Structural features of brown seaweed fucoidans differentially modulate gut microbiota and metabolic profiles in vitro

Fucoidan has attracted increasing attention as a functional food ingredient due to its potential role in modulating gut microbiota. However, the structure of fucoidan varies considerably among different brown algae. There remains a limited understanding of how specific structural characteristics of fucoidans, including backbone composition, sulfation pattern, and molecular accessibility, determine their microbial utilization and downstream metabolic outcomes. In this study, structural characterization combined with 16S rRNA sequencing, short-chain fatty acid (SCFA) analysis, and untargeted metabolomics was performed on fucoidans from Undaria pinnatifida (UPP), Laminaria japonica (LJP) and Ascophyllum nodosum (ANP). UPP exhibited the greatest fermentability, with total sugar content decreasing by 33.17 ± 0.31%, compared with 7.05 ± 0.97% for LJP, whereas no significant change was observed for ANP (P < 0.05). 16S rRNA sequencing analysis showed that UPP and LJP induced more similar community shifts, while LJP significantly enriched Lactobacillus compared with UPP and ANP (P < 0.05). Untargeted metabolomics further revealed distinct metabolic profiles among the three fucoidan fermentation systems. After in vitro fermentation, UPP promoted greater accumulation of several metabolites, including ricinoleic acid, hyocholic acid, kyotorphin, azelaic acid, acetic acid and valeric acid. All three fucoidans significantly increased acetate and propionate production (P < 0.05), while butyrate, isobutyrate, and isovalerate showed no significant changes; valerate increased significantly only after UPP fermentation (P < 0.05). These findings enhance our understanding of the roles played by structurally distinct fucoidans in modulating intestinal microbiota function and provide insights into the structure-function relationships of fucoidans for potential functional food applications.