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CD38 — the enzyme
that consumes NAD+,
and why it rises with age.
The NAD+ pool declines with age from two directions simultaneously. NAMPT — the enzyme that makes NMN — becomes less active. CD38 — the enzyme that destroys NAD+ — becomes more active. CD38 is an NAD+ase expressed on immune cells, driven by the chronic low-grade inflammation of aging. Understanding CD38 is understanding the other half of why NAD+ falls — and why the same inflammatory environment that ages the immune system also depletes the cellular resource that NMN research is designed to address.
I
What CD38 is —
and what it does to NAD+.
CD38 is a multifunctional enzyme expressed primarily on the surface of immune cells — including T cells, B cells, natural killer cells, monocytes, and dendritic cells — that uses NAD+ as its primary substrate. Its enzymatic activity includes NAD+ glycohydrolase (cleaving NAD+ to nicotinamide and ADP-ribose), ADP-ribosyl cyclase (converting NAD+ to cyclic ADP-ribose, a calcium-mobilizing second messenger), and hydrolase activity on cyclic ADP-ribose. From the perspective of NAD+ biology, CD38's most consequential activity is its NAD+ase function: it is the largest single consumer of NAD+ in aging tissue, degrading NAD+ at rates that vastly exceed the stoichiometric consumption of sirtuins and PARPs combined.
The scale of CD38's NAD+ consumption is what makes it significant. Sirtuins consume one NAD+ per deacetylation reaction — a carefully regulated, stoichiometric consumption that produces the nicotinamide that the Salvage Pathway recycles. CD38 consumes NAD+ at rates that are orders of magnitude higher, and unlike sirtuin consumption, the NAD+ degraded by CD38 is not efficiently recycled: the nicotinamide produced can be recaptured by NAMPT, but only if NAMPT has the capacity to process it — and NAMPT's capacity, as described in the preceding articles in this series, declines with age. The combination of CD38-driven NAD+ degradation and reduced NAMPT-mediated NMN synthesis is the compounding mechanism that produces the net NAD+ decline measured across aging tissues. NMN research addresses the production side of this equation — supplying NMN to bypass the NAMPT bottleneck. Understanding CD38 is understanding why that supply faces a persistent demand that does not diminish on its own.
CD38's normal physiological role is in calcium signaling: the cyclic ADP-ribose it produces is a calcium-mobilizing molecule from intracellular stores, playing important roles in muscle contraction, secretory cell function, and immune cell activation. It is not an enzyme whose existence is pathological — it is a normal component of cellular signaling whose expression happens to scale with the inflammatory environment that aging produces. This is a critical distinction: CD38 does not "malfunction" with age. It responds normally to the signals it receives — signals that, in the context of chronic inflammaging, are sustained at a level that was never present in youth and that the NAD+ system was not designed to accommodate indefinitely. Studies referenced were conducted independently and did not involve any specific Codeage product.
CD38 does not malfunction with age.
It responds normally
to the signals it receives.
The problem is that the signals —
the chronic inflammation of aging —
are sustained at a level
the NAD+ system
was not designed to accommodate.
What CD38 Produces From NAD+
Two enzymatic products — one for calcium signaling,
one that is simply NAD+ consumed.
CD38 Product 01 · Calcium signaling
cADPR
Cyclic ADP-ribose — second messenger
When CD38 acts as an ADP-ribosyl cyclase, it converts NAD+ into cyclic ADP-ribose (cADPR) — a calcium-mobilizing second messenger that acts on ryanodine receptors on the endoplasmic and sarcoplasmic reticulum to release calcium stores into the cytoplasm. cADPR-mediated calcium signaling is important in cardiac muscle contraction, pancreatic beta cell insulin secretion, T cell activation, and other processes requiring rapid intracellular calcium elevation. This is CD38's original, normal physiological role — calcium signaling in contexts where rapid cellular activation is required. The problem is that producing cADPR requires consuming one NAD+ molecule per cADPR molecule produced, and in conditions of chronic immune activation — which chronic inflammaging provides — this consumption is sustained continuously rather than transiently.
CD38 Product 02 · NAD+ hydrolysis
ADPR + Nam
ADP-ribose + nicotinamide — net NAD+ loss
When CD38 acts as an NAD+ glycohydrolase, it cleaves NAD+ directly into ADP-ribose and nicotinamide — a simple hydrolysis that destroys the NAD+ structure without producing the calcium-signaling cADPR. This is the dominant activity of CD38 under most conditions, and it represents a net loss to the NAD+ pool: unlike sirtuin-mediated NAD+ consumption (where the nicotinamide is efficiently recycled back to NMN by NAMPT in the Salvage Pathway), the ADP-ribose produced by CD38 hydrolysis is not efficiently converted back to NAD+. The nicotinamide can potentially be recycled by NAMPT, but ADP-ribose is metabolized through a separate pathway that does not feed back into the Salvage Pathway. CD38's glycohydrolase activity is therefore a true drain on the NAD+ pool — one that grows as CD38 expression on immune cells rises with the inflammatory state of aging. Studies were conducted independently and did not involve any specific Codeage product.
