CD38: The Drain That Grows With Age

If you only learn one enzyme name in this field, make it this one. CD38 is the single biggest reason NAD+ falls as you age, and it is the reason adding more precursor is a less complete answer than it sounds.

The one-paragraph version

CD38 is an enzyme that destroys NAD+ as part of calcium signalling. Its levels rise substantially with age, driven by the chronic low-grade inflammation that accumulates as senescent cells build up. The result is a drain that grows over time — and because CD38 can also degrade NMN, a rising drain may reduce the efficiency of the very supplements people take to counter it.

What CD38 does

CD38 is an NAD+ glycohydrolase. It breaks NAD+ down and, in doing so, produces signalling molecules — cyclic ADP-ribose and related compounds — that cells use to mobilise calcium. Calcium signalling is fundamental to immune function among much else, so this is useful work, not malfunction.

The problem is the exchange rate. CD38 is an inefficient consumer in the sense that matters here: it destroys a large number of NAD+ molecules relative to the signalling product it generates. As a drain on the NAD+ pool it is substantial.

Why it rises with age

CD38 expression increases markedly in multiple tissues across the lifespan, and the driver appears to be inflammation rather than age as such.

The sequence runs roughly like this. Senescent cells — cells that have stopped dividing but have not been cleared — accumulate in tissue over time. They secrete a mix of inflammatory signals. Immune cells responding to that environment express more CD38. More CD38 means faster NAD+ destruction.

That makes falling NAD+ substantially a downstream consequence of inflammation, rather than an independent process of running out. It is a useful reframe, because it explains why the decline accelerates rather than tracking age linearly.

The experiment that made the case. Animals lacking CD38 entirely maintain dramatically higher NAD+ levels than normal ones, and are substantially protected from the age-related decline. That is about as direct a demonstration as you get that CD38 is a dominant consumer rather than one of many.

The part that matters for your supplement

Here is the practical sting, and it rarely appears on product pages.

CD38 does not only degrade NAD+. It can also act on NMN. So in an older person with elevated CD38, some proportion of a supplemental NMN dose may be degraded by the same enzyme responsible for the deficit being corrected.

This does not mean precursor supplementation is pointless — it reliably raises measured NAD+, elevated CD38 notwithstanding. What it means is that the picture is not simply "top up the tank." You are adding to a system with an enlarged outflow, and some of what you add goes out through it.

Why there is no CD38 product to buy

The obvious thought once you understand this is: rather than adding substrate, why not reduce the drain? It is a reasonable thought and it is an active research direction. It is not a shelf product.

Several compounds inhibit CD38 in laboratory settings, including some flavonoids found in foods and sold as supplements. Two cautions apply before anyone draws a conclusion from that.

Laboratory inhibition is not the same as inhibition in a person. Concentrations achievable in a dish routinely exceed anything oral dosing produces in tissue.
CD38 does real work. It is central to calcium signalling and immune function. Suppressing it broadly is not self-evidently desirable, and the research-stage inhibitors are being studied with that in mind.

If you see a supplement marketed as a CD38 inhibitor, the mechanism it is invoking is real. The claim that the product achieves it at the dose you are taking is the part that has not been shown.

What to take from this

The honest summary is that CD38 explains why NAD+ decline happens and simultaneously limits how much a precursor can do about it. Precursors help you keep pace with a faster drain. They do not reduce the drain.

That is still a reasonable thing to do, and it is a more modest claim than the one usually made for it. The thing that would genuinely change this picture is a safe way to address the inflammation driving CD38 upward in the first place — which is a much larger problem than a supplement category.

Last reviewed: 3 October 2026.