NAD+ Explained: What It Is, Why Levels Decline, and What the Science Actually Says

By Charles Fowler, Founder of Portal to Wellness · Last reviewed: June 2026 · ~6 min read

A quick note on why I wrote this: I’m not a doctor — I’m a founder who spends my days deep in the wellness and research-compound space, reading the literature and vetting what we carry. I built this guide to give you the same straight, no-hype picture I’d want before looking into anything myself. Everything below is educational only and reflects published research, not medical advice. Please talk to a qualified healthcare professional before changing your health routine.

NAD+ has moved from biochemistry textbooks to wellness headlines in just a few years. If you’ve seen it referenced alongside terms like “longevity,” “cellular energy,” or “healthy aging” and wondered what’s substance versus hype, this guide walks through what NAD+ is, what it does in the body, why levels change as we age, and where the research currently stands — including its limits.

What NAD+ actually is

NAD+ stands for nicotinamide adenine dinucleotide. It’s a coenzyme found in every living cell, and it’s one of the most abundant and essential molecules in human metabolism. A coenzyme is a helper molecule: it partners with enzymes to make biochemical reactions happen. Without NAD+, hundreds of those reactions simply stall.

NAD+ exists in two interchangeable forms — an oxidized form (NAD+) and a reduced form (NADH). Cells constantly shuttle between the two as they move electrons around during metabolism. That ability to accept and donate electrons is what makes NAD+ so central to turning the food we eat into usable energy.

The two big jobs of NAD+

Researchers generally group NAD+’s roles into two categories.

The first is energy metabolism. NAD+ is a key player in the chemical pathways — glycolysis, the citric acid cycle, and oxidative phosphorylation — that convert carbohydrates, fats, and proteins into adenosine triphosphate (ATP), the cell’s energy currency. In this role NAD+ is recycled rather than consumed: it picks up electrons, becomes NADH, drops them off, and returns to NAD+ to do it again.

The second role is as a substrate that gets used up by certain “NAD+-consuming” enzymes. Two families get the most research attention:

• Sirtuins are a group of enzymes (SIRT1 through SIRT7 in humans) that depend on NAD+ to function. They’re involved in regulating gene activity, mitochondrial function, and cellular stress responses. Because sirtuins need NAD+ to work, NAD+ availability acts as a kind of metabolic signal for these pathways.
• PARPs (poly-ADP-ribose polymerases) use NAD+ during DNA repair. When cells experience DNA damage, PARP activity rises and draws down NAD+ as part of the repair process.

This second category matters for the aging conversation, because these enzymes don’t just borrow NAD+ — they spend it. That sets up a supply-and-demand dynamic that shifts over a lifetime.

Why NAD+ levels decline with age

A consistent finding across animal and human studies is that intracellular NAD+ concentrations tend to fall with age in many tissues. Reported figures vary by tissue and measurement method, but the overall direction — a meaningful decline from younger to older adulthood — is well documented in the scientific literature.

Several mechanisms appear to contribute. On the demand side, accumulated DNA damage and chronic low-grade inflammation can increase the activity of NAD+-consuming enzymes like PARPs, pulling more NAD+ out of circulation. On the supply side, the activity of NAMPT — the rate-limiting enzyme in the body’s main NAD+ recycling route — appears to decline with age, so cells regenerate NAD+ less efficiently. Researchers often describe this as a self-reinforcing loop: lower NAD+ can mean less sirtuin activity, which is associated with further DNA damage and inflammation, which in turn consumes still more NAD+.

It’s worth being precise here. The fact that NAD+ declines with age, and that this decline correlates with various features of aging, does not by itself prove that restoring NAD+ reverses or slows aging in people. Correlation and causation are different questions, and that distinction is where much of the active research is focused.

How the body makes NAD+

The body produces NAD+ through three main routes, and most cells rely heavily on the recycling option.

The salvage pathway is the dominant route in most tissues. It recycles nicotinamide (a form of vitamin B3) back into NAD+, with NAMPT as the key enzyme. The de novo pathway builds NAD+ from scratch starting with the amino acid tryptophan. And the Preiss-Handler pathway uses nicotinic acid (another B3 form) as a starting point.

This is why classic vitamin B3 nutrition matters: niacin and related compounds feed these pathways. Severe B3 deficiency causes pellagra, a serious condition, which underscores how fundamental adequate NAD+ precursors are to baseline health.

NAD+ precursors: NR, NMN, and others

Because NAD+ itself is a large molecule that isn’t efficiently taken up by cells when consumed directly, much of the supplement and research interest centers on precursors — smaller building blocks that cells can convert into NAD+. The two most discussed are:

• Nicotinamide riboside (NR) — a form of vitamin B3 that cells can convert to nicotinamide mononucleotide (NMN) and then to NAD+.
• Nicotinamide mononucleotide (NMN) — one step further along the pathway; cells convert it to NAD+ in a single enzymatic step.

