The hypothesis posits that the epigenetic age of the gut microbiome—not merely its taxonomic composition—directly regulates host transcription factor activity through modulation of histone deacetylase (HDAC) enzymes by microbial short‑chain fatty acids (SCFAs). As the microbiome ages, its epigenetic clock advances, marked by altered DNA methylation patterns at genes governing SCFA biosynthesis (e.g., butyrate‑producing pathways in Faecalibacterium prausnitzii). This shift reduces luminal butyrate and propionate levels, diminishing HDAC inhibition in intestinal epithelial cells and enteroendocrine cells. Consequently, HDAC activity rises, leading to increased acetylation of histones at enhancers of pro‑inflammatory transcription factors such as NF‑κB p65 and AP‑1, amplifying their target gene expression (e.g., IL‑6, TNF‑α, CCL2). The resulting cytokine surge compromises gut barrier integrity, promotes endotoxin translocation, and activates vagal afferents that relay inflammatory signals to the brain, where they prime microglia via TLR4 signaling and exacerbate neurodegeneration.

Key mechanistic steps:

1. Microbiome epigenetic age predicts loss of SCFA‑producing taxa independent of host genetics.
2. Reduced SCFA levels relieve HDAC inhibition, increasing NF‑κB/AP‑1 transcriptional activity in gut epithelium.
3. Heightened gut‑derived inflammatory mediators drive systemic inflammaging and microglial activation.
4. Brain inflammation further dysregulates the HPA axis, elevating cortisol, which reshapes the microbiome toward a more aged epigenetic state, closing a feedback loop.

Testable predictions:

• In longitudinal human cohorts, baseline microbiome epigenetic age (measured via microbial DNA methylation clocks) will predict future increases in serum IL‑6 and TNF‑α, as well as declines in cognitive performance, even after adjusting for host age and genetics.
• Colon biopsies from older individuals with high microbiome epigenetic age will show elevated HDAC activity and increased NF‑κB p65 binding at inflammatory gene enhancers compared to those with a younger microbiome epigenotype.
• Fecal microbiota transplantation (FMT) from young donors into aged mice will reset the recipient microbiome’s epigenetic age, restore luminal butyrate, decrease colonic HDAC activity, reduce NF‑κB target expression, lower systemic cytokines, and improve memory performance.
• Pharmacological HDAC inhibition (e.g., with butyrate or selective HDAC3 inhibitors) in aged mice with a persistently old microbiome epigenotype will attenuate inflammaging markers and neuroinflammation, demonstrating that HDAC activity is a downstream effector of microbiome aging.

Falsifiability: If microbiome epigenetic age shows no correlation with host inflammatory transcripts or if FMT does not alter host HDAC activity and inflammatory outcomes, the hypothesis would be refuted. Conversely, supportive data would establish the microbiome’s epigenetic state as a causal regulator of host gene regulatory networks in aging, providing a mechanistic bridge between microbial aging and inflammaging that can be targeted therapeutically.