Hypothesis

Age-related decline in gut microbial tryptophan catabolism raises circulating phenylacetic acid (PAA), which drives endothelial senescence and impairs brain perfusion. We hypothesize that extending daily fasting to 20–24 h boosts microbial production of indole derivatives that activate the aryl hydrocarbon receptor (AhR) in intestinal epithelium, thereby tightening gut and blood‑brain barriers, lowering PAA translocation, and breaking the inflammaging loop.

Mechanistic Rationale

Aging reduces Lactobacillus and Bifidobacterium and enriches taxa that convert phenylalanine to PAA {Phenylalanine to PAA conversion}. Simultaneously, fasting alters enteroendocrine secretion of neuropeptide F, reshaping nutrient availability in the lumen {Enteroendocrine neuropeptide F secretion}. Indole, generated by tryptophanase‑positive bacteria, is a known AhR ligand that promotes expression of tight‑junction proteins (occludin, claudin‑5) and mucin secretion, reducing permeability {AhR barrier enhancement}. When AhR signaling is strong, fewer microbial antigens and metabolites like PAA reach the circulation, lessening endothelial NF‑κB activation and senescence {Endothelial senescence link}. Conversely, weak AhR activity permits barrier leak, PAA‑induced senescence, and downstream neuroinflammation via vagal afferents {Vagus‑mediated neuroinflammation}. Thus, fasting‑induced indole/AhR signaling offers a mechanistic bridge that links nutrient timing to microbial metabolism and barrier integrity.

Testable Predictions

1. Mice subjected to 20‑h daily fast will show higher fecal indole concentrations and increased AhR target gene expression in colonic epithelium compared with 16‑h fast controls.
2. Elevated AhR activity will correlate with reduced serum PAA levels and lower endothelial senescence markers (p16^INK4a^, SA‑β‑gal) in aortic tissue.
3. Blocking AhR with a specific antagonist during the extended fast will abolish the protective effects on barrier permeability and PAA levels, restoring the inflammaging phenotype.
4. Transplanting feces from extended‑fast donors into germ‑aged recipients will transfer the barrier‑enhancing phenotype, confirming microbial mediation.

Experimental Approach

• Use male C57BL/6 mice aged 18 months. Assign to ad libitum, 16‑h fast, or 20‑h fast groups for 8 weeks.
• Measure fecal indole (LC‑MS), colonic AhR targets (Cyp1a1, Il22) by qPCR, serum PAA (HPLC), endothelial senescence (p16 immunostaining, SA‑β‑gal), and gut permeability (FITC‑dextran assay).
• In a subset, co‑administer the AhR antagonist CH‑223191.
• Perform fecal microbiota transplantation from fasted donors to young germ‑free recipients and assess barrier indices.
• Statistical analysis via ANOVA with post‑hoc Tukey; power analysis set to detect 20 % change with α = 0.05, β = 0.2.

Falsification: If extended fasting fails to raise indole/AhR signaling, or if AhR blockade does not rescue PAA‑driven senescence despite unchanged fasting duration, the hypothesis is refuted.