Other compounds in the conversation include nicotinamide, nicotinic acid, and intravenous or topical NAD+ formulations. Each is handled differently by the body, and how efficiently any of them raises NAD+ in specific tissues is an area of ongoing study. Recent research has even examined whether some precursors are absorbed and processed partly through the gut and liver before reaching the rest of the body — a reminder that the picture is still being refined. (For a deeper comparison, see NMN vs. NR: how these two NAD+ precursors compare.)

What the research does and doesn’t show

Here’s an honest summary of the current state. Studies in cells and animals have generated genuine interest: in various models, raising NAD+ has been associated with improvements in measures of mitochondrial function, metabolism, and certain markers of healthy aging. Human studies to date show that some precursors can raise blood NAD+ levels and have generally been well tolerated in the doses studied. (For more on tolerability, see Is NAD+ safe? What the research shows.)

What’s less settled is whether those increases translate into meaningful, durable health or longevity benefits in humans. As a 2023 review in Endocrine Reviews put it, NAD+ biology holds real promise but still has “many unknowns” — many human studies so far are relatively small, short, or focused on specific markers rather than long-term outcomes. Larger and longer trials are underway, and the field is moving quickly.

The responsible takeaway: NAD+ is real, important, and biologically fascinating, and the research is promising enough to follow closely — but it is not a proven anti-aging cure, and claims that outrun the evidence should be treated with skepticism.

The bottom line

NAD+ is a foundational coenzyme that powers energy metabolism and fuels key enzymes involved in DNA repair and cellular regulation. Its levels tend to decline with age, which has made it a major focus of longevity science and a driver of interest in precursors like NR and NMN. The science is active and encouraging, but still maturing. If you’re exploring this category, the smartest approach is to stay curious, read primary research, be wary of overblown claims, and consult a qualified healthcare provider about what’s right for you.

Related reading

• NMN vs. NR: How These Two NAD+ Precursors Compare
• Is NAD+ Safe? What the Research Shows — and What’s Still Unknown

Frequently asked questions

Is NAD+ the same as NMN or NR?
No. NAD+ is the active coenzyme your cells use. NMN and NR are precursors — smaller molecules the body can convert into NAD+ through its salvage pathway.

Does NAD+ really decline with age?
The scientific literature consistently reports that NAD+ levels fall with age across many tissues. What’s still being studied is exactly how much that decline drives aging versus simply accompanying it.

Can you get NAD+ from food?
Your body makes NAD+ using building blocks from your diet, especially vitamin B3 (niacin) and the amino acid tryptophan, found in foods like meat, fish, dairy, and whole grains. Severe B3 deficiency is what causes pellagra.

Is taking an NAD+ precursor proven to make you live longer?
Not in humans, no. Animal and early human studies are encouraging, but large, long-term human trials are still needed before anyone can claim a proven longevity benefit. Be cautious of marketing that says otherwise.

About the author

Charles Fowler is the founder and researcher behind Portal to Wellness. After a career in management consulting, he has spent several years exploring health optimization — the last three focused on the research-compound space — sourcing products, vetting suppliers, and reading the primary literature daily. He isn’t a physician or a scientist, and he’ll tell you so. His goal is to translate what the research actually says into plain English so customers can make informed decisions. When the evidence is uncertain, he says so.

How we research our content

Our Learn Center articles are written from peer-reviewed scientific literature and reputable references, and we date every article so you know how current it is. We don’t make therapeutic, dosing, or “cure” claims, and we flag clearly where the science is still unsettled. If we get something wrong or new evidence emerges, we update the article. Have a correction or a question? Reach out — we read every message.

References

1. Bhasin S, Seals D, Migaud M, Musi N, Baur JA. Nicotinamide Adenine Dinucleotide in Aging Biology: Potential Applications and Many Unknowns. Endocrine Reviews. 2023;44(6):1047–1073. https://pubmed.ncbi.nlm.nih.gov/37364580/
2. Yang Y, et al. An Updated Review on the Mechanisms, Pre-Clinical and Clinical Comparisons of Nicotinamide Mononucleotide (NMN) and Nicotinamide Riboside (NR). Food Frontiers. 2025. https://iadns.onlinelibrary.wiley.com/doi/10.1002/fft2.511
3. The NAD+/PARP1/SIRT1 Axis in Aging. (Review.) https://pubmed.ncbi.nlm.nih.gov/28537485/
4. NAD+ Metabolism in Cardiac Health, Aging, and Disease. Circulation. https://www.ahajournals.org/doi/10.1161/CIRCULATIONAHA.121.056589
5. The Plasma NAD+ Metabolome Is Dysregulated in “Normal” Aging. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6482912/

📚 Part of our Energy & Longevity research guide.

This article is for informational and educational purposes only. It is not medical advice and does not describe the diagnosis, treatment, cure, or prevention of any disease. Portal to Wellness products are sold for research purposes only. Always consult a qualified healthcare professional before making changes to your health routine.