Probiotic-Driven Metabolic Transformation in Dark Tea: Enhanced Bioactives and Gut Microbiota Modulation Underlie Its Alleviation Effect Against Constipation

Constipation is a widespread gastrointestinal condition with complex causes and limited therapeutic efficacy of current treatments. Probiotics and natural-function foods like dark tea (DT) have emerged as promising alternatives. We investigated the protective effects of dark tea (DT) and <i>Lactiplantibacillus paracasei</i> K34-fermented dark tea (FDT) against constipation induced by loperamide (LOP) in mice. FDT exhibited a superior effect over DT in alleviating constipation, as evidenced by increased fecal water content, reduced defecation time, and accelerated small intestinal transit. FDT also restored serum gastrointestinal neurotransmitters and repaired intestinal barrier damage more effectively. FDT intervention significantly increased the richness and altered the composition of gut microbiota, notably elevating <i>Lactobacillus</i>, <i>Ligilactobacillus</i>, and Actinobacteria, while reducing <i>Prevotellaceae</i>_UCG-001. Correlation analysis linked these microbial shifts with improved motility and anti-inflammatory responses. Untargeted metabolomics indicated that fermentation drastically enriched key bioactive compounds in FDT compared to DT, including cyclic dipeptide (cyclo-(Gly-Pro), 132.4-fold) and hydroxylated linoleic acid derivatives (N-(17-hydroxy-9,12-octadecadienoyl)-glutamine, 123.8-fold). KEGG pathway analysis suggested fermentation upregulated tryptophan metabolism and branched-chain amino acid biosynthesis, while downregulating flavone biosynthesis. Collectively, probiotic fermentation enhances the anti-constipation efficacy of DT, which is achieved via a synergistic mechanism involving neuroendocrine modulation, intestinal barrier repair, gut microbiota composition, and transformation of bioactive metabolites.