You've Probably Seen the NAD+ Aging Graph
It usually looks something like this:
Young age → High NAD+
↓
Middle age → Lower NAD+
↓
Older age → Very low NAD+
It's simple.
It's memorable.
And it makes an intuitive case for trying to increase NAD+ as we age.
But there's a problem:
Human biology doesn't necessarily follow one universal downward line.
NAD+ is unquestionably important to cellular metabolism and signaling.
Research in animals has also produced substantial evidence that NAD+ metabolism changes with aging.
But when scientists ask a more specific question —
Does NAD+ actually decline consistently across human blood, muscle, brain, liver, skin and other tissues as we get older?
— the evidence becomes much less straightforward.
And newer human research is making that distinction increasingly important.
First: What Is NAD+?
NAD+ stands for nicotinamide adenine dinucleotide.
It's a coenzyme found throughout the body.
NAD+ participates in fundamental metabolic reactions that allow cells to process nutrients and generate energy.
In simplified terms:
Food
↓
Cellular metabolism
↓
NAD+ / NADH reactions
↓
ATP production
But NAD+ isn't simply involved in energy metabolism.
It is also consumed by enzymes involved in processes such as:
DNA-damage responses
Cellular signaling
Metabolic regulation
Stress responses
These include enzymes such as sirtuins, PARPs and CD38.
So there are good biological reasons for researchers to ask whether changes in NAD+ metabolism contribute to aging.
The problem begins when that complex research question becomes simplified into:
"Everyone loses NAD+ as they get older."
Where Did the NAD+ Decline Idea Come From?
A substantial part of the NAD+-and-aging story comes from preclinical research.
Studies in organisms including yeast, worms, rodents and other experimental models have reported age-associated alterations in NAD+ metabolism.
In aged animals, researchers have also observed lower NAD+ concentrations in certain tissues.
These findings helped generate an important hypothesis:
Could declining NAD+ availability contribute to age-associated physiological changes?
Researchers then investigated ways to increase NAD+ availability using precursors including NMN and nicotinamide riboside (NR).
In experimental animals, NAD+ augmentation has produced interesting effects across metabolic, mitochondrial and other outcomes.
But there's a crucial distinction:
Animal evidence
is not automatically
human evidence.
And even within one animal — or one human — different tissues may behave differently.
NAD+ Isn't One Single Pool
This is one of the most important concepts for understanding NAD+ research.
We often talk about "NAD+ levels" as though your entire body has one NAD+ tank.
It doesn't.
NAD+ metabolism is compartmentalized.
Different tissues can have different:
- NAD+ concentrations
- Metabolic demands
- NAD+ synthesis pathways
- NAD+-consuming enzymes
- Responses to aging
- Responses to NAD+ precursors
Even within a cell, NAD+ biology involves distinct compartments such as the:
Nucleus
Cytosol
Mitochondria
That means measuring NAD+ in one place doesn't necessarily tell us what's happening everywhere else.
A blood measurement, for example, cannot automatically tell us the NAD+ status of:
Brain tissue
Skeletal muscle
Liver
Skin
Heart
or a specific population of cells within those tissues.
What Does Human Research Actually Show?
This is where the picture becomes much more nuanced.
A 2025 review published in Nature Metabolism specifically examined clinical evidence surrounding NAD+ decline and NAD+ precursor supplementation in human aging.
Its conclusion deserves attention:
Evidence for an age-related decline in human NAD+ has been consistently observed only in a limited number of studies.
The authors also emphasized that published data describing NAD+ dynamics across human tissues remain sparse.
In other words:
There isn't yet a comprehensive map showing NAD+ steadily declining across every major human tissue as we age.
The authors argued that extrapolating results from rodents to humans isn't straightforward and called for more tissue-specific human research.
PMID: 41083806
Read → NAD+ Precursor Supplementation in Human Ageing: Clinical Evidence and Challenges
Then a 2026 Study Challenged the Blood-NAD+ Story
In 2026, researchers published another important piece of the puzzle.
Instead of assuming NAD+ declines with age, they directly examined whole-blood NAD+ across seven independent human cohorts.
They used a validated analytical method designed to account for real-world measurement variability.
What did they find?
Whole-blood NAD+ remained remarkably stable with age.
The researchers also examined lifestyle interventions and again found whole-blood NAD+ remained relatively stable.
However, NAD+ did respond to nicotinamide riboside supplementation, demonstrating that their analytical system could detect expected changes.
Their conclusion challenged the usefulness of whole-blood NAD+ as a straightforward biomarker of aging.
