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Shelly Albaum

Editor, Science of NAD

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Hope v. Hype: A Response to Dr. Stanfield

There's more evidence to be considered when comparing NMN and other NAD precursors

Here at Science of NAD we like Dr. Brad Stanfield. We consider ourselves kindred spirits in trying to sort competing claims in the context of trying to help real people make rational decisions. And we especially agree with him that the right way to do this is by looking hard at the actual science.

In a new video that Dr. Stanfield released yesterday (January 28) called "NMN Supplements: Hope v. Hype?, and which he characterizes as his most comprehensive analysis of NMN yet, he builds toward three conclusions about NMN.

  1. It's not obvious that NAD levels decline with age

  2. NR and NMN are not directly absorbed by tissues

  3. NR and NMN don't raise NAD levels in tissues

That's pretty hard-hitting, but I think the weight of the science is against all three points. Let's look at each one.

To be clear, Dr. Stanfield is not saying that Vitamin B3 isn't effective, or that NAD replenishment isn't a good idea. He's all for it. He is only arguing that Niacin is good enough, and there is inadequate evidence that the newer precursors nicotinamide riboside (NR) and nicotinamide mononucleotide (NMN) provide a significant additional benefit.

Let's see.

1. Do NAD Levels Decline with Age? Actually, they do.

To show that "It's not obvious that NAD levels decline with age," Dr. Stanfield references a study of muscle NAD from last year that found NAD levels indeed declined in older humans, but not if the older human exercised regularly.

That finding is cold comfort to people who do not exercise enough, or at least do not exercise as much as the people in the study did. The finding is also limited to muscle NAD, and not NAD levels in every other kind of tissue, like skin, eye, heart, liver, kidney, and nerves, which is problematic for reasons we will see shortly.

But most important, this study runs contrary to a much larger body of studies that shows that NAD levels do decline with age, in all kinds of organisms, including organisms that get plenty of exercise.

It has been well established that NAD+ levels decline with age...

Aging is also known to be linked to a decline in NAD+ levels in muscle tissue in lower organisms and mammals...

There is also evidence of decreased NAD+ in aged human tissues. Specifically, in vivo NAD+ assays have been used to demonstrate that intracellular NAD+ declines with age in the human brain. Also, NAD+ in post-pubescent males and females negatively correlates with age. Together, these data suggest that there may be a universal age-dependent decrease of cellular NAD+ across species... [emphasis added]

...Total NAD+ levels were once considered extremely stable. Recently, however, it has become clear that a steady decline in total NAD+ levels over time is a natural part of life for all species, from yeast to humans. This decline, along with the decreased activity of NAD+ signaling proteins, is believed to be one of the major reasons organisms, including humans, age.

And there are even more studies than that showing that NAD levels decline with age.

I do think it is an important idea that exercise might partially or entirely resist the natural reduction in NAD levels, at least in some people. But it's too big a jump to conclude from that that NAD levels do not decline with age -- and certainly not based on a single study looking just at muscle NAD levels. And most important, that conclusion is against the weight of the science.

2. Does NMN Get Absorbed Directly? Yes.

We must clarify here that throughout the video Dr. Stanfield refers to NMN and NR interchangeably, but usually calling them "NMN." That's an okay shortcut, because he knows, just as we know, that NMN must be converted to NR before it can enter most or all cells, and so the two precursors are likely to behave in similar ways.

But the point he is trying to make is that whether the supplement starts as NR or gets converted to NR, the transformations don't end there. Before it reaches the cell, says Dr. Stanfield, it gets further converted to Niacin (NA) and/or Nicotinamide (NAM), and therefore you might as well just use those less-expensive forms.

The studies Dr. Stanfield is relying on, however, do not justify his very strong conclusion. And indeed, if you look at additional studies, a different picture comes into focus.

Dr. Stanfield compares two studies -- one of which states that oral NR or NMN will be almost fully transformed to NA by the gut microbiome, the other states that NR or NMN will be almost entirely converted to NAM and the effect will be "indistinguishable from oral NAM."

It's obvious that these studies can't both be right -- your oral NMN supplement can't be both entirely converted to NAM and entirely converted to NA. So something is wrong here.

