Long Oral Presentation Nutrition Society of Australia 50th Annual Scientific Meeting 2026

Probiotic supplementation has been proposed to influence bone and metabolic health via modulation of the gut microbiome,⁽¹⁾ but whether sustained supplementation alters gut microbial diversity or composition in postmenopausal women remains unclear. This study examined the effect of 12-month probiotic supplementation on gut microbiota diversity, composition and taxonomic abundance in early postmenopausal women participating in a randomised, placebo-controlled trial, in which effects of the same intervention on bone density have previously been reported.⁽²⁾

Stool samples were collected at baseline and 12 months from 100 women (control n = 49, probiotic n = 51; 200 paired samples) and analysed using full-length 16S rRNA sequencing. Alpha diversity (Shannon diversity, observed ASV richness, Faith’s phylogenetic diversity and Pielou’s evenness) and beta diversity (Bray–Curtis, Jaccard, and weighted and unweighted UniFrac distances) were compared using linear mixed-effects models and PERMANOVA, respectively. Differential abundance at the genus level was assessed using MaAsLin2, ANCOM-BC2 and DESeq2. Associations between host characteristics and overall community composition were examined using environmental vector fitting. P-values were adjusted for multiple comparisons using the Benjamini–Hochberg (BH) false discovery rate procedure.

Alpha and beta diversity did not differ between treatment groups at baseline, and no significant Treatment × Timepoint interactions were observed during follow-up (alpha diversity: all BH-adjusted p ≥ 0.93; beta diversity: all BH-adjusted p ≥ 0.55). Within-person compositional change from baseline to 12 months was also similar between groups (all BH-adjusted p = 0.78), with no genus showing a significant Treatment × Timepoint interaction across the three differential abundance methods (all BH-adjusted p ≥ 0.178). In contrast, dietary fibre intake, the bone turnover markers CTX and PINP, age and years since menopause were each significantly associated with overall community composition (all p < 0.05), independent of treatment allocation.

Twelve months of probiotic supplementation did not produce detectable changes in gut microbial diversity, composition or taxonomic abundance beyond those occurring with placebo. In contrast, host factors, particularly dietary fibre intake and bone turnover markers, were more strongly associated with microbiome composition than treatment allocation. Nevertheless, the potential effects of probiotic supplementation on bone health via microbial functions and metabolic pathways may not be captured by taxonomic profiling alone.