Dietary supplementation with phlorotannin-lowered Eisenia nipponica increases caecal antioxidant activity and promotes Akkermansia in mice

Abstract

Eisenia nipponica (EN), a traditional Japanese edible brown alga, is rich in storage polysaccharides (laminaran) and polyphenols (phlorotannins) during summer. Laminaran is a water-soluble dietary fibre readily fermented in the gut, whereas the phlorotannins in EN exhibit potent antibacterial activity against Gram-positive bacteria. To distinguish the effects of algal fibre from those of soluble polyphenols, we prepared a phlorotannin-lowered EN via 70% ethanol washing (WEN). Either EN or WEN was added at 5% (w/w) to a high-sucrose, low-fiber (NF) diet and administered to male ICR mice for 14 days. Compared with mice fed NF diet, those fed EN and WEN showed approximately four- and three-fold increase, respectively, in caecal total phenolic content (TPC) alongside increased antioxidant activities (superoxide anion radical scavenging and Fe-reducing power). 16S rDNA amplicon sequencing revealed that both EN and WEN suppressed the caecal relative abundance of Gram-positive bacteria, such as Allobaculum . While both EN and WEN increased the abundance of laminaran-degrading Bacteroides intestinalis- like bacteria, other laminaran-degrading Bacteroides uniformis -like bacteria increased in mice fed WEN but not EN. Furthermore, while laminaran- and alginate-degrading Bacteroides acidifaciens- like bacteria were suppressed by EN, Akkermansia muciniphila -like bacteria were most abundant in mice fed WEN (14%). KEGG functional predictions indicated reduced amino acid-related metabolism and increased glycan degradation (ko00511) in both EN and WEN groups. These results suggest that phlorotannin-lowered EN may exert beneficial effects through its antioxidant properties and promotion of next-generation probiotics such as B. uniformis and A. muciniphila.