Hypothesis

Baseline circulating indole-3-propionic acid (IPA) predicts future mortality and age‑related disease incidence because it maintains intestinal barrier integrity via aryl hydrocarbon receptor (AhR)‑dependent up‑regulation of tight‑junction proteins and mucin secretion. Low IPA reflects a leaky gut, leading to elevated systemic endotoxin (LPS), chronic low‑grade inflammation, and accelerated biological aging.

Rationale

• IPA is depleted in Alzheimer’s, osteoporosis, and coronary artery disease cohorts [1][2][3].
• In mice, IPA extends lifespan, improves muscle and bone, and reduces oxidative stress [4][5].
• IPA activates AhR in intestinal epithelial cells, driving expression of claudin‑5, occludin, and MUC2 [6].
• Human data are cross‑sectional; no prospective cohort links baseline IPA to hard outcomes with effect sizes.

Novel Mechanistic Insight

AhR activation by IPA not only modulates Wnt/β‑catenin and NF‑κB pathways in systemic tissues but also directly reinforces the intestinal epithelial barrier. Enhanced tight‑junction assembly reduces bacterial translocation, lowering circulating LPS and downstream Toll‑like‑4‑mediated inflammation. This creates a feedback loop: less inflammation preserves AhR signaling, sustaining barrier function. When IPA falls, barrier permeability rises, endotoxemia increases, inflammaging accelerates, and age‑related pathology follows.

Testable Predictions

1. Prediction 1: In a community‑dwelling cohort (≥5 y follow‑up), participants in the lowest tertile of baseline serum IPA will have a hazard ratio (HR) ≥1.8 for all‑cause mortality compared with the highest tertile, after adjusting for age, sex, BMI, and comorbidities.
2. Prediction 2: Low baseline IPA will be associated with elevated serum zonulin and LPS‑binding protein (LBP) at baseline, and these mediators will statistically mediate ≥30 % of the IPA–mortality relationship.
3. Prediction 3: Oral IPA supplementation (10 mg/day for 12 mo) in a randomized, double‑blind trial will reduce zonulin and LBP levels by ≥20 % relative to placebo, concomitant with a decrease in high‑sensitivity CRP.
4. Prediction 4: The mortality benefit observed in Prediction 1 will be attenuated in participants who concurrently receive a broad‑spectrum antibiotic that diminishes Clostridium sporogenes abundance, confirming the microbial origin of the effect.

Study Design (Outline)

• Cohort arm: Recruit 3 000 adults aged 45‑75 from existing biobanks; measure serum IPA, zonulin, LBP, LPS, and inflammatory cytokines at baseline; follow for mortality, incident dementia, cardiovascular events, and osteoporosis.
• Intervention arm: Parallel RCT (n = 200) of IPA vs. placebo for 12 mo, with primary outcomes of change in zonulin/LBP and secondary outcomes of cognitive and physical performance scores.
• Microbial sub‑study: Shotgun metagenomics on stool to quantify C. sporogenes fldC abundance; test interaction with IPA levels.

Falsifiability

If low IPA does not predict higher mortality, or if IPA supplementation fails to improve barrier markers, the hypothesis is refuted. Conversely, confirming the mediation pathway would elevate IPA from a correlative biomarker to a causal modulator of gut‑driven inflammaging and a viable therapeutic target.

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