beta diversity of the intestinal microbiota between lactulose and placebo ingestion. The percentage of Bifidobacterium in the faecal bacteria returned to its original level within a week of the end of intervention with lactulose.
Keywords: constipation; daily change; immediate effect; indigestible oligosaccharide; intestinal microbiota; prebiotic.