Dietary patterns and gut microbial clusters jointly stratify frailty in community-dwelling older Japanese adults: a cross-sectional study in the Kyotango longevity area.

BACKGROUND: Habitual diet and the gut microbiota each influence frailty in older adults, but whether they contribute jointly and whether the fiber-frailty association is statistically mediated by gut microbial composition remain poorly characterized in Asian populations. OBJECTIVE: To quantify the joint contribution of dietary patterns and gut microbial clusters to frailty and to test whether a butyrate-producer-rich microbial axis statistically mediates the cross-sectional fiber-frailty association in older Japanese adults. METHODS: In 785 community-dwelling participants aged ≥65 years from Kyotango, a Japanese longevity area, BDHQ-derived intakes (27 food groups) and 16S rRNA gene amplicon sequencing of fecal samples (47 CLR-Z genus-level taxa) were each summarized by Varimax-rotated PCA and k-means clustering, yielding four dietary patterns and four microbial clusters. A 32-item deficit-accumulation frailty index (FI; frail = FI ≥ 0.21) was the outcome. Regression models adjusted for age, sex and energy intake; a pre-specified mediation model tested whether Microbe PC2 - a butyrate-producer-rich axis dominated by Faecalibacterium, Fusicatenibacter and Eubacterium eligens - statistically mediated the fiber-FI association; indirect effects used 5,000-bootstrap percentile 95% CIs. RESULTS: Frail prevalence was 15.2% overall but varied nine-fold across the 16 diet × microbiota cells, from 4.0% (Pattern 1 [Vegetable, soy and dairy] × Ruminococcus-Faecalibacterium cluster) to 36.4% (Pattern 3 [Seafood and washoku] × Lachnoclostridium-Ruminococcus gnavus cluster). Total fiber intake was inversely associated with FI (β = -0.142, p < 0.0001), and Microbe PC2 statistically mediated this cross-sectional association (indirect β = -0.016, 95% CI -0.031 to -0.005; Sobel p = 0.009; proportion mediated 11.4%). CONCLUSIONS: Dietary patterns and gut microbial clusters jointly stratify frailty in older Japanese adults, with a small but consistent statistical mediation of the fiber-frailty association via a butyrate-producer-rich microbial axis. Combined dietary-microbial profiling may refine frailty risk assessment.