Some NR gets through as NR, and it is enough to make a difference.
Bacteria in the gut, and enzymes in the bloodstream and in the liver convert some NR to NAM, NA, or NAR, all of which can replenish intracellular NAD, but in different places and under different circumstances. The better question isn't how much NR gets converted to other precursors, but how much NR does not get converted and makes it through as NR?
TL;DR: It is clear that orally ingested NR gets broken down into a number of different precursors that replenish NAD through different pathways, which is a good thing. It is equally clear that some of the NR gets through to cells as NR and replenishes NAD through the Brenner pathway, which is an even better thing. Exactly how much NR gets through to which cells and how quickly is not clear, which is probably why the recommended dose of NR is about 20x higher than the RDA for Niacin, and why even higher doses of NR like 1,000 mg or more are used in most clinical research -- to make sure that enough gets through to have a measurable effect.
Does NR Get Broken Down in the Digestive Tract?
In one simulation, NR was subjected to the kind of chemical environments found in the stomach and the intestines. It was discovered that most of the NR exposed to acids and bases remained intact even after 24 hours:
The 24-hour simulation of the GI ["only"] 71.69 (±1.92)% of NRCl remained at the end of the experiment.
Twenty-four hours to degrade is a long time, especially because it turns out that NAD precursors are absorbed and distributed to tissues in minutes, not hours:
Shockingly, NA and NAM are found in circulation at rather low concentrations....tracer analyses have revealed how NR and NMN disappear from the bloodstream and engage NAD+ synthesis within minutes after intravenous delivery
That means that degradation of NR by stomach or intestinal liquids is minimal before the NR gets distributed.
That rapid distribution and assimilation is one reason that it is difficult to know precisely how where the NR goes and how it gets there. NAD precursors are very quickly taken up by cells everywhere in the body, and we can't dissect humans to find out where it all went.
Absorption versus Degradation
It would seem that if the NR doesn't get quickly degraded by acid and basic conditions in the GI tract, and is instead rapidly distributed to and absorbed by tissues throughout the body, then we could be confident that the NR was getting through intact.
It is not as simple as that, though, because the NR can get degraded in three other ways: First, the gut microbiome itself can consume the NR. Second, the liver can transform the NR into NAM. Third, circulating enzymes can also degrade NR to NAM. So it's difficult to know exactly what is going on.
Still, we have some clues.
One study measured what happened to NR in rodent blood plasma and found that some degraded to NAM in minutes, and most degraded after one hour:
NR incubation in [rodent] plasma leads to relatively quick degradation, with ∼10% of NR degraded after 10 min and ∼66% degraded after 1 h
So if distribution begins within minutes, but complete degradation requires more than an hour, does that mean that much of the NR gets distributed before it gets degraded, rather than degraded before it gets distributed?
Both Absorption AND Degradation
Another study answered this question and said by the time an hour has passed all the NR has already been absorbed:
NR quickly disappears from the bloodstream, and is almost undetectable 1 h after intraperitoneal administration...
If degradation of NR takes more than an hour, then there should still be plenty of intact circulating NR after an hour, but instead the study found that there is almost none, which strongly suggests that much of the NR arrived at its destination intact. Indeed, that is exactly what this same group of scientists went on to conclude:
...Nevertheless, evidence indicates that NR is directly used as a NAD+ precursor during this time.
Another team looking at the problem concluded that NR absorption occurs in two phases. First, some NR is quickly absorbed as NR and replenishes NAD through the Brenner pathway. Later, additional NR gets degraded to NAM and NA in different locations and replenishes NAD through two additional pathways:
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]
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, 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.
This research is telling us that oral supplementation with nicotinamide riboside not only uses the expected Brenner/NR pathway to replenish NAD, but also generates a variety of different NAD precursors all capable of replenishing NR through various pathways -- that would seem like a very good outcome.
Or Is It Only Degradation?
Nonetheless, a number of studies have drawn strongly negative conclusions about the bioavailability of NR, asserting that it is mostly converted to NAM. For example, this mouse study using isotope tracing found that:
Readily detectable concentrations of intact NR were observed in the blood following i.v., but not after oral, administration, indicating nearly complete first-pass metabolism...which likely result in [NR and NMN] having systemic effects similar to or indistinguishable from oral NAM.
