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.