CD38 is an enzyme found on the surface of many cells — especially immune cells — that actively breaks down NAD+ and its precursor NMN. It's often described as the body's main NAD+-degrading enzyme, meaning that as CD38 activity rises, it directly reduces the NAD+ available in surrounding tissue.15 This matters because NAD+ levels aren't just about how much you produce — they're a balance between synthesis and consumption, and CD38 sits on the consumption side alongside other NAD+-using enzymes like sirtuins and PARPs.
The connection to aging is well documented in research from the Mayo Clinic and others: CD38 expression increases in white adipose tissue and the liver with age, driven largely by an accumulation of CD38-positive immune cells, particularly a type of pro-inflammatory macrophage.16 In other words, aging brings more low-grade inflammation, inflammation recruits more CD38-expressing immune cells, and those cells then consume more of the body's available NAD+ — a self-reinforcing cycle, since age-related cellular senescence itself drives the buildup of these inflammatory cells.17
Researchers have also shown this process is at least partly reversible at the cellular level: clearing senescent cells or blocking their inflammatory signals reduces CD38 and partially restores NAD+, and directly inhibiting CD38's enzymatic activity increases NAD+ through an NMN-dependent mechanism.16
Why this matters if you're considering precursors: taking NMN or NR adds more raw material to the NAD+ pathway, but if your CD38 activity is elevated — which tends to happen with age and chronic inflammation — some of that added precursor may get broken down faster than in a younger or less inflamed body. This is why researchers are now investigating CD38 inhibitors (found naturally in compounds like quercetin) as a complementary strategy alongside precursor supplementation, rather than treating precursor dosing as the whole picture.
CD38 also plays a role in normal immune function, so if you're on immune-modulating medication or have an autoimmune condition, discuss any CD38-targeting supplement strategy with your doctor first.
"The Role of CD38 in the Pathogenesis of Cardiorenal Metabolic Disease and Aging" — https://www.ncbi.nlm.nih.gov/pmc/articles/PMC9954496/
Chini et al., "CD38 ecto-enzyme in immune cells is induced during aging and regulates NAD+ and NMN levels," Nature Metabolism (2020) — https://www.nature.com/articles/s42255-020-00298-z
"Aging-related inflammation driven by cellular senescence enhances NAD consumption via activation of CD38+ pro-inflammatory macrophages" — https://www.biorxiv.org/content/10.1101/609438.full.pdf
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