II
Why CD38 rises with age —
the inflammaging connection.
CD38 expression is regulated by NF-κB — the master transcription factor of the inflammatory response. When NF-κB is activated by inflammatory cytokines (TNF-α, IL-1β, IL-6) or by pattern recognition receptor signaling, it drives transcription of a broad set of pro-inflammatory genes — and CD38 is among them. In acute inflammation, this makes functional sense: CD38 upregulation on activated immune cells supports the calcium signaling and oxidative burst capacity needed for an effective immune response. In chronic low-grade inflammation — the inflammaging state of aging tissue — NF-κB activation is sustained at a low but continuous level, and CD38 expression on immune cells accumulates accordingly.
The result is an age-related rise in CD38 expression and activity that parallels the age-related rise in inflammatory markers. Studies in aged mice and aged human tissue have documented substantially higher CD38 protein levels in immune cells and in some non-immune tissues compared to young controls — with CD38 activity in aged tissue sufficient to account for a significant fraction of the measured NAD+ decline. The relationship is not merely correlational: CD38 knockout mice, which cannot express functional CD38 protein, maintain substantially higher NAD+ levels with age than wild-type controls and show preservation of NAD+-dependent metabolic function in aging tissues. This genetic evidence establishes CD38 as a causal contributor to age-related NAD+ decline — not merely an age-associated marker. Studies were conducted independently and did not involve any specific Codeage product.
The CD38–inflammaging loop also runs in the other direction. SIRT1 and SIRT6 — the NAD+-dependent sirtuins that deacetylate and thereby suppress NF-κB target genes — lose activity as NAD+ falls. When CD38 drives the NAD+ pool down, the sirtuins that would otherwise restrain the NF-κB activation driving CD38 expression are less able to do so. This creates a self-reinforcing cycle: inflammaging activates NF-κB → NF-κB drives CD38 expression → CD38 depletes NAD+ → NAD+ depletion reduces SIRT1/SIRT6 activity → SIRT1/SIRT6 can no longer restrain NF-κB → NF-κB drives more CD38 expression. Understanding this loop is understanding why the NAD+ decline of aging accelerates rather than plateaus as the inflammatory state worsens — and why it is mechanistically connected to the same immune aging biology covered in this series.
The Inflammaging–CD38–NAD+ Loop
Five steps — the self-reinforcing cycle
through which aging drives NAD+ decline.
This is a documented mechanistic loop in the aging biology literature — a cycle where each step worsens the next. Understanding it explains why NAD+ decline accelerates with age rather than simply tracking it. All studies were conducted independently and did not involve any specific Codeage product.
Inflammaging establishes chronic NF-κB activation in aging immune cells and tissue
As senescent cells accumulate and immune surveillance becomes less effective, the SASP-driven inflammatory environment of aging tissue produces sustained, low-level NF-κB activation across immune and non-immune cells. Unlike the acute NF-κB activation of a normal immune response — which is resolved within days — the NF-κB activation of inflammaging is persistent and unresolved, driven by the same senescent cell burden and immune remodeling described in earlier articles in this series.
NF-κB drives CD38 expression on immune cells — continuously, at a level proportional to inflammatory tone
CD38 is a direct NF-κB target gene. As NF-κB activation rises with inflammaging, CD38 expression on T cells, B cells, NK cells, monocytes, and macrophages rises in proportion. The scale of this effect is significant: in aged mice, CD38 protein levels in immune-rich tissues are documented at 2–3 times the levels found in young controls. The elevated CD38 then degrades NAD+ — both through glycohydrolase activity (direct NAD+ → ADPR + nicotinamide) and through cyclase activity (NAD+ → cADPR + nicotinamide) — at a rate that exceeds what the Salvage Pathway can replenish.
CD38 degrades NAD+ faster than the aging Salvage Pathway can produce it — the pool falls
The Salvage Pathway — already running at reduced capacity because NAMPT activity has declined with age — now faces a degradation load from elevated CD38 activity that it cannot match. The NAD+ pool falls. The magnitude of the fall is the compound result of both forces acting simultaneously: less NMN synthesized per unit time (NAMPT decline) and more NAD+ degraded per unit time (CD38 rise). This is why the NAD+ decline of aging is so consistent and so pronounced across tissues — it is driven by two independent mechanisms that both worsen with age and that both worsen with the same underlying inflammatory state.
Reduced NAD+ attenuates SIRT1 and SIRT6 activity — the sirtuins that normally restrain NF-κB
SIRT1 deacetylates the p65 subunit of NF-κB, reducing its transcriptional activity at inflammatory gene promoters. SIRT6 deacetylates H3K9ac at NF-κB target gene promoters, creating a repressive chromatin state. Both activities require NAD+ as a stoichiometric cofactor. When the NAD+ pool falls, SIRT1 and SIRT6 activity falls proportionally — and their restraint of NF-κB signaling is attenuated. The inflammatory genes that SIRT1 and SIRT6 would otherwise silence become more accessible, and the transcriptional activation of those genes — including CD38 itself — increases.
