Why Does NAD+ Decline With Age?

What the evidence actually shows

A number of human and animal studies have reported lower NAD+ or NAD+-related metabolite measurements in older subjects compared with younger ones.[1] This is genuine and widely discussed in the scientific literature. What is not settled is a single, universal figure for how much NAD+ declines with age. You may encounter claims online stating something like “NAD+ declines 50% by age 50” — treat any such figure with skepticism unless the source names a specific study, the tissue measured, the population studied, and the measurement method used. Reported magnitudes vary considerably across the literature, and no single number applies to every tissue, every study, or every person.

Why magnitude varies so much

Several factors explain why studies report different numbers. NAD+ has been measured in different tissues — blood, skin, muscle, liver — which do not necessarily age in the same way or at the same rate. Studies also use different measurement methods (for example, mass spectrometry versus enzymatic assays), which do not always agree closely with one another. Many studies are cross-sectional (comparing different people at one point in time) rather than longitudinal (following the same people over years), and sample sizes are often small. All of these are reasons a responsible summary of this topic avoids collapsing the evidence into one headline percentage.

Proposed mechanisms behind the decline

Researchers have proposed several biological explanations for age-related differences in NAD+, largely based on animal and cell studies. These include increased activity of NAD+-consuming enzymes such as CD38, which has been linked to NAD+ decline in animal models of aging,[2] along with chronic low-grade inflammation and potential changes in NAMPT activity, the rate-limiting enzyme of the NAD+ salvage pathway.[3] These mechanisms are best described as mechanistic hypotheses supported by preclinical evidence — plausible and actively researched, but not fully confirmed as causal explanations for NAD+ changes in humans.

What this does — and doesn’t — mean for supplementation

The observed association between age and lower NAD+ measurements is part of the scientific rationale researchers cite for studying NAD+ precursors like NMN in human trials, which this hub reviews in a dedicated evidence section. But an observed association is not the same thing as proof that any specific intervention reverses the decline or produces a defined health benefit in a given person. This page describes what has been observed and proposed as explanation; it does not claim that taking NMN reverses aging or restores youthful NAD+ levels.