Hypothesis

Intermittent fasting combined with NAD+ precursor supplementation produces a detectable GrimAge reduction that predicts improved functional healthspan only when it concurrently lowers circulating senescence-associated secretory phenotype (SASP) factors, reflecting a chromatin‑based mechanism whereby NAD+-dependent SIRT1 deacetylates histone H3K9 at SASP gene promoters, rendering those loci more susceptible to senolytic clearance and thereby locking in a youthful epigenetic state.

Mechanistic Rationale

NAD+ fuels SIRT1 activity, which removes acetyl groups from histone H3K9, promoting a more compact chromatin state at pro‑inflammatory SASP loci (e.g., IL6, GDF15). In mouse models, this deacetylation correlates with reduced SASP expression after senolytic treatment. We propose that in humans, intermittent fasting elevates hepatic NAD+, boosting SIRT1‑mediated H3K9 deacetylation in circulating immune cells. When SASP‑producing cells are subsequently cleared by senolytics (or naturally reduced via fasting‑induced autophagy), the epigenetic change becomes stable, measurable as a GrimAge shift. If SASP remains high, the NAD+‑SIRT1 signal is insufficient to lock in the epigenetic state, and GrimAge change fails to forecast functional outcomes.

Testable Predictions

1. In a 2‑year randomized trial of middle‑aged adults receiving intermittent fasting plus nicotinamide riboside (NR) versus control, GrimAge will decline significantly at 12 months in the intervention arm.
2. Participants who show a ≥10 % GrimAge reduction will exhibit improved gait speed, grip strength, and cognitive composite scores at 36 months only if their plasma SASP panel (IL‑6, TNF‑α, GDF15, MCP‑1) drops ≥20 % over the same period.
3. Participants with GrimAge improvement but unchanged or elevated SASP will not show functional benefits, falsifying the predictive value of the clock under those conditions.
4. Mechanistic substudy: peripheral mononuclear cells from responders will display increased SIRT1 activity and reduced H3K9 acetylation at SASP gene promoters measured by ChIP‑qPCR, whereas non‑responders will not.

Falsifiability

If the intervention yields GrimAge reductions that do not correlate with SASP changes, or if GrimAge improvements predict functional gains irrespective of SASP levels, the hypothesis is falsified. Likewise, absence of SIRT1‑dependent H3K9 deacetylation in responders would refute the proposed chromatin mechanism.