What's actually going on, and you can see here all the studies that show it, although I will also summarize them below, is that (1) some of the NR or NMN gets converted to NA or NAR in the gut microbiome, (2) some of the NR or NMN gets converted to NAM in the liver and in circulation, and (3) some of the NR or NMN reaches cells as NR and replenishes NAD through the NR kinase pathway. This was summarized by a November 2021 study published in Nature Communications:

In the present study, we show that orally administered NR increases NAD+ levels by two different mechanisms. In the early phase, NR is directly absorbed from the small intestine and contributes to NAD+ generation through the NR salvage pathway...while in the late phase, NR was hydrolyzed to nicotinamide (NAM) by bone marrow...and was further metabolized by the gut microbiota to nicotinic acid, contributing to generate NAD+ through the Preiss–Handler pathway. [emphasis added]

In other words, oral NR delivers to cells all three precursors: NR, NAM, and NA. The study goes on to suggest that the situation turns out to be even more complicated than that. In the first phase, NR gets absorbed as NR. What's left gets converted to NAM and then NA in the gut. But some of the NA in the gut then gets converted to NAR (nicotinic acid riboside), and this team speculates that the NAR may then get converted back to NR again to replenish NAD through the Brenner pathway if the Preiss-Handler pathway is unavailable to replenish NAD!

These results indicated that NR-derived NA is mainly converted to NAD+ through the Preiss–Handler pathway, but the base-exchange reaction from NAR to NR may function as a backup route when the Preiss–Handler pathway is impaired.

So the important question isn't whether NR gets delivered directly to tissues, but how much, and is it enough? The reason we know that some NR gets delivered to tissues as NR is because when the the NAM pathway was blocked NR still replenished NAD, which would not have happened if all the NR were getting converted to NAM:

the oral administration of NR improved functional deficits and restored muscle mass in muscle-specific Nampt knockout mice.

This team compared NR with NAM and NA head to head, and saw a difference:

NR boosts hepatic NAD+ and NAD+- consuming activities to a greater degree than Nam or NA...These data establish that oral NR has clearly different hepatic pharmacokinetics than oral Nam.

Indeed, one of the isotope teams pointed out that their data actually precluded the possibility that NR works exclusively as NAM:

These data exclude the possibility that all three vitamins are utilized through the Preiss-Handler pathway in liver or that oral NR is used exclusively as Nam. [emphasis added]

So Dr. Stanfield is not quite right when he says that NR and NMN do not reach the tissues intact, especially when he cites two studies that contradict each other.

What is correct is that NMN and NR activate at least three metabolic pathways, including the NR pathway, and there is an open question about what percent of the NR gets through as NR.

The answer to that open question, I think we will learn, is that it is some small percent, like, 5% or 10%, gets through. And that, I believe, is why NR and NMN supplements are recommended at dosages of 300 or 600 mg, and tested at 1,000 or 2,000 mg.

Five percent getting through isn't a crazy guess, either. That's what one of the isotope labeling study measured:

After two hours, 54% of the NAD+ and 32% of the NADP+ contained at least one heavy atom while 5% of the NAD+ and 6% of the NADP+ incorporated both heavy atoms. (emphasis added)

Dr. Stanfield says at the end of his video that he only takes 50mg of Niacin, and he thinks that's enough. But if he took 500mg of NR and only 10% got through as NR, then he would be delivering just as much NR to his tissues as Niacin. Plus, he would be getting whatever additional benefits come from also delivering NAM, NA, and NAR to whatever tissues might have those pathways metabolically upregulated.

3. Does NMN Get Delivered to Tissues? Probably.

Dr. Stanfield correctly and reasonably notes that most of the studies measuring NAD levels are measuring blood NAD, and just assume that if the NAD is circulating then it is also getting distributed. It is perfectly reasonable to challenge that condition, and to ask whether tissue NAD levels are in fact increased?

He then cites a study showing that NAD supplements did NOT do anything for muscle NAD, and correctly notes that numerous studies have shown disappointing effects of NAD supplements in muscle tissue.

So far so good.

The fact that NAD precursors don't seem to do much for muscles isn't going to be enough to support the conclusion that NMN supplements do not increase NAD levels in tissues more generally though.

Muscles turn out to be a difficult case. In 2016, researchers reported that they had completely deleted NAMPT in mouse skeletal muscles, which prevented the muscles from synthesizing NAD from NAM. You would think that dropping NAD+ levels by 85% would have catastrophic results, and they did:

Knockout mice exhibited a dramatic 85% decline in intramuscular NAD content, accompanied by fiber degeneration and progressive loss of both muscle strength and treadmill endurance. Administration of the NAD precursor nicotinamide riboside rapidly ameliorated functional deficits and restored muscle mass despite having only a modest effect on the intramuscular NAD pool.

