Hypothesis

Baseline serum indole‑3‑propionic acid (IPA) concentration predicts slower epigenetic aging acceleration in humans, but only when estimated glomerular filtration rate (eGFR) exceeds 60 mL/min/1.73 m². This association is mediated by hepatic activation of the aryl hydrocarbon receptor (AhR) and downstream Nrf2‑dependent antioxidant signaling, independent of intestinal Clostridium sporogenes abundance.

Rationale

Preclinical work shows that IPA improves grip strength, bone microarchitecture, reduces oxidative stress and senescence, and extends lifespan in mice and Drosophila via AhR/PXR activation [1][2]. IPA crosses the blood‑brain barrier, inhibits lipid peroxidation, protects DNA and mitochondria, and modulates immune responses through AhR/PXR pathways [2]. Observational human data link low IPA to unhealthy aging, type 2 diabetes, obesity, and cardiovascular risk [4][5], yet these are narrative associations without quantified effect sizes. A polyphenol‑rich dietary intervention raised serum IPA only in older adults with preserved kidney function and higher baseline Clostridiales abundance, highlighting eGFR as a potential effect modifier [3].

We propose that renal clearance determines circulating IPA bioavailability: when eGFR falls below ~60 mL/min/1.73 m², increased tubular reabsorption or reduced excretion alters IPA half‑life, diminishing its ability to sustain AhR activation in extra‑renal tissues. Simultaneously, hepatic AhR activation by IPA triggers Nrf2 translocation, up‑regulating antioxidant enzymes (e.g., HO‑1, NQO1) that attenuate epigenetic drift measured by clocks such as DunedinPACE and PhenoAge. This mechanistic chain bypasses the need for high C. sporogenes levels because hepatic AhR can be stimulated by IPA irrespective of its microbial origin, provided sufficient systemic exposure.

Predictions

1. In cohorts with eGFR > 60 mL/min/1.73 m², a 1‑SD increase in baseline serum IPA will associate with a −0.05 unit slower DunedinPACE (≈5 % reduction in pace of aging) after adjusting for age, sex, BMI, smoking, and diet.
2. The IPA‑aging association will attenuate or reverse in participants with eGFR ≤ 60 mL/min/1.73 m², showing no significant correlation or a positive association with accelerated epigenetic aging.
3. Mediation analysis will reveal that hepatic AhR target gene expression (e.g., CYP1A1) and Nrf2‑dependent antioxidant scores account for ≥40 % of the IPA‑epigenetic aging link, while fecal C. sporogenes qPCR will explain <10 % of IPA variance.
4. Longitudinal follow‑up will show that baseline IPA predicts incident frailty and mortality only in the high‑eGFR stratum, with hazard ratios comparable to established biomarkers (e.g., IL‑6, CRP).

Experimental Design

• Population: Leverage existing longitudinal cohorts (e.g., UK Biobank, Framingham Heart Study) with baseline metabolomics, eGFR, and epigenetic clock data (n ≥ 5 000).
• Measurements: Quantify serum IPA via LC‑MS/MS, determine eGFR using CKD‑EPI creatinine, assess DunedinPACE/ PhenoAge, quantify hepatic AhR activity via circulating CYP1A1 mRNA (extracellular vesicles) or surrogate markers, measure Nrf2 target expression in peripheral blood mononuclear cells, and quantify stool C. sporogenes by 16S rRNA gene sequencing or qPCR.
• Analysis: Use linear regression for cross‑sectional IPA‑clock associations, stratified by eGFR > 60 vs ≤ 60 mL/min/1.73 m². Conduct mediation analysis (counterfactual approach) to estimate indirect effects via AhR/Nrf2 pathways. Validate findings in an independent replication cohort.
• Falsifiability: If no significant IPA‑clock association appears in the high‑eGFR group, or if mediation via AhR/Nrf2 accounts for <10 % of the effect, the hypothesis is refuted.

Potential Confounders

Dietary tryptophan intake, hepatic dysfunction, and medications that modulate AhR (e.g., flavonoids, pollutants) could influence IPA levels or signaling. We will adjust for dietary tryptophan (food frequency questionnaire), liver enzymes (ALT/AST), and use of AhR‑modulating drugs. Sensitivity analyses excluding participants with CKD stage 3b or worse will test robustness of the eGFR threshold.

By integrating renal function, hepatic AhR/Nrf2 signaling, and epigenetic aging metrics, this hypothesis moves IPA from a correlative metabolite to a mechanistically grounded, clinically actionable biomarker of human aging.