Hypothesis

Baseline serum indole-3-propionic acid (IPA) levels predict slower epigenetic aging in humans, independent of renal function, through activation of hepatic farnesoid X receptor (FXR) and downstream fibroblast growth factor 21 (FGF21) signaling.

Mechanistic Rationale

IPA, a tryptophan metabolite produced by certain gut Clostridiales, crosses the portal vein and reaches the liver. Preclinical work shows IPA activates neuronal PXR and suppresses NFκB 2. We propose that in hepatocytes IPA also binds FXR, a bile‑acid sensor known to regulate metabolism and stress resistance. FXR activation induces FGF21 secretion, which elevates NAD+ levels, activates sirtuins (SIRT1/3), and promotes deacetylation of histone and non‑histone proteins, leading to a more youthful epigenetic profile. This pathway links gut‑derived IPA to epigenetic clocks measured by DNA methylation age.

Testable Predictions

1. In a middle‑aged to older cohort, higher baseline serum IPA will be associated with a lower epigenetic age acceleration (EAA) after adjusting for age, sex, BMI, smoking, and eGFR.
2. The association will be mediated by hepatic FXR activity, proxied by circulating FGF21 or bile‑acid ratios, such that FGF21 levels significantly attenuate the IPA‑EAA relationship in mediation analysis.
3. Individuals with genetically reduced FXR function (e.g., FXR loss‑of‑function variants) will show a weakened or absent IPA‑EAA association, providing a gene‑environment interaction test.
4. Experimental manipulation—acute IPA supplementation in a randomized crossover trial—will increase serum FGF21 within 24 h and, after 12 weeks, reduce EAA relative to placebo, but only in participants with normal renal function.

Experimental Design

• Observational cohort: Recruit 1,500 participants aged 45‑80 from existing biobanks (e.g., UK Biobank, Framingham). Measure baseline serum IPA (LC‑MS/MS), eGFR, fasting FGF21, bile‑acid profile, and collect blood for DNA methylation arrays (e.g., Illumina EPIC). Compute epigenetic age using PhenoAge or GrimAge. Use linear regression for IPA → EAA, mediation analysis for FGF21, and interaction testing for FXR rs SNPs.
• Intervention trial: Double‑blind, placebo‑controlled crossover (n=120, aged ≥ 60, normal eGFR). Participants receive 500 mg IPA or placebo for 12 weeks, with a 4‑week washout. Primary outcome: change in epigenetic age acceleration; secondary: serum FGF21, NAD+/NADH ratio, inflammatory cytokines.

Potential Confounders and Falsifiability

If IPA levels show no association with EAA after full adjustment, or if FGF21 does not mediate the effect, the hypothesis is falsified. Likewise, lack of interaction with FXR variants or failure of IPA supplementation to raise FGF21 and improve epigenetic markers would refute the proposed mechanism. Conversely, confirmation across both observational and interventional arms would support IPA as a functional longevity biomarker acting through hepatic FXR‑FGF21 signaling.

References (inline)

[1] https://pubmed.ncbi.nlm.nih.gov/41540161/ [2] https://www.science.org/doi/10.1126/sciadv.adw8410 [3] https://onlinelibrary.wiley.com/doi/full/10.1002/mnfr.202100349 [4] https://isappscience.org/insights-into-healthy-aging/ [5] https://clinicaltrials.gov/study/NCT06674018