Hypothesis\nIntermittent fasting (IF) increases colonic tryptophan metabolism by Clostridium sporogenes through upregulation of the fldC gene, raising circulating indole‑3‑propionic acid (IPA). Elevated IPA activates the pregnane X receptor (PXR) in intestinal epithelial cells, which triggers a downstream signaling cascade that boosts hepatic fibroblast growth factor 21 (FGF21) secretion and enhances systemic SIRT1 activity. This IPA‑PXR‑FGF21‑SIRT1 axis improves NAD+ bioavailability and promotes favorable DNA methylation patterns, thereby decelerating epigenetic age as measured by GrimAge and PhenoAge clocks.\n\n## Mechanistic Rationale\n1. IF → ↑ C. sporogenes fldC expression → ↑ IPA (supported by [7]).\n2. IPA is a potent PXR agonist; PXR activation in gut epithelium drives expression of cytochrome P450 enzymes and transporters that alter bile acid pools and stimulate enteroendocrine L‑cells to release FGF21 (extrapolating from PXR‑FGF21 links in liver and intestine [5,6]).\n3. FGF21 acts on hypothalamus and adipose tissue to increase NAD+ biosynthesis via NAMPT upregulation and to activate SIRT1 deacetylase activity (known FGF21‑SIRT1 crosstalk [8]).\n4. Elevated SIRT1 deacetylates histone H3K9 and promotes DNMT1 inhibition, leading to a younger methylome (SIRT1‑DNA methylation relationship demonstrated in aging models).\n5. Consequently, individuals with higher fasting‑induced IPA show slower epigenetic aging.\n\n## Testable Predictions\n- In a human crossover trial, participants undergoing 16:8 IF for 4 weeks will exhibit a ≥2‑fold increase in plasma IPA relative to baseline, accompanied by a rise in intestinal PXR target gene expression (measured in stool‑derived RNA) and higher circulating FGF21.\n- Mediation analysis will show that the IF‑induced change in GrimAge Δ is significantly accounted for by the increase in IPA (indirect effect p<0.05).\n- Pharmacologic blockade of PXR (using ketoconazole) during IF will abolish the rise in FGF21 and prevent the epigenetic age benefit, despite unchanged IPA levels.\n- Germ‑free mice colonized with an fldC knockout C. sporogenes strain will fail to elevate IPA, FGF21, or SIRT1 activity under IF and will not show the epigenetic age delay seen with wild‑type colonized mice.\n\n## Experimental Design (Human)\nRecruit 60 middle‑aged volunteers, randomize to IF or isocaloric control for 4 weeks, crossover after 4‑week washout. Collect fasting plasma at baseline and week 4 for IPA (LC‑MS/MS), FGF21, NAD+ metabolites, and peripheral blood mononuclear cell RNA for PXR targets. Perform epigenetic clock analysis (GrimAge, PhenoAge) on DNA from same samples. Use mixed‑effects models to test interaction of time·condition, and causal mediation analysis (IPA as mediator). Include a substudy where 20 participants receive oral ketoconazole (PXR inhibitor) during the IF phase.\n\n## Falsifiability\nIf IF does not raise plasma IPA, or if IPA elevation fails to correlate with changes in FGF21/SIRT1 activity, or if PXR inhibition does not blunt the epigenetic age improvement despite adequate IPA increase, the hypothesis would be falsified. Conversely, a consistent chain of effects across these levels would support the proposed mechanistic link.\n\n## Potential Confounders\nDietary tryptophan intake, baseline microbiota composition, and liver function could affect IPA levels; we will control tryptophan via standardized meals and stratify sequencing‑based enterotypes.\n\nCitations: [1] [2] [3] [4] [5] [6] [7] [8]