Other teams summarizing these results characterized it like this:
The majority of orally delivered NR has been shown to be rapidly cleaved to NAM before entering circulation and peripheral tissues
NR is largely transformed to NAM and NA in the gut and circulation, which would curtail a large part of the potential advantages of using NR vs. classical NAD+ precursors.
If these studies are right, then there isn't much reason to take NR because its conversion to NAM is "nearly complete" and it would likely be "indistinguishable from oral NAM" and "a large part of the potential advantages of using NR" would be curtailed.
More Likely, Some NR Gets Through As NR
But there is good reason to doubt the breadth of these conclusions.
First, they are measuring NR levels in the blood stream (in mice), not in each of the various tissues that is the destination. So the fact that the NR vanishes does not by itself mean that it got degraded. It could have been incorporated into NAD, which is actually what is desired.
Second, the evidence of degradation is incomplete.
We know that NR ends up as NAD. The question is whether it gets broken in half first. The reason that matters is because the unique property of NR is that it doesn't need NAMPT to add a ribose, because it already has one. Bypassing NAMPT means that NR can be used to make NAD even when NAMPT is scarce and the NAM pathway is downregulated:
[nicotinamide riboside] can be incorporated into NAD without breaking the nicotinamide-ribose linkage, allowing them to bypass the gating NAMPT reaction, which is subject to feedback inhibition by NAD
So the researchers wanted to measure whether the entire NR molecule gets incorporated into NAD whole, or whether it gets broken in half first and must be re-built, requiring the help of NAMPT.
The way they did that is to attach two radioactive isotopes to the NR, one to each end of the NR molecule and then measure if the resulting intracellular NAD had both tags present, or whether only one tag was present. If both tags were present, that would suggest that the entire NR molecule got turned into NAD in one step. But if only one tag was present, that would likely mean that the NR got broken in half first and only one half was turned into NAD.
We employed versions of NR and NMN that are isotopically labeled on both the nicotinamide and ribose moieties. This allowed us to distinguish NAD made directly from NR or NMN (M+2) versus NAD made from NAM-derived NR or NMN (M+1)
They found some double-tagged NAD, but also a lot of single-tagged NAD. From that, the researchers concluded that the NR mostly does not get incorporated as NR, but instead must first be getting broken into NAM, and then the NAM gets matched with some new non-tagged ribose molecule. Because the double-tagged NR was getting absorbed by the liver and not as much elsewhere, even after they quadrupled the dose, the researchers concluded that probably the liver was converting most of it to NAM:
Examination of tissue NAD labeling indicated some direct assimilation of oral NR and NMN into liver NAD, based on M+2 labeling, which made up a minority of the signal, but was nonetheless readily detectable. The active formation of liver NAD from NR and NMN is consistent with both compounds being subject to substantial hepatic first-pass metabolism. In contrast, extrahepatic tissues displayed minimal M+2 NAD, suggesting that orally delivered NR and NMN are converted into NAM before reaching the systemic circulation. Intravenous injection of NR or NMN, on the other hand, resulted in substantial M+2 NAD in both the liver and kidney. To explore whether higher oral doses of NR were also cleared by the liver, we examined the dynamics of tissue NAD labeling from 200 mg/kg i.v. or oral doubly labeled NR. Similar to the 50 mg/kg dose, the 200 mg/kg dose resulted in M+2 NAD in the liver but not the muscle or kidney after oral administration (Figure S7A). Thus, even very high doses of oral NR do not reach tissues intact.
That could be so, that "even very high doses of oral NR do not reach tissues intact," but alternative explanations are possible.
One alternative is that the the NR could be forming NAD quickly, but then the different parts of the NR molecule get split and swapped during the normal flux of NAD turnover in the cell -- although the researchers do not think so. They think they got their measurements before the flux could have interfered, at least with respect to many tissues:
NAD turnover rates varied dramatically across tissues. In several tissues, NAD turnover was substantially faster than in any of the cultured cell lines that we examined. On the flip side, in the skeletal muscle, it was substantially slower...Turnover of M+2 NAD within a tissue could in principle produce M+1 NAD after direct NR or NMN assimilation, but our independent measurements of tissue NAD turnover revealed that these fluxes are too slow to account for the lack of M+2 tissue NAD.
