Hypothesis

The post‑70 reversal of the Firmicutes/Bacteroidetes (F/B) ratio does not merely reflect a loss of butyrate producers; it actively drives a shift in colonocyte epigenetic state through diminished histone deacetylase (HDAC) inhibition. Reduced luminal butyrate lowers HDAC inhibition, increasing acetylation of NF‑κB p65 and promoting a pro‑inflammatory transcriptional program that exacerbates epithelial barrier dysfunction and contributes to inflammaging.

Mechanistic Reasoning

1. Butyrate as an HDAC inhibitor – At concentrations achieved in the colon of younger adults (≈5–10 mM), butyrate enters colonocytes via monocarboxylate transporter‑1 (MCT1) and inhibits class I HDACs, leading to hyperacetylation of histones and transcriptional regulators that sustain tight‑junction gene expression (e.g., OCLN, CLDN1) and suppress NF‑κB activity【2】【3】.
2. Age‑related F/B shift – After age 70 the median F/B ratio falls below 0.7, mirroring a loss of butyrate‑producing Firmicutes (Faecalibacterium, Roseburia, Ruminococcaceae) and a relative rise of Bacteroidetes that generate lower SCFA yields【1】【4】.
3. Epigenetic consequence – With luminal butyrate dropping to <1 mM in the elderly, HDAC inhibition wanes, causing decreased histone H3K27ac at barrier‑gene promoters and increased RelA (p65) acetylation, which augments NF‑κB DNA binding and transcription of IL‑6, TNF‑α, and CXCL8.
4. Feedback loop – Elevated cytokine secretion further alters mucosal oxygenation and mucin composition, favoring Bacteroidetes expansion and suppressing Firmicutes, thereby locking the low‑F/B, low‑butyrate state.

Testable Predictions

• Prediction 1: Colon biopsies from individuals >70 years with F/B < 0.7 will show significantly lower histone H3K27ac and higher acetyl‑p65 levels at NF‑κB target promoters compared with age‑matched individuals >70 years whose F/B > 1.0 (adjusted for medication, BMI, diet).
• Prediction 2: Ex vivo incubation of elderly colonocytes with physiological butyrate (5 mM) for 24 h will restore HDAC inhibition, reduce acetyl‑p65, and increase OCLN/CLDN1 mRNA to levels comparable to those from younger donors.
• Prediction 3: Longitudinal cohorts that receive a prebiotic blend designed to boost Firmicutes (e.g., resistant starch + inulin) and raise fecal butyrate >5 mM will exhibit a measurable increase in F/B ratio and a concomitant decrease in colonic epithelial acetyl‑p65 and inflammatory cytokines over 12 months.

Experimental Approach

• Collect paired stool and sigmoid colon biopsy samples from three age strata (20‑39, 60‑69, ≥70) with stratifying by F/B ratio.
• Perform Western blot / ELISA for acetyl‑H3K27, acetyl‑p65, and total HDAC activity; RNA‑seq for barrier and NF‑κB target genes.
• Conduct ex vivo treatment assays with sodium butyrate at 0, 1, 5, 10 mM.
• In a pilot RCT, administer daily resistant‑starch/inulin supplement to ≥70 year olds with low F/B; monitor stool SCFA, F/B ratio (16S rRNA sequencing), and biopsy epigenetics at baseline, 6, and 12 months.

Falsifiability

If epigenetic markers (acetyl‑H3K27, acetyl‑p65) and barrier‑gene expression do not differ between low‑ and high‑F/B elderly groups, or if butyrate supplementation fails to modify HDAC inhibition or inflammation in elderly colonocytes, the hypothesis would be refuted. Conversely, confirmation would support a causal link between age‑related F/B decline, loss of butyrate‑mediated HDAC inhibition, and epigenetically driven epithelial dysfunction.