The hypothesis is that β-hydroxybutyrate (BHB) exerts longevity effects only when its concentration oscillates in sync with feeding–fasting cycles, creating windows of selective HDAC inhibition that permit histone β-hydroxybutyrylation (Kbhb) at circadian‑regulated promoters. Constant high BHB, as achieved by continuous supplementation or unchecked ketogenic diets, triggers compensatory HDAC up‑regulation and erodes the epigenetic rhythm, abolishing healthspan benefits and potentially activating stress‑sensitive pathways such as AMPK‑p53‑mediated senescence in young tissues.

Mechanistically, intermittent BHB peaks (~2–5 mM) reached during fasting inhibit class I HDACs with sufficient potency to increase acetylation at FOXO3a and Nrf2 promoters, as shown in invertebrate models4 and biochemical studies3. These peaks also provide the substrate for the lysine β-hydroxybutyryl transferase activity of p300/CBP, leading to Kbhb marks that recruit co‑activators and stabilize the transcriptional program of circadian regulators (e.g., Bmal1, Clock). The resulting chromatin state enhances mitochondrial respiration, thermotolerance, and proteostasis without chronically suppressing deacetylase activity that is required for DNA repair and cell‑cycle checkpoints.

In contrast, sustained BHB elevation leads to homeostatic feedback: HDAC expression rises via HDAC‑responsive elements, and excess Kbhb may sterically hinder acetyl‑transferase access, flattening the epigenetic landscape. This dampens the amplitude of circadian gene expression, reduces NAD+ salvage flux, and shifts AMPK signaling toward p53‑dependent senescence, especially in proliferative compartments such as intestinal stem cells where DNA damage accrues5.

Testable predictions:

1. Mice receiving timed BHB boluses aligned with the dark phase (fasting) will show increased histone acetylation and Kbhb at circadian promoters, heightened expression of Bmal1‑driven metabolic genes, and improved healthspan metrics (grip strength, glucose tolerance) compared with mice receiving continuous BHB infusion of equal total dose.
2. Continuous BHB treatment will elevate HDAC1/2 protein levels in liver and muscle after 4 weeks, correlating with reduced Kbhb occupancy and increased p53‑p21 staining in young animals.
3. Genetic disruption of the Kbhb writer (p300 KAT domain) will abolish the healthspan benefits of intermittent BHB without affecting its HDAC inhibitory capacity, confirming the necessity of the histone modification.
4. In aged mice, intermittent BHB will blunt the age‑related decline in circadian amplitude, whereas continuous BHB will fail to rescue or may exacerbate fragmentation of rest–activity cycles.

Experimental design: Use C57BL/6J mice split into four groups (ad libitum control, continuous BHB via osmotic pump, intermittent BHB via timed gavage, and intermittent BHB + p300 KAT inhibitor). Monitor lifespan, frailty index, circadian rhythm (wheel running), tissue‑specific HDAC activity, western blot for acetyl‑ and Kbhb‑histone marks, and senescence markers. A significant interaction between BHB regimen and p300 inhibition on healthspan outcomes would falsify the hypothesis that rhythmic Kbhb, not mere HDAC inhibition, underlies BHB’s longevity potential.

This framework bridges the mechanistic promise of BHB with the translational gap highlighted by the lack of large‑scale mammalian validation1 and the modest tolerability focus of ongoing trials2. It offers a clear, falsifiable path to determine whether timing, rather than mere presence, of ketone signaling dictates its impact on aging.