Improvement effect of synbiotics intake on stool frequency in constipation-prone healthy participants: placebo-controlled, randomised, double-blind, parallel-group comparison.

Synbiotics exert various physiological effects by complementing the physiological functions of both probiotics and prebiotics. Chronic constipation is a prevalent global health concern, and despite several clinical studies utilising synbiotics, definitive conclusions regarding its efficacy remain elusive. This study aimed to investigate the effect of a synbiotic combination of Lacticaseibacillus paracasei strain Shirota (LcS: formerly known as Lactobacillus casei strain Shirota) and galactooligosaccharides (GOS), both of which independently improve defecation symptoms, on defecation frequency in constipation-prone healthy Japanese adults. This study employed a placebo-controlled, randomised, double-blind, parallel-group comparison design over a 4-week consecutive intake period. The primary objective was to evaluate the efficacy of the synbiotic intervention on bowel movement frequency in healthy adults prone to constipation. The synbiotic group exhibited a significantly increased defecation frequency at week 2 ( P = 0.048) but not at week 4, whereas the number of defecation days showed only a trend at week 2 ( P = 0.058). This represents a key limitation of this study. A significant positive correlation was observed between faecal LcS counts and defecation parameters in the synbiotic group, with no equivalent correlation observed for endogenous Lactobacillaceae in either group. This suggests a strain-specific effect of LcS beyond a general class effect of Lactobacillaceae. Additionally, increases in faecal Bifidobacterium counts and a decrease in isobutyric and isovaleric acid levels were observed along with improvements in defecation-related symptoms. These findings suggest that the combination of LcS and GOS may contribute to improvements in constipation-related outcomes in healthy individuals prone to constipation. The study was registered by the University Hospital Medical Information Network: registration number: UMIN000039236.