Hypothesis

The epigenetic age of the gut microbiome, not host cellular senescence, initiates the inflammaging cascade that accelerates brain aging; prolonged fasting reverses this by inducing autophagy-dependent remodeling of the microbial epigenome.

Mechanistic Reasoning

• Age‑related shifts in gut microbiota increase production of metabolites such as indole‑propionic acid and secondary bile acids that act as histone deacetylase inhibitors in host cells, leading to NF‑κB activation and microglial priming.
• These microbial metabolites also alter DNA methylation patterns within the microbiome itself, creating a measurable “microbiome epigenetic clock” that predicts host systemic inflammation better than host telomere length.
• Fasting triggers neuronal autophagy via mTOR inhibition and AMPK/SIRT1 activation, which concurrently upregulates gut epithelial autophagy, enhancing barrier function and reducing metabolite leakage.
• Importantly, autophagy in intestinal epithelial cells promotes turnover of mucus‑associated microbes, allowing recolonization by youth‑associated taxa whose epigenetic state is reset through reduced exposure to host‑derived oxidative stress, thereby lowering the microbiome epigenetic age.

Testable Predictions

1. In cross‑sectional human cohorts, the microbiome epigenetic age (derived from methyl‑seq of stool DNA) will correlate positively with plasma IL‑6, TNF‑α, and CSF p‑tau, independent of host age.
2. Participants undergoing a 4‑week intermittent fasting regimen will show a significant reduction in microbiome epigenetic age alongside decreased circulating AGEs and improved performance on hippocampal‑dependent memory tasks.
3. Germ‑free mice colonized with stool from old, high‑microbiome‑epigenetic‑age donors will develop heightened microglial activation and amyloid‑β deposition, whereas colonization with stool from fasting‑treated, low‑epigenetic‑age donors will protect against these phenotypes.
4. Pharmacological inhibition of autophagy in gut epithelium (e.g., with chloroquine) will block the fasting‑induced reduction in microbiome epigenetic age and abolish the associated anti‑inflammatory effects.

Experimental Design (outline)

• Human arm: Recruit 120 adults aged 60‑80, baseline stool methyl‑seq, plasma cytokines, CSF biomarkers, and cognitive testing. Randomize to intermittent fasting (16:8) or control diet for 12 weeks. Repeat measurements at 6 and 12 weeks.
• Mouse arm: Perform fecal microbiota transplantation (FMT) from young, old, and fasting‑treated old donors into germ‑free aged mice. Assess microglial Iba1 staining, brain AGE accumulation, and behavior over 8 weeks. Include autophagy‑deficient intestinal epithelial‑specific Atg5 knockout mice to test necessity.
• Readouts: Microbiome epigenetic age (using CpG sites identified from longitudinal stool methyl‑seq), serum AGE levels (ELISA), gut permeability (FITC‑dextran), and neuroinflammatory markers.

Falsifiability

If microbiome epigenetic age does not predict inflammaging markers, or if fasting fails to lower this epigenetic age despite improvements in barrier function and cognition, the hypothesis would be refuted. Likewise, if transplanting a “young” microbiome epigenetic state does not mitigate brain aging in germ‑free recipients, the causal link would be questioned.

This framework shifts focus from host‑centric inflammaging to a measurable, modifiable microbial epigenetic driver, positioning fasting‑induced autophagy as a bidirectional reset button for the gut‑brain aging loop.