There could also be problems with the experimental conditions -- the labeling or measuring might not have worked as well as expected. Or maybe they did not wait long enough for the oral NR to clear the gut. But we aren't in a position to critique the experimental conditions. As far as we know the experiment was executed perfectly and worked as intended.
Indeed, an earlier research team also using isotope labeling came up with somewhat similar results: lots of NAD metabolites included at least one tagged atom, but only about 5% of the NAD itself ended up double-tagged:
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.
Interestingly, and consistent with the theory that oral NR supplementation generates a cascade of NAD precursors that support multiple metabolic pathways, the research team found that NR supplementation increased the levels of many different NAD metabolites, with the peculiar exception of NAM, presumably because the NAM was immediately incorporated into other molecules:
With the exception of Nam, the levels of which were unaffected by NR, NR produced or tended to produce dose-dependent elevation of the entire NAD+ metabolome
In the liver, at least, there seems to be no question, because both isotope studies agree that NR makes it to the liver intact:
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. The...ability of NR to elevate ADPR [a measure of sirtuin and other NAD+-consuming activities] exceeded that of Nam by ∼3-fold. This validates NR as the favoured NAD+ precursor vitamin for increasing NAD+ and NAD+-consuming activities in liver.
But despite the impressive evidence of the effect of NR supplementation on liver conditions like NAFLD, most people interested in NAD precursors are hoping to achieve a broader effect than that.
There remain two sources of uncertainty about the low rates for direct incorporation of NR observed in the isotope labeling experiments. First, the tracing study did not actually keep track of where all the NR ended up before concluding that oral NR does not reach tissues intact, so they do not actually know:
Study Limitations: Although we can infer NAD turnover in vivo based on its labeling kinetics from isotopic NAM, we did not dissect the consuming enzymes involved. In addition, we did not quantify the terminal excretion of the NAM ring from the body. Establishing mass balance for whole-body nicotinamide production and consumption is an important future objective.
Second, other studies have come up with results inconsistent with this study's conclusion. It is a bold thing to predict that oral NR will have effects "indistinguishable from oral NAM." But that's easy to test, and it's been tested a number of times, and the two supplements actually do not behave the same. They are distinguishable. So it's fair to believe something is not quite right with this study or with its conclusion, even if we don't know exactly what.
For example, the same team that certified that NR is largely transformed to NAM and NA in the gut and circulation conceded in the same paper that there is nonetheless evidence that NR gets through intact:
After IP injection part of the action of NR on NAD+ levels was blunted in NRK1 KO mice, indicating that NR is still reaching tissues in significant amounts to increase NAD+ levels.
These scientists concluded that even if NR can get to some tissues, others are more difficult targets, like muscle tissue, which probably relies more on NAM. And yet, they also note that even muscle tissue does absorb some NR as NR after oral administration.
These results provide demonstration that direct utilization of NR by the muscle can occur after oral administration, albeit at very marginal levels.
So the broad general conclusions we read do acknowledge a fair amount of uncertainty.
Other studies also find that oral NR does get delivered to cells intact, and/or that its effects are in fact distinguishable from oral NAM:
For example, this study observed that when the Brenner pathway was blocked, NAD replenishment by NR was reduced. That would not happen if the NR were all getting converted to NAM, because the NAM pathways would be equally available in both instances:
the increase in NAD+ levels observed in multiple tissues after intraperitoneal administration of NR was largely compromised in the NRK1-deficient mice.