And yet, it took a long time for the problems to show up. The researchers were surprised that after 3 months the knockout mice still fatigued at the same rate as controls, and the weights of their muscles used for running were the same as controls, too, and the expected signs of muscle weakness were not obvious.

Surprisingly, mNKO mice aged 3 months appeared to fatigue at the same rate as littermate controls. Nampt-deficient mice were noted to be slightly smaller at this young adult age, apparently due to an overall reduction in lean mass. However, this difference was not reflected in the weights of the major hindlimb muscles used for running...A clinical hallmark of muscle weakness is decreased density and altered architecture of the bones to which muscles routinely apply tensile force. Upon examination of the tibial cortex by microcomputerized tomography (microCT), we found no significant decrease in bone mineral density or in the number or spacing of trabecular plates from 3-month-old mice

This isn't to say that starving muscles of NAD has no effect. At three months, differences were difficult to observe. By seven months, the knockout mice were in a lot of trouble.

But what this does counsel is that muscles have back-up systems for preserving performance, even if you almost entirely wipe out their NAD system. And three or seven months for a mouse would be years for a human. The human studies involving muscle performance have been run at weeks or months, not years, so we may not be able to measure the effect in muscles. And we certainly would not, because the differences in NAD levels that we are trying to observe in humans are not nearly as catastrophic as what we saw in the mice -- we are not genetically deleting NAMPT in human muscles.

So based on this, it would be reasonable to conclude that muscle tissue is not going to be a very good way to evaluate whether NAD precursors are effectively increasing NAD levels in most tissues. Indeed, before all of the disappointing human studies involving muscle tissue we saw a run of disappointing rodent studies involving muscle tissue:

Intravenous nicotinamide riboside elevates mouse skeletal muscle NAD + without impacting respiratory capacity or insulin sensitivity

Nicotinamide riboside supplementation confers marginal metabolic benefits in obese mice without remodeling the muscle acetyl-proteome

Nutraceutical and pharmaceutical cocktails did not preserve diaphragm muscle function or reduce muscle damage in D2-mdx mice

[Although in full disclosure, there have occasionally been some positive results with muscle tissue in both humans and mice.]

So if you're really interested in determining whether NR or NMN raise NAD levels in tissues, muscle tissue may be an outlier in a number of respects, and we might do better looking at results in liver, kidney, heart, and neurons. There, the studies seem to show that NAD levels are increased, and the increase is effective.

Heart. In this human heart study, NR increased blood NAD, and the effect decreased inflammation and increased mitochondrial respiration.

This randomized, placebo-controlled trial of HF patients demonstrates that NR approximately doubles whole blood NAD+ levels, and that relative increases in blood NAD+ levels correlate with both increased PBMC mitochondrial respiratory function and decreased inflammatory gene (NLRP3) expression. Therefore, larger studies of patients with HFrEF that are sufficiently powered to assess the effect of NR on clinically relevant, surrogate endpoints as well as inflammation are warranted.

What we don't know, which is Dr. Stanfield's point, is whether NAD levels in the heart muscle itself were increased. But it is a little much to ask that the researchers biopsy the hearts of the human participants, and NR had already been shown to be effective in rodent models. So it's not reasonable to assume that the NAD is not being replenished in the tissue, even though we don't know definitely. Certainly the researchers involved in the study don't assume that; on the contrary -- they are recommending larger studies.

Liver. There is much less doubt that NR is delivered to the liver. And the effects of NR supplementation on the liver are easier to measure, and they don't look like merely raising blood NAD.

Liver biopsies of humans with alcohol-related liver disease showed significantly reduced NAD levels:

We used human liver samples to quantify the NAD+ metabolome in ArLD...These data constitute the first report of the NAD+ metabolome in human liver biopsies. Using samples from explanted and resected liver tissue, we show that the concentration of NAD+ and precursor molecules are significantly lower in ArLD compared to other liver diseases and NL tissue. The concentration of NAD+ is inversely correlated with disease activity...

