What is NAD+ (nicotinamide adenine dinucleotide), and what do human studies show?

What are the key points in brief?

NAD+ is a coenzyme for more than 500 enzymes¹.
Precursors such as NR and NMN probably raise NAD+ levels in blood^2,3.
According to a 2026 systematic review, clinical benefit for anti-aging and wellness has not been established².
Whether NAD+ in blood falls with age is disputed^4,5,6.

What is NAD+?

NAD+ stands for nicotinamide adenine dinucleotide. It is a coenzyme for more than 500 enzymes. It plays a central role in energy production, metabolism, signaling inside cells and the repair of genetic material¹.

NAD+ occurs mainly inside cells. In blood plasma, the level is about 500 times lower than in certain white blood cells¹.

What do human studies show?

A systematic review from 2026 evaluated 113 studies, 33 of them in humans². In humans, oral precursors such as NR and NMN consistently raised NAD-related levels in blood or cells. Over weeks to months they were mostly well tolerated².

Effects on function, metabolism, blood vessels and other health markers were inconsistent and often showed no difference². The authors regard clinical efficacy for anti-aging and wellness as not established².

A critical review of 25 human studies on NR paints a similar picture. It sees few clinically relevant effects and a tendency in the literature to overstate results⁷.

That the blood level rises is shown by a randomized trial in healthy overweight adults. Depending on the dose level, NR raised whole-blood NAD+ by 22, 51 and 142 percent³. A higher blood level alone does not show a health benefit².

Does NAD+ in blood fall with age?

It is often said that NAD+ falls with age. For human blood, the data are inconsistent⁶.

In seven independent cohorts, whole-blood NAD+ stayed remarkably stable across age and under lifestyle interventions⁶.
In 299 healthy people aged 18 to 70, the blood level did not change with age⁵.
In a group of 1,518 adults, the level fell with age in men only, according to the authors⁴.

The question therefore appears to be open.

How does a medical academy assess anti-aging products?

In a 2026 report, the French National Academy of Medicine writes that, on current knowledge, neither measuring biological age nor using so-called geroprotectors can be recommended for clinical practice. It advises giving priority to lifestyle measures: exercise, nutrition, sleep and social contact⁸.

What does this mean for you?

If you think of NAD+ because of persistent fatigue, you can first have the common causes checked by a doctor⁹. The page What can lie behind fatigue before considering NAD+ infusions? shows what that involves. The five-step preparation helps with the appointment.

If you are pregnant or breastfeeding, take medication regularly or have or had cancer, the page Does NAD+ have side effects? has notes on this.

Which sources does this page rely on?

Saqr AHA et al. (2024). Optimized protocol for quantification of extracellular nicotinamide adenine dinucleotide: evaluating clinical parameters and pre-analytical factors for translational research. Frontiers in Medicine. DOI: 10.3389/fmed.2023.1278641
The study examines which steps in blood collection and sample handling affect the measurement of NAD+ in plasma. It is a pre-analytical study on human blood samples. Long or fast-braking centrifugation lowered the measured values, which suggests these steps should be standardised.
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. DOI: 10.1016/j.arr.2026.103057
The review asks what studies on NAD+ products show for ageing and well-being. It systematically assesses 113 studies, 33 of them in humans. NR and NMN may raise NAD levels and appear well tolerated, while a clinical benefit remains open, and suitable trials of NAD+ infusions were lacking.
Conze DB, Brenner C, Kruger CL (2019). Safety and Metabolism of Long-term Administration of […] (Nicotinamide Riboside Chloride) in a Randomized, Double-Blind, Placebo-controlled Clinical Trial of Healthy Overweight Adults. Scientific Reports. DOI: 10.1038/s41598-019-46120-z
The trial examined how healthy adults with overweight tolerate nicotinamide riboside over eight weeks and how their blood values change. It was randomised, double-blind and placebo-controlled. Blood NAD+ may rise depending on the amount taken, and adverse events were about as frequent as with placebo.
Yang F et al. (2022). Association of Human Whole Blood NAD+ Contents With Aging. Frontiers in Endocrinology. DOI: 10.3389/fendo.2022.829658
The study asks whether whole-blood NAD+ declines with age. It is a cross-sectional study of 1,518 adults from a community in China. A decline with age appeared only in men, which may point to differences between the sexes.
Euro L et al. (2026). Dynamics of blood NAD and glutathione in health, disease, aging, and under NAD+-booster treatment. Redox Biology. DOI: 10.1016/j.redox.2026.104343
The study developed a standardised measurement of NAD and glutathione from a single whole-blood sample. It is a methods and cohort study with 299 healthy adults. Levels appeared stable with age and rose to a different extent from person to person under NAD+ boosters.
Trętowicz MM et al. (2026). Human whole-blood NAD+ levels do not vary with age or lifestyle interventions. Nature Metabolism. DOI: 10.1038/s42255-026-01537-5
The study tests whether whole-blood NAD+ changes with age or with lifestyle interventions. It used a validated mass spectrometry method in seven independent cohorts. Levels stayed largely stable and changed with nicotinamide riboside, which may call into question blood NAD+ as a marker of ageing.
Damgaard MV, Treebak JT (2023). What is really known about the effects of nicotinamide riboside supplementation in humans. Science Advances. DOI: 10.1126/sciadv.adi4862
The review checks what is actually established about nicotinamide riboside in humans. It critically assesses 25 published human studies. Clinically relevant effects were rare, and reported effects may often be overstated in the literature.
Académie nationale de médecine (2026). Approche clinique de la longévité en bonne santé. Report of 13 January 2026, in French. www.academie-medecine.fr/wp-content/uploads/2026/01/RAPPORT-Approche-clinique-de-la-longevite-en-bonne-sante-APRES-VOTE.pdf
The report asks what medical practice can do today to support a long and healthy life. It is a position paper of the French National Academy of Medicine. Based on current knowledge, it does not recommend measuring biological age or using anti-ageing agents in practice and gives priority to lifestyle.
Maisel P, Baum E, Donner-Banzhoff N (2021). Fatigue as the Chief Complaint. Deutsches Ärzteblatt International. DOI: 10.3238/arztebl.m2021.0192
The paper summarises how family doctors may assess persistent fatigue. It is a review based on a literature search. Its authors suggest that history, physical examination and simple laboratory tests may usually be enough, with further tests mainly when other findings point that way.