PMID: 42135539
Read → Human Whole-Blood NAD+ Levels Do Not Vary With Age or Lifestyle Interventions
Does That Mean NAD+ Doesn't Decline With Age?
No.
And that's an equally important distinction.
The 2026 study measured whole blood.
It did not prove that NAD+ remains unchanged in:
- Skeletal muscle
- Brain
- Liver
- Skin
- Heart
- Adipose tissue
- Every individual cell type
A finding in whole blood cannot automatically be generalized to every tissue.
So replacing:
"NAD+ declines everywhere with age"
with:
"NAD+ never declines with age"
would simply replace one oversimplification with another.
A more accurate statement is:
Evidence suggests NAD+ metabolism changes with aging, but the direction and magnitude of those changes may depend on the tissue, cell type, biological compartment and measurement method. Human evidence remains incomplete.
That's less catchy.
But it's much closer to the current science.
Why Can Different Tissues Produce Different Results?
Different tissues have very different jobs.
A neuron doesn't have the same metabolic demands as a skin cell.
A skeletal-muscle cell doesn't function like a liver cell.
And blood contains multiple populations of circulating cells.
Those differences matter because NAD+ is continuously:
Produced
↓
Used
↓
Recycled
↓
Transported between metabolic pathways and cellular compartments
Aging can potentially influence several parts of this system.
For example, researchers investigate whether aging alters:
NAD+ synthesis
or
NAD+ consumption
or
the enzymes controlling NAD+ metabolism
or
the composition of the tissue being measured.
Two tissues could therefore respond differently even within the same person.
Measuring NAD+ Is Harder Than It Sounds
Another issue is technical.
You might imagine NAD+ measurement as:
Take blood → measure NAD+ → get answer.
Research is more complicated.
Results can be affected by:
How the sample is collected
How quickly it is processed
How it is stored
Which tissue is analyzed
Which cells are present
Which analytical technique is used
Whether NAD+ or related metabolites are being measured
NAD metabolites can also be chemically unstable during sample handling.
That means differences between studies aren't necessarily caused only by biological differences between participants.
Some can potentially arise from differences in study design and analytical methodology.
A major challenge identified by researchers is therefore developing standardized and validated methods capable of accurately measuring NAD+ across specific tissues, cell types and even subcellular compartments.
Blood NAD+ Is Not the Same as Muscle NAD+
This distinction becomes especially important when discussing supplements.
Imagine a clinical trial reports:
Blood NAD+ increased after supplementation.
That's evidence of biochemical target engagement.
It tells researchers that the intervention affected NAD-related biology in the compartment being measured.
But it doesn't automatically mean:
Muscle NAD+ increased by the same amount.
It doesn't automatically mean:
Brain NAD+ increased.
And it doesn't automatically mean:
Physical function improved.
Those are separate research questions.
This is why well-designed human trials go beyond measuring NAD-related biomarkers.
Researchers also examine endpoints such as:
Walking speed
Muscle strength
Exercise capacity
Insulin sensitivity
Sleep
Metabolic outcomes
Physical function
The pathway researchers actually need to establish is:
NAD+ precursor
↓
NAD-related biology changes
↓
Target tissue changes
↓
Physiology changes
↓
Function changes
↓
Meaningful health outcome
Each arrow requires evidence.
Why Simple NAD+ Decline Graphics Can Be Misleading
A graph showing one smooth downward line from age 20 to age 80 suggests several things simultaneously:
NAD+ declines continuously.
The decline happens at roughly the same rate.
It occurs throughout the body.
Every person experiences a similar pattern.
A single measurement represents systemic NAD+ status.
Current human evidence doesn't justify assuming all of those things.
A more scientifically realistic picture might look less like:
AGE ↑
NAD+ ↓ ↓ ↓ ↓
and more like:
That's harder to fit into an advertisement.
But it's a much better representation of the research question.
Human Evidence and Animal Evidence Answer Different Questions
Animal research is extremely valuable.
It allows scientists to collect tissue samples that would be difficult or unethical to obtain repeatedly from healthy humans.
Researchers can directly examine organs including:
Brain
Liver
Muscle
Heart
Adipose tissue
They can also tightly control diet, genetics, environment and experimental interventions.
That's one reason our mechanistic understanding of NAD+ aging biology is much richer in animals.
But humans differ from laboratory animals in:
Lifespan
Metabolism
Genetics
Diet
Environment
Disease history
Medication exposure
and countless other variables.
The 2025 Nature Metabolism review specifically cautioned that extrapolating rodent NAD+ findings directly to humans is not straightforward.
