Background/Objectives: Functional constipation (FC) commonly affects middle-aged and older adults, but current pharmacological treatments have limitations. Postbiotics may offer safety advantages, but clinical evidence is limited. This randomized controlled trial evaluated the efficacy and safety of a heat-inactivated two-strain Lacticaseibacillus paracasei fermented milk in adults with FC. Methods: One hundred adults aged 45–75 years with Rome IV-defined FC received the fermented milk or placebo for 4 weeks. The primary outcome was the change in weekly spontaneous bowel movement (SBM) frequency from baseline to week 4. Secondary outcomes included complete spontaneous bowel movement (CSBM) frequency, whole-gut transit time (WGTT), constipation symptom scores, quality of life, serum biomarkers, and adverse events. Primary analysis was per-protocol (n = 96); intention-to-treat analysis was applied to primary and key secondary outcomes. Results: Dropout was 4% (n = 4, 2 per group), and adherence was >80% in both groups. The intervention showed no significant benefit over placebo for the primary outcome or for most secondary clinical outcomes. Although both groups improved within-group, no significant between-group differences were observed at week 4 for changes in SBM (MD = −0.14, 95% CI: −0.85, 0.57; p = 0.683), CSBM (MD = 0.27, 95% CI: −0.61, 1.15; p = 0.543), or WGTT (MD = −1.55 h, 95% CI: −7.65, 4.55; p = 0.614). Symptom and quality-of-life scores also did not differ between groups. Exploratory biomarker analyses showed significantly greater increases in serum VIP and ACh in the intervention group (VIP: MD = 105.23 ng/L, p < 0.001; ACh: MD = 42.95 ng/L, p = 0.035). No adverse events were reported. Conclusions: Four weeks of this postbiotic was safe but did not significantly improve bowel function or symptoms in the overall FC population. The increases in serum VIP and ACh suggest engagement of neurotransmitter-related pathways; however, these exploratory findings do not imply causation or clinical efficacy and warrant confirmation in longer-duration trials (Clinical Trial Registry: ChiCTR2500111771).

Background/Objectives: The preschool period is critical for neurodevelopment, yet evidence investigating fortified formula’s effect and potential microbiota–gut–brain axis mechanisms in this age group is limited. To evaluate fortified formula milk’s effect on neurodevelopment and explore potential microbiota–gut–brain axis mechanisms in preschool children. Methods: In this 9-month cluster-randomized, double-blind, controlled trial, 120 healthy children aged 3–6 years from four kindergarten classes were stratified by grade and randomly allocated (1:1) to receive either multi-nutrient fortified formula (intervention, n = 60) or standard control milk (n = 60). Neurocognitive function was assessed using the Wechsler Preschool and Primary Scale of Intelligence, Fourth Edition (WPPSI-IV). Safety was evaluated through anthropometry and blood biochemistry. Gut microbiota (16S rRNA sequencing) and fecal metabolomes (untargeted LC-MS) were analyzed at baseline and 9 months. Results: The intention-to-treat (ITT) analysis showed no significant difference in Full Scale Intelligence Quotient (adjusted mean difference: 1.05 points; 95% CI: −1.42, 3.52; p = 0.400). However, the intervention group significantly improved the Processing Speed Index (adjusted mean difference: 5.91 points; 95% CI: 1.88, 9.93; p = 0.004), increased gut microbial alpha diversity (Shannon index) and Bifidobacterium abundance. Metabolomic analysis revealed elevated fecal 2-hydroxybutyric acid (2-HB), a marker of propanoate metabolism. Increases in both Bifidobacterium and 2-HB levels showed a positive association with PSI improvement (both p < 0.05). All children maintained normal growth and safety parameters. Conclusions: Fortified formula milk improved processing speed in preschoolers, a benefit associated with gut ecosystem modulation characterized by Bifidobacterium enrichment and upregulated microbial propanoate metabolism. These results offer preliminary evidence for the role of the microbiota–gut–brain axis in nutritional cognitive programming during early childhood. (Clinical Trial Registry: ChiCTR2400084211).

Background: Functional constipation is a common gastrointestinal disorder that impairs quality of life. Although probiotics have shown benefits in constipation, whether live and heat-killed forms of the same strain act through similar mechanisms remains unclear. This study compared the effects of live and heat-killed Lacticaseibacillus paracasei K56 in experimental constipation. Methods: Loperamide-treated mice received live or heat-killed L. paracasei K56. Gastrointestinal motility, colonic histology, neurotransmitters, aquaporin expression, gut microbiota, short-chain fatty acids, and metabolomic profiles were evaluated. Results: Both preparations relieved constipation, as reflected by shorter defecation latency, increased fecal output and moisture, and faster intestinal transit, with live K56 producing greater improvement. Colonic injury was alleviated, accompanied by reduced Aqp2 and Aqp3 expression and normalization of 5-hydroxytryptamine, substance P, and vasoactive intestinal peptide. The treatments increased Lactobacillus and Lachnospiraceae, promoted butyrate production, and altered tryptophan metabolism. Heat-killed K56 was mainly associated with changes in phenylalanine/tyrosine- and methionine-related metabolism, whereas live K56 showed more pronounced alterations in arginine/polyamine, tryptophan, and bile acid metabolism. Conclusions: Live and heat-killed L. paracasei K56 improved constipation through shared and distinct mechanisms involving gut microbiota, microbial metabolism, enteric neurotransmission, and intestinal water homeostasis. Live K56 produced stronger effects, whereas heat-killed K56 retained substantial biological activity.