Hypothesis: Age‑dependent upregulation of ileal ASBT increases systemic conjugated primary bile acids (CPBAs) and ammonia, which sensitize peripheral nociceptors and central pain‑processing circuits, thereby reducing pain threshold. Consequently, individual differences in pain sensitivity serve as a functional readout of intestinal bile‑acid dysregulation and predict biological age more accurately than current epigenetic clocks.

Mechanistic Rationale

1. Bile Acid Signaling on Sensory Neurons – CPBAs activate TGR5 and FXR receptors expressed on dorsal root ganglia (DRG) neurons. Chronic TGR5 signaling elevates intracellular cAMP, potentiating TRPV1‑mediated heat and chemical nociception. FXR activation alters lipid membrane composition, increasing neuronal excitability. Elevated serum CPBAs from ASBT hyperabsorption thus chronically prime nociceptive pathways, lowering the stimulus intensity required to elicit pain.
2. Ammonia‑Induced Oxidative Stress – Increased ileal reabsorption raises portal ammonia, which escapes hepatic urea cycle clearance in aging. Ammonia crosses the blood‑brain barrier and accumulates in DRG and spinal dorsal horn, where it stimulates NADPH oxidase, generating reactive oxygen species. Oxidative modification of Nav1.7 sodium channels prolongs action‑potential duration, enhancing pain signaling.
3. Microglial Priming via Gut‑Brain Axis – CPBAs and ammonia activate hepatic Kupffer cells, releasing IL‑1β and TNF‑α that signal via the vagus nerve to the spinal cord. This cytokine milieu primes microglia toward a pro‑inflammatory phenotype, lowering the threshold for central sensitization and amplifying pain perception.
4. Feedback Loop with Vagal Tone – Chronic pain reduces vagal efferent activity, diminishing cholinergic anti‑inflammatory signaling in the gut. Reduced vagal tone further impairs intestinal barrier integrity, exacerbating bile‑acid‑driven dysbiosis and permeability, creating a vicious cycle that accelerates epigenetic aging.

Testable Predictions

• Prediction 1: In a cross‑sectional cohort (ages 20‑80), serum concentrations of tauro‑ and glyco‑conjugated cholic acid will positively correlate with lower pressure‑pain thresholds (measured by algometry) after adjusting for sex, BMI, and comorbidities.
• Prediction 2: Individuals in the highest quartile of pain sensitivity will show significantly greater ileal ASBT mRNA expression (via biopsy‑derived organoids or non‑invasive MRI‑based transport proxies) and higher fecal ammonia levels than those in the lowest quartile.
• Prediction 3: Pharmacological inhibition of ASBT (e.g., elobixibat) for 12 weeks will increase pain thresholds by ≥15% and concomitantly decrease serum CPBAs and ammonia, with the magnitude of pain‑threshold improvement mediating the change in Horvath epigenetic age acceleration score.
• Prediction 4: In vitro DRG neurons exposed to physiological concentrations of tauro‑cholate (10‑50 µM) will exhibit increased capsaicin‑evoked calcium fluxes via TRPV1, an effect blocked by TGR5 antagonism; ammonia (0.5‑2 mM) will further potentiate this response through oxidative modulation of Nav1.7.

Falsifiability If serum CPBAs and ammonia show no correlation with pain thresholds, or if ASBT inhibition fails to alter pain sensitivity despite robust reductions in bile‑acid reabsorption, the hypothesis would be refuted. Similarly, if DRG neuronal excitability is unresponsive to bile‑acid or ammonia exposure under controlled conditions, the proposed peripheral mechanism would be invalidated.

Implications Linking intestinal bile‑acid hyperabsorption to pain perception offers a novel, clinically accessible biomarker of biological age that integrates gut‑liver‑brain axis dysregulation. It also suggests that targeting ASBT could simultaneously ameliorate age‑related pain sensitivity and attenuate systemic aging processes.