A human clinical trial involving a cocktail that included NR showed significant effects, which suggests that the NAD precursor was in fact being delivered to the tissues:

In animal experiments and human kinetic study, we found that administration of combined metabolic activators (CMAs) promotes the oxidation of fat, attenuates the resulting oxidative stress, activates mitochondria, and eventually removes excess fat from the liver. Here, we tested the safety and efficacy of CMA in NAFLD patients in a placebo-controlled 10-week study. We found that CMA significantly decreased hepatic steatosis and levels of aspartate aminotransferase, alanine aminotransferase, uric acid, and creatinine, whereas found no differences on these variables in the placebo group after adjustment for weight loss.

Indeed, the isotope labeling study in rodents actually found that the liver preferred to get NR over NAM:

In terms of elevation of mouse liver NAD+, we discovered that NR is more orally bioavailable than Nam, which is more orally bioavailable than NA...This validates NR as the favoured NAD+ precursor vitamin for increasing NAD+ and NAD+-consuming activities in liver.

Kidney. Again, the preclinical studies show that NAD levels are depressed in kidney injury, and that NAD precursors are effective. In a clinical test of 41 patients, researchers found that the NAD precursor NAM appeared to be effectively delivered to the kidney:

To test the preclinical finding that exogenous Nam can ameliorate AKI when QPRT levels are suppressed, we designed a small, placebo-controlled, blinded and randomized trial of peri-operative oral Nam (1 g or 3 g daily) in patients undergoing on-pump cardiac surgery, which causes renal ischaemia...Evidence of conversion of Nam to NAD+ was ascertained...

The fact that NAM was delivered to kidneys does not mean that NR would be delivered to kidneys. But we already showed that NR can be bioavailable at higher doses, and there is absolutely no reason to believe that NR would be less able to enter kidney cells than NAM. The clinical tests with NR and kidney disease haven't been done yet (except to show safety), so we don't know, but the science here is not giving the kind of pause that we saw with the muscle studies.

Neurons. Perhaps the strongest argument that NAD precursors get effectively delivered to tissues can be found in the Phase 2 Alzheimer's study published this week which shows that a cocktail including NR led to enhanced cogniative functions and improved clinical parameters associated with phenomics, metabolomics, proteomics, and imaging analysis. Those are good signs that NR was successfully delivered to the neurons in the brain.

To test the effect of CMA in AD patients, we performed a double-blinded, randomised, placebo-controlled phase 2 study and screened 89 adults diagnosed with AD...Our results suggested that oral administration of CMA for 84 days has a considerable effect on cognitive function in AD patients...

Overall. It is still possible in the face of all these studies to disbelieve that NR is being delivered to tissues. Maybe the heart benefits will turn out not to be related to NAD replenishment. Maybe the kidney will prove receptive to NAM but not to NR. Maybe the results of the liver and neuron studies will turn out to have been entirely caused by an ingredient in the cocktail other than the NAD booster.

But such a position would be one of extreme skepticism, and not founded on any clear theory of why these tissues would not be susceptible to having their NAD levels raised by NR. Certainly that kind of skepticism cannot be justified by, and generalized from, skeletal muscle studies.


I know as well as anyone how difficult it is to describe the science of NAD in terms that non-specialists can understand. You have to omit a lot of detail, and you have to overlook a lot of research, because there are so many studies, often with conflicting, noisy results.

But in his latest effort, I think Dr. Stanfield has overlooked too much, and extrapolated a little too far.

Specifically, I think it IS obvious that NAD levels decline with age, even though some people through exercise may be able to resist that trend to some extent.

I also think that the evidence shows us that NR DOES get absorbed directly, and in quantities sufficient to have a significant impact, even if as much as 90% or 95% of the orally administered NR will get degraded to NAM, NA, and NAR.

Finally, I think that the weight of the evidence suggests that NR DOES get delivered to tissues and raises NAD in those tissues, even though the results in muscle are not great. But there is reason to believe that skeletal muscle is an outlier compared to other tissues, and it would not be proper to generalize only from skeletal muscle tissues to all other tissues.

Clinical trials that show positive effects from NAD boosting with NAM include the skin cancer trial (which Dr. Stanfield mentions) and a glaucoma trial (which he does not mention). These early results are telling us that NAD boosting works. There is reason to believe that NR and NMN will work even better. The clinical trials that might prove this haven't been done, but the pre-clinical results do not warrant such deep skepticism.

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