The study also noted the inverse, which was that when 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:
Nam is the only vitamin precursor of NAD+ that produces elevated hepatic Nam 15 min after oral administration and, as shown in, NA is the only precursor that produces elevated NA 15 min after oral administration. 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]
Another research team fed mice a high-fat diet and blocked the Brenner pathway for NAD metabolism in the liver found that the mice's livers were damaged in a number of ways by not being able to use NR, including impaired mitochondrial function, glucose intolerance, and liver damage. But what was interesting, was that when they tried to make up for the lack of NR by supplementing with NAM, it didn't work. The mice needed some source of NR to protect their livers from the effects of the high fat diet:
This work provides evidence that (1) endogenous NR metabolism is required to sustain hepatic NAD+ levels in situations of metabolic damage and lipotoxicity and, (2) the inability to use NR as a NAD+ precursor leads to mitochondrial dysfunction and amplifies the detrimental effects of HFD...There is no compensation through NAM to generate NMN and maintain hepatic NAD+ in this situation...The administration of NAM to NRK1 LKO mice neither increased NMN and NAD+ levels nor recovered their metabolic defects. Instead, the supplemented NAM seemed to be largely diverted towards clearance methylation/oxidation paths...
Another research team modified the muscles of mice to block the conversion of NAM to NAD. They then gave the mice NR in their drinking water, and the muscles recovered more than when they gave the mice NAM:
A single week of NR supplementation was sufficient to dramatically restore exercise capacity in mNKO mice...our results indicate that NR is more effective than NAM for reversing mNKO phenotypes
This team created a mouse model of malnutrition, which caused liver damage. NAM and NR supplementation both helped the mice by increasing Sirt1, but only NR, not NAM, raised NAD levels:
In our study, NAM treatment did not significantly restore NAD+ levels whereas NR did...
In a mouse model of aging, one research team noted very different results from NR versus NA. They did not compare NAM, but the result is a reminder that the various precursors act differently, and at least counters the suggestion that NR is entirely degraded to NA by the gut microbiome:
whereas NA specifically delayed the onset of imbalance, NR significantly extended lifespan, beyond the maximum age reached by any of the other drugs tested
Another team noted that oral NR raises liver NAD more effectively than NAM or NA, which precludes the possibility NR is entirely converted to either one or the other in the gut or the liver.
Recent animal studies have indicated that equimolar oral NR is superior to NA and NAM in elevating NAD+ content in the liver
This team found that NR provided heart protection, but NAM did not:
Boosting NAD+ content using the NAD+ precursor NR but not NAM increased peak INa in heterologous expression systems and RNCMs through both acetylation-independent and acetylation-dependent mechanisms.
-- Journal of Molecular and Cellular Cardiology, March 21, 2020
A different type of refutation of the claim that NR gets entirely converted to NAM is based on genetics. Experiments have shown that the NR pathways to synthesize NAD get upregulated under some situations in which the body urgently needs NAD, for example in cases of heart failure:
NAD+ homeostasis is altered at an early stage in the heart of SRFHKO mice with a striking induction of the NMRK2 pathway for NAD+ synthesis...The gene expression pattern (Nt5e and Nmrk2 up with Pnp and Nampt down) suggested that cardiac tissue is attempting to mobilize and utilize NR as an NAD+ precursor while not increasing NAM usage...Consistent with this hypothesis, myocardial NAD levels were preserved by NR but not by NAM...We show that Nmrk2 gene can be activated in response to NAMPT inhibition and activation of the energy stress sensor AMPK.
If NR were too fragile to be present in circulation, there would be no reason for the failing heart to be specifically looking for NR, because it would never find any. Therefore, NR must not always be totally degraded and must be bioavailable.
But the details of all this are quite controversial. While one team says that NR is getting converted to NAM, and that the NAM is replenishing NAD through the salvage pathway, another team finds that the NAM itself is being converted to NAR by the gut microbiome and is instead replenishing NAD through the Preiss-Handler pathway:
our findings demonstrate that gut microbiota-mediated deamidation of NAM starts in the small intestine, continues in the colon, and is responsible for the bulk of NAD synthesis in the colon, liver, and kidney
That team goes on to point out that the same gut microbiota that convert NAM to NAR also convert NR to NAR, and therefore the NAD replenishing effect of NR is also largely diverted to the Preiss-Handler pathway. Moreover, NR does NOT get converted to NAM if the gut microbiome is clean:
In addition to NAM, we further found that gut microbiota is essential for the NAD boosting effect of orally supplemented NR...However, the conversion of NR to NAM was reduced 3-fold in the germ-free mice, suggesting that conversion of NR to NAM is largely carried out by the gut microbiota.
