Dietary Dried Laver (Porphyra tenera) Modulates Gut Microbiota Composition and Diversity in Older Women with and Without Metabolic Syndrome: An Exploratory Pilot Study

Background: Metabolic syndrome (MetS) is a cluster of cardiometabolic abnormalities linked to increased risk of type 2 diabetes and cardiovascular disease. Emerging evidence implicates gut microbiota dysbiosis in MetS pathophysiology; however, human clinical data on seaweed-based dietary interventions remain scarce. Methods: Twenty-four older women were stratified into a MetS group (n = 13) and a control group (n = 11) per NCEP-ATP III criteria with Korean-specific waist circumference cutoffs. All participants consumed 4 g of dried laver (Porphyra tenera) per day for 12 weeks. Fecal DNA was subjected to 16S rRNA gene amplicon sequencing (V4 region; Illumina iSeq 100). Bioinformatic processing used QIIME2 and MicrobiomeAnalyst; alpha diversity was quantified by Chao1 and Fisher indices; beta diversity by Bray–Curtis, Jensen–Shannon, and UniFrac metrics; and differential abundance by LEfSe. Results: The Firmicutes/Bacteroidetes ratio declined 0.81-fold in the control group and 0.54-fold in the MetS group. Alpha diversity (Chao1 and Fisher indices) increased significantly in both groups (p < 0.001 and p < 0.05, respectively). Unweighted UniFrac distance showed significant compositional differences (R2 = 0.141, p = 0.001). LEfSe identified four FDR-significant genera: CAG_873 in the control group and Muribaculaceae, Paraprevotella, and Tyzzerella in the MetS group. Conclusions: Twelve-week dried laver supplementation produced measurable shifts in gut microbial diversity and community composition in older women, with potentially greater responsiveness in those with MetS. These preliminary findings justify adequately powered randomized controlled trials to evaluate laver as a gut-microbiome-targeted dietary strategy for MetS.