Research board · The consuming enzymes
Sirtuins and the NAD+ axis
Three enzyme families spend NAD+ from one pool that shrinks with age. The mechanism the rest of this board is organized around.
The short version
Sirtuins (cellular-maintenance enzymes), PARPs (DNA-repair enzymes) and CD38 all run on NAD+ (the fuel-handling helper molecule), and they all draw from the same shared supply. As we age that supply shrinks while the demand does not — and one enzyme, CD38, is the main reason the supply falls. This page walks through that competition: who spends NAD+, why the pool drops, and how the body's daily clock paces its rebuild. It summarizes published research on the NAD+ axis; it is not advice and recommends no product or dose.
Sirtuins: the maintenance crew that runs on NAD+
Sirtuins (SIRT1–SIRT7) are NAD+-dependent deacylase enzymes — they cannot work without NAD+, which they consume as they strip chemical tags off target proteins to regulate metabolism, stress resistance and DNA repair [5]. The mitochondrial member, SIRT3, regulates mitochondrial biogenesis through the AMPK–PGC-1α axis and deacetylates metabolic enzymes and the antioxidant SOD2; SIRT3 loss impairs ATP production and antioxidant defense and tracks with disease pathology in mouse models [9].
Because sirtuin activity rises and falls with NAD+ availability, the size of the NAD+ pool is not a passive number — it sets how much maintenance work the crew can do. That coupling is the mechanistic heart of why "NAD+ decline" is treated as consequential rather than cosmetic [5].
PARP1 and CD38: the other two claimants
Sirtuins are not the only enzymes spending the pool. PARP1 (poly(ADP-ribose) polymerase 1) is a DNA-repair enzyme that consumes large amounts of NAD+ when it is activated by DNA damage [5]. And CD38 — an NAD-consuming ectoenzyme — rises with age and inflammation and is the principal driver of the age-related fall in tissue NAD+ [5].
The CD38 evidence is the most direct. CD38-knockout mice are protected against age-related NAD+ decline and retain SIRT3 activity and mitochondrial function with age [2]. A potent, specific CD38 inhibitor reversed age-related metabolic dysfunction in aged mice by restoring tissue NAD+ through an SIRT3-dependent mechanism [8]. Even the dietary flavonoid apigenin inhibits CD38 (IC50 about 10.3 µmol/L in vitro) and raised intracellular NAD+ and improved glucose handling in high-fat-diet mice at 100 mg/kg IP [10].
NAMPT, exercise and the circadian clock
On the supply side, NAMPT (nicotinamide phosphoribosyltransferase) is the rate-limiting enzyme of the salvage pathway — the bottleneck for rebuilding NAD+ from nicotinamide [5]. Two things tune it. Exercise: aerobic training induces NAMPT protein about 2-fold higher in athletes than sedentary subjects, and transgenic NAMPT overexpression gives mice roughly 3-fold higher exercise endurance [7]. And the clock: CLOCK:BMAL1 directly drives circadian expression of NAMPT, so intracellular NAD+ oscillates over 24 hours, with SIRT1 binding the Nampt promoter to amplify its own coenzyme synthesis — a feedback loop absent in clock-deficient mice [6].
That circadian biology is real and well documented [6]. It is also frequently over-read into "take NAD+ at this time of day," which the literature does not support — the oscillation is mechanism, not a human dosing schedule.