Resolving the Conflicting Findings
If you find this confusing and confounding, I do too. We just spent paragraphs trying to make sense of one group of scientists' declaration that oral NR is almostly completely metabolized to NAM in the liver in the first pass. Then we find another group that says, no, actually, the NR is almost completely transformed to NAR in the gut. They can't both be right. One of the leading NAD researchers commented on this development:
A fascinating twist has been recently added to the NR story by suggesting that orally administered NR, as well as NMN or NAM, will be almost fully transformed to NA by the gut microbiome, which then accounts for the effects on NAD+ synthesis in mouse tissues...The clinical translation of pre-clinically successful molecules is most often a bumpy road, and this is being no exception for NR. Nevertheless, most data indicates that NR plays unique and differentiated roles compared to other NAD+ precursors. [emphases added]
I think this is fun for the researchers because it makes the story they are telling interesting and engaging, and reminds them how much remains to be learned.
But for us it is at once a confounding and liberating insight. It means that when you take NR, some will get through, some will get converted to NAM, some will get converted to NA and NAR, and all three biosynthetic pathways will be engaged, but in proportions that depend on what is happening with your gut microbiome. And everyone's gut microbiome is different. So we can expect different results in different people in different times in unpredictable ways.
We might be tempted to resent the gut microbiome for making the analysis infinitely complex. But the microbiome may actually be doing us a favor when it comes to NR. And we might be smart to do it a favor, too, by letting it participate in the NAD replenishment.
This study found that NR supplementation actually improved the health of mice by changing the composition of the gut microbiome:
Our findings show that NR dietary supplementation protects against alcohol-induced depression-like behaviours, possibly by altering the composition of the gut microbiota
Another group found the same thing, that NR prevented alcohol-induced liver disease by restructuring the gut microbiome:
NR intervention changed the microbial community structure at the phylum, family and genus levels, and the species abundances returned to a level similar to these of the normal control group...Therefore, NR supplementation has the potential to prevent ALD, and its mechanism may be related to regulating lipid metabolism disorders and the gut microflora-bile acid axis.
And so did a third team recently, concluding that a group of metabolic activators including nicotinamide riboside had the "capacity to recover beneficial bacteria levels and prevent harmful bacterial growth" in the gut, which, in turn, helped resist non-alcoholic fatty liver disease.
Yet another direct benefit to NR supplementation in the gut is to gut stem cells. Another team found that NR rejuvenated gut stem cells, which allowed the gut to repair damage:
The treatment with the NAD(+) precursor nicotinamide riboside (NR) rejuvenates Intestinal Stem Cells (ISCs) from aged mice and reverses an impaired ability to repair gut damage.
So in the end we are left to wonder if the NR actually gets degraded, if so how much, where, how quickly, and is that a good thing or a bad thing? Instead of resenting the partial degradation of NR, we might celebrate it.
It may turn out that the reason we take such high doses of NR -- most studies run at more than 50x the RDA for Niacin -- is because we need enough NR for the gut, plus enough to get converted to other precursors, and still have some left over to act as NR. If we just injected NR into our veins directly, we might get a greater dose of NR, but then we would lose the various benefits of NR that are only mediated by the gut. So the wisdom of those trying to bypass the GI tract by injections, IV drips, nasal sprays, and sublingual applications in order to maximize the delivery of pure NR to tissues may be open to question.
Conclusion
The bioavailability of NAD precursors is an exceedingly complex topic that defies general statements like, "It mostly gets converted to NAM in circulation," or "It mostly gets converted to NA in the gut" or "It is better to inject NR intravenously to avoid the gut." The smarter view may be that human biology knows lots of good things to do with NR, but the process cannot (and should not) be precisely managed.
It is clear that orally ingested NR gets broken down into a number of different precursors that replenish NAD through different pathways, which is a good thing. It is equally clear that some of the NR gets through to cells as NR and replenishes NAD through the Brenner pathway, which is an even better thing. Exactly how much NR gets through to which tissues and how quickly is not clear, which is probably why the recommended dose of NR is about 20x higher than the RDA for Niacin, and why even higher doses of NR like 1,000 mg or more are used in most clinical research -- to make sure that enough gets through to have a measurable effect.