Hypothesis

Experimental pain threshold, particularly to noxious heat, reflects NAD+-dependent mitochondrial function in sensory neurons and thus mirrors systemic NAD+ decline, a core hallmark of aging. We hypothesize that baseline heat pain threshold predicts future epigenetic age acceleration (measured by GrimAge) independent of chronological age, physical activity level, and circulating inflammatory markers. Furthermore, a short‑term NAD+ precursor boost will transiently raise heat pain threshold and attenuate GrimAge increase over 6 months, whereas placebo will not.

Mechanistic Rationale

• Sensory neurons rely on mitochondrial oxidative phosphorylation to maintain ion gradients that set nociceptor firing thresholds. NAD+ is a cofactor for sirtuins and PARPs that regulate mitochondrial efficiency and DNA repair. Declining NAD+ with age reduces ATP production, increases ROS, and lowers the threshold for heat‑evoked pain (see 1 for age‑related pain tolerance loss).
• Systemic NAD+ decline drives inflammaging via NF‑κB activation and impairs vagal tone, linking to the observed correlation between chronic pain and brain‑PAD (3).
• Physical activity elevates muscle NAD+ via NAMPT upregulation, which may explain why active individuals show higher cold tolerance (1) but does not directly alter neuronal NAD+ in nociceptors.

Testable Predictions

1. In a cohort of adults aged 40‑70, baseline heat pain threshold (measured by a 3‑second contact thermode at 48 °C) will inversely correlate with baseline GrimAge (r ≈ ‑0.30, p < 0.01) after adjusting for age, sex, BMI, and leisure‑time MET‑hours/week.
2. After adjusting for baseline sTNF‑RII and CRP, the pain‑threshold–GrimAge association will remain significant, indicating independence from inflammation.
3. A double‑blind, placebo‑controlled trial giving nicotinamide riboside (NR) 500 mg BID for 12 weeks will increase heat pain threshold by ~15 % relative to placebo and will blunt the 6‑month GrimAge rise (ΔGrimAge_NR < ΔGrimAge_placebo, p < 0.05).

Falsifiability

If baseline pain threshold shows no correlation with GrimAge after controlling for covariates, or if NR fails to modify either pain threshold or epigenetic age change, the hypothesis is falsified. Likewise, if the association disappears when adjusting for sTNF‑RII or CRP, inflammation would be deemed the primary driver, refuting the neuronal NAD+‑centric mechanism.

References (inline)

[1] https://pmc.ncbi.nlm.nih.gov/articles/PMC10208467/ [2] https://pmc.ncbi.nlm.nih.gov/articles/PMC12670344/ [3] https://pmc.ncbi.nlm.nih.gov/articles/PMC6752890/ [4] https://pmc.ncbi.nlm.nih.gov/articles/PMC8037327/ [5] https://www.frontiersin.org/journals/public-health/articles/10.3389/fpubh.2020.00172/full [6] https://www.acc.org/latest-in-cardiology/articles/2025/07/02/15/19/the-relationship-between-exercise-and-longevity