The two evidence streams should therefore work together.
Animal studies help explain mechanisms.
Human studies tell us whether those mechanisms translate.
What About NMN and Other NAD+ Precursors?
None of this means research into NMN becomes irrelevant.
Quite the opposite.
NMN is an intermediate in NAD+ biosynthesis:
Researchers can administer NMN and investigate whether it changes NAD-related metabolites and physiological outcomes.
Human trials have shown that oral NMN can alter NAD-related biomarkers under specific study conditions.
A broader 2026 systematic review similarly found that oral NMN and NR generally demonstrated biochemical target engagement in human studies.
But effects on functional, metabolic, vascular and other healthspan-related outcomes were much more heterogeneous and were often endpoint-specific or null.
PMID: 41655607
That distinction matters.
The research question isn't simply:
"Can we increase an NAD-related biomarker?"
It's:
If NAD+ metabolism changes in a particular tissue, does changing that biology produce a meaningful health benefit?
That is the much harder question.
What Do We Actually Know?
Fairly well established:
NAD+ is essential to human cellular metabolism and signaling.
Aging affects NAD+ biology in experimental models and appears to influence NAD+ metabolism in tissue-specific ways.
NAD+ precursors such as NMN and NR can alter NAD-related biomarkers in human studies.
Still uncertain:
Whether NAD+ consistently declines across all human tissues with age.
Which tissues experience the most biologically meaningful changes.
How NAD+ changes within specific cell types and cellular compartments.
Whether blood NAD+ accurately reflects NAD+ status elsewhere in the body.
Whether increasing NAD+ biomarkers translates into meaningful long-term healthy-aging outcomes.
These uncertainties don't weaken NAD+ research.
They define the questions researchers still need to answer.
A Better Way to Think About NAD+ and Aging
Instead of:
That model is more complicated.
But aging biology is complicated.
The Bigger Lesson: Don't Confuse a Diagram With Human Biology
Scientific diagrams are useful because they simplify complex ideas.
But simplification becomes a problem when the diagram starts being treated as proof.
A downward NAD+ curve can illustrate a hypothesis supported by parts of the aging literature.
It shouldn't automatically be interpreted as a literal map of what happens in every human tissue, every year, in every person.
The latest research gives us a more interesting question than:
"How fast does NAD+ decline?"
It asks:
Where does NAD+ metabolism change with age, in which cells and tissues, why does it change, and does modifying those changes improve human health?
Those are the questions that matter.
And they're exactly why NAD+ remains such an active area of longevity research.
Continue Exploring NAD+ and NMN Research
Want to understand where NMN fits into this picture?
Continue to the Aeternum NAD+ & NMN Research Hub to explore:
How NAD+ works
↓
How NMN enters the NAD+ pathway
↓
What human NMN trials actually measure
↓
Which outcomes have changed
↓
Which results remain uncertain
↓
How to evaluate NMN formulation and product-quality information
Continue to the NAD+ & NMN Research Hub →
Next article → NAD+ and Aging: Why Researchers Study NMN
Start with the biology. Then explore the clinical evidence.
Research Featured in This Article
Vinten KT, et al. (2025).
NAD+ precursor supplementation in human ageing: clinical evidence and challenges.
Nature Metabolism. PMID: 41083806.
This review concluded that consistent evidence of an age-related decline in human NAD+ exists only in a limited number of studies and emphasized that human tissue data remain sparse.
PubMed: PMID 41083806
Trętowicz MM, et al. (2026).
Human whole-blood NAD+ levels do not vary with age or lifestyle interventions.
Nature Metabolism. PMID: 42135539.
Researchers analyzed NAD+ across seven independent human cohorts and reported that whole-blood NAD+ remained remarkably stable with age, challenging its use as a general biomarker of aging.
PubMed: PMID 42135539
Peluso AA, et al. (2022).
Age-Dependent Decline of NAD+—Universal Truth or Confounded Consensus?
Ageing Research Reviews. PMID: 35010977.
This earlier critical review found that evidence supporting a universal age-related NAD+ decline was limited, particularly in humans, and often came from individual tissues or cell types.
PubMed: PMID 35010977
Gallagher C, Emmanuel OO. (2026).
NAD+ supplementation for anti-aging and wellness: A PRISMA-guided systematic review of preclinical and clinical evidence.
Ageing Research Reviews. PMID: 41655607.
The review found clear biochemical activity from oral NAD+ precursors in human studies, while clinical effects on healthspan-related outcomes remained heterogeneous and inconclusive.
PubMed: PMID 41655607