Antioxidant and immunomodulatory properties of aqueous acetic acid macromolecular extract from Saccharina japonica holdfast and its effects on the caecal microbiota of mice fed a high-sucrose and low-fibre diet

A large quantity of Saccharina japonica holdfast (SjH) is discarded as an unused by-product of the cultivation process. In this study, we isolated a functional fucoidan extract (FE) from SjH as an aqueous acetic acid macromolecular (>3 kDa) fraction. The main compounds in FE were fucoidans and phenolic compounds (phlorotannins). FE demonstrated notable superoxide anion radical-scavenging capacity in vitro, promoted nitric oxide (NO) secretion in RAW264.7 cells in the absence of lipopolysaccharide (LPS) from E. coli O111, and exerted an inhibitory effect on NO secretion in the presence of LPS. Mice were fed a high-sucrose diet containing no fibre (NF), 0.5% FE (FL) or 1.0% FE (FH) for 14 days. Compared with that in the spleen tissue of NF group mice, pro-inflammatory cytokines such as interleukin (IL)-1β, IL-6, and tumour necrosis factor-α were suppressed in FL- and FH-fed mice. Interferon-γ was suppressed by FH. The caecal microbiota was analysed using 16S rDNA (V4) amplicon sequencing. Allobaculum- and Bacteroides acidifaciens–like bacteria were abundant in FL- and FH-fed mice. In contrast, the abundances of Desulfovibrionaceae, which are inflammation- and gut dysbiosis–related microbes, and Prevotellaceae-like bacteria were reduced in FH-fed mice. These results suggest that SjH FE is a functional food with antioxidant and immunomodulatory properties that correlates with beneficial gut microbiota composition.