Hypothesis

Beta-hydroxybutyrate (BHB) acts as an endogenous signal that lowers cellular NAD+ turnover by inhibiting class I HDACs, thereby reducing expression of NAD+-consuming enzymes such as CD38 and PARP1. This creates a NAD+-sparing state that mimics the metabolic downshift seen in caloric restriction, positioning NAD+ decline as a downstream adaptive response rather than a primary driver of aging.

Mechanistic Rationale

BHB inhibits HDAC1, HDAC3, and HDAC4 at physiologic concentrations achieved during fasting (1). HDAC inhibition increases histone acetylation at stress‑resistance gene promoters (e.g., Foxo3a, Mt2). We propose that a parallel epigenetic effect occurs at the promoters of Cd38 and Parp1. Deacetylated histones at these loci favor a repressive chromatin state; HDAC inhibition by BHB shifts the balance toward acetylation, which paradoxically reduces transcription of these NAD+-consuming genes in certain contexts because acetylation can recruit bromodomain‑containing repressors or interfere with activator binding. Lower CD38 and PARP1 expression diminishes NAD+ cleavage, conserving the NAD+ pool despite unchanged biosynthesis. Consequently, cells exhibit lower NAD+ flux without a rise in NAD+ concentration, uncoupling NAD+ levels from epigenetic aging markers.

Testable Predictions

1. In human peripheral blood mononuclear cells, BHB supplementation (achieving ~3 mM plasma) will decrease CD38 and PARP1 mRNA and protein levels after 4 weeks, independent of changes in NAD+ biosynthesis enzymes (NAMPT, NAPRT).
2. Epigenetic age (e.g., GrimAge) will improve alongside reduced HDAC activity, while total NAD+ concentration remains statistically unchanged.
3. If NAD+ decline were causal, raising NAD+ via NR or NMN would reverse epigenetic aging even when HDAC activity is high; we predict that NAD+ boosting will not further improve epigenetic age when BHB‑mediated HDAC inhibition is already maximal.

Experimental Design

• Population: n = 1200 adults aged 50‑80, stratified by baseline NAD+ levels.
• Intervention: Randomized, double‑blind, placebo‑controlled; BHB ester delivering ~3 mM plasma BHB vs. iso‑caloric placebo for 12 weeks.
• Measurements (baseline, 6 weeks, 12 weeks): plasma BHB, NAD+ metabolomics (targeted LC‑MS), CD38/PARP1 protein (flow cytometry, Western blot), HDAC activity assay, histone acetylation ChIP‑seq at Cd38 and Parp1 promoters, epigenetic clocks, physical function (gait speed, grip strength), and cognitive battery.
• Analysis: Mixed‑effects models to test interaction between BHB exposure and changes in NAD+ consumption enzymes on epigenetic age, adjusting for age, sex, BMI, and baseline NAD+.

Potential Outcomes and Falsifiability

• Supportive outcome: BHB reduces CD38/PARP1 expression, lowers HDAC activity, improves epigenetic age, yet NAD+ concentration does not rise significantly. This would indicate that NAD+ sparing via reduced consumption, not increased production, underlies the epigenetic benefit, supporting the view that NAD+ decline is an adaptive downstream signal.
• Falsifying outcome: BHB fails to alter CD38/PARP1 levels or HDAC activity, or any epigenetic improvement is tightly coupled to a rise in NAD+ concentration. In this case, the data would suggest that NAD+ availability, rather than its consumption, drives epigenetic aging, challenging the adaptive downshift hypothesis.

By directly linking ketone‑mediated HDAC inhibition to the regulation of NAD+-consuming enzymes, this hypothesis extends the current mechanistic insight beyond correlative observations and offers a clear, falsifiable path to resolve whether NAD+ loss drives aging or merely reflects a strategically lowered metabolic ambition.