Reduced sirtuin restraint amplifies NF-κB signaling — which drives more CD38 expression — completing the loop
With SIRT1 and SIRT6 less active, NF-κB drives its target genes — including CD38, pro-inflammatory cytokines, and SASP components — at higher levels. More CD38 is produced. More NAD+ is degraded. The NAD+ pool falls further. SIRT1 and SIRT6 activity falls further. NF-κB is restrained even less. The loop closes and accelerates. This is the molecular mechanism by which the inflammatory and NAD+ dimensions of aging are coupled — not through correlation or indirect association, but through a specific, documented biochemical feedback loop in which each element drives the next. The loop explains why interventions that supply NMN address only the production side of the equation — they are working against an active degradation mechanism whose underlying driver (inflammation) continues to operate independently.
CD38 and NAD+ Decline in Numbers
What the CD38–NAD+ relationship
looks like as documented biology.
2–3×
Higher CD38 protein levels in immune-rich aging tissue compared to young controls — documented in aged mouse studies and paralleled in human aging data
Multiple studies in aged rodents have documented CD38 protein levels 2–3 times higher in immune-rich tissues compared to young controls, with CD38 activity in aged tissue accounting for a significant fraction of the measured NAD+ decline. CD38 knockout mice — which cannot express functional CD38 — maintain substantially higher NAD+ levels with age and show preserved NAD+-dependent metabolic function. This genetic evidence establishes CD38 as a causal contributor to age-related NAD+ decline, not merely a correlated marker. Studies were conducted independently and did not involve any specific Codeage product.
2
Simultaneous and independent mechanisms driving NAD+ decline with age — NAMPT down (less production) and CD38 up (more degradation) — both worsened by the same inflammatory environment
The NAD+ decline of aging is driven by two forces that are both independently documented and that both worsen with the same underlying process (inflammaging). This is why simple interventions targeting only one side of the equation face the other side's continued operation: supplying NMN addresses the production deficit but not the degradation excess; addressing inflammation alone may attenuate CD38 expression but does not immediately recover the NAMPT activity that has already declined. The compound nature of the deficit is why the research literature increasingly considers NAD+ precursor strategies alongside anti-inflammatory approaches as complementary rather than alternative.
5
Steps in the documented inflammaging–CD38–NAD+–sirtuin feedback loop — a self-reinforcing cycle in which each element drives the next toward further NAD+ depletion
The five-step feedback loop — inflammaging → NF-κB → CD38 → NAD+ decline → SIRT1/SIRT6 attenuation → NF-κB amplification → CD38 → NAD+ decline — is not a theoretical model but a sequence of documented molecular events, each step supported by experimental evidence in cell culture, animal models, and in several cases human studies. The loop's self-reinforcing character explains the acceleration of NAD+ decline observed in aged tissue, and provides the mechanistic context within which NMN research — and the broader field of NAD+ precursor biology — is best understood. All steps are documented independently; the complete loop as described reflects current understanding in an active research field.
III
What CD38 biology tells us
about NMN research and the NAD+ system.
Understanding CD38 completes the picture of why the NAD+ pool declines with age. The NAMPT story, covered in the preceding article, explains the production side: the rate-limiting enzyme of the Salvage Pathway becomes less active with age, producing less NMN per unit time. The CD38 story explains the degradation side: an NAD+ase whose expression scales with inflammatory activation degrades NAD+ at rates that rise as inflammaging progresses. Together, these two mechanisms — one pulling production down, one pushing degradation up — produce the compound NAD+ deficit that the research literature consistently measures across aging tissues.
The CD38–NAD+–sirtuin feedback loop also explains the most important structural feature of the NAD+ decline: it is self-amplifying. Unlike a simple linear decline, the loop produces a system in which each increment of decline creates conditions for further decline — making the trajectory increasingly difficult to address through endogenous mechanisms as age advances. This is the biological context in which NMN research has developed: not as a response to a simple depletion, but as an attempt to supply the upstream precursor that bypasses the NAMPT bottleneck and feeds the pool that CD38 is simultaneously working to drain. For the NAMPT article covering the production side of this equation, and for how NMN and sirtuins connect through the NAD+ pool that both NAMPT and CD38 compete to determine, those articles provide the complementary mechanistic context.
The CD38 story also situates NMN research within a broader landscape of NAD+ biology research that includes CD38 inhibition as a separate but complementary strategy. Compounds studied as CD38 inhibitors — including apigenin, luteolin, and quercetin, which have been examined in cell culture and animal models for their effects on CD38 activity and NAD+ levels — represent a research approach that addresses the degradation side rather than the production side. These two approaches — NMN supplementation and CD38 modulation — are not mutually exclusive; the research literature has examined their potential combination in animal models with results that reflect the additive logic of addressing both sides of the NAD+ deficit simultaneously. This research is ongoing, and what is described here reflects the current state of a literature that continues to develop. All studies were conducted independently and did not involve any specific Codeage product.
NAMPT down. CD38 up.
Sirtuins attenuated.
NF-κB less restrained.
CD38 rising further.
The NAD+ decline of aging
is not a simple depletion.
It is a self-reinforcing loop.
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