Hypothesis

We propose that the sharp decline in the Firmicutes/Bacteroidetes (F/B) ratio after age 70 results from age‑dependent loss of specific host‑derived mucosal glycans that selectively support Firmicutes adhesion and growth. This glycan depletion reduces butyrate production, lowering colonocyte mitochondrial respiration and weakening the epithelial barrier, thereby contributing to inflammaging.

Mechanistic basis

• Colonocytes rely on butyrate from Firmicutes for β‑oxidation, which maintains hypoxia in the lumen and favors obligate anaerobes. Reduced butyrate elevates luminal oxygen, shifting the community toward facultative anaerobes such as Proteobacteria, a shift observed in elderly microbiota[Differences in the gut Firmicutes to Bacteroidetes ratio across age groups in healthy Ukrainian population].
• Age‑related decline in expression of glycosyltransferases (e.g., FUT2, B4GALNT2) alters the O‑glycan landscape of the colonic mucus layer[The gut microbiota and aging: interactions, implications, and interventions]).
• Firmicutes such as Faecalibacterium prausnitzii possess glycan‑binding protein modules that recognize these mucin O‑glycans; loss of these ligands decreases their competitive advantage versus Bacteroidetes, which utilize a broader range of dietary polysaccharides[The Firmicutes/Bacteroidetes Ratio: What It Means for Gut Health, Hormones, and Overall Wellness].
• Consequently, butyrate flux drops, impairing colonocyte peroxisome proliferator‑activated receptor‑γ (PPARγ) signaling and mitochondrial fatty‑acid oxidation, leading to increased epithelial permeability and systemic endotoxin translocation.

Predictions and experimental design

1. Human cohort – Measure fecal mucin O‑glycan profiles (via LC‑MS) and F/B ratio across ages (20‑90). Expect a negative correlation between specific sulfated/sialylated glycans and Firmicutes abundance, strongest after 70[Differences in the gut Firmicutes to Bacteroidetes ratio across age groups in healthy Ukrainian population]).
2. Intervention – Supplement aged mice with purified mucin O‑glycans (e.g., sialyl‑Lewis^x) and assess: (a) Firmicutes abundance, (b) fecal butyrate, (c) colonocyte oxygen consumption (Seahorse), (d) barrier integrity (FITC‑dextran assay). Predict restoration of Firmicutes and butyrate to mid‑life levels.
3. Mechanistic test – Use germ‑free mice colonized with a defined Firmicutes strain lacking the mucin‑binding protein; glycan supplementation will not rescue its abundance, confirming the ligand‑receptor specificity.
4. Falsifiability – If glycan supplementation fails to increase Firmicutes or butyrate in aged subjects, or if mucin glycan loss does not correlate with the F/B decline, the hypothesis is refuted.

Broader implications

Linking host glycan biosynthesis to microbial ecology provides a testable, host‑centric mechanism for the post‑70 F/B shift, moving beyond correlative ratios toward causal interventions that could preserve colonocyte health and mitigate inflammaging.