NR, Nicotinamide, and NAD -- The Emerging Scientific Puzzle
A new study published April 19, 2018, in Cell Metabolism ("the Isotope study" or "The Rabinowitz Study") says that orally ingested nicotinamide riboside (NR) and nicotinamide mononucleotide (NMN) get almost entirely converted to Nicotinamide (NAM) in the liver.
They used isotopes to track the molecules, and compared orally ingested NR and NMN with intravenously delivered NR and NMN.
As a result, the study concludes, (1) "NR and NMN are effectively delivered to tissues by i.v., but not oral, administration," and (2) "Nearly complete first-pass metabolism of oral NR and NMN [likely results] in these compounds having systemic effects similar to or indistinguishable from oral NAM."
Well that's great news for NR customers who can get 50 grams of Nicotinamide (NAM) on Amazon for $28, whereas the same amount of Tru Niagen on Amazon would cost over $240.
Who wouldn't want to save 90% and get "indistinguishable systemic effects?"
But not so fast. Several other recent studies complicate the matter, and I am not sure how to reconcile them. At the end, I will ask the biochemists in the audience to kindly weigh in.
First, the finding that NR gets converted to NAM isn't news. A study published in Cell Metabolism in August 2016 ("the Mouse Muscle study") found that NR helped mice whose muscles had been starved of NAD, but "oral NR dosing increased circulating NAM ∼40-fold," and "the majority of the orally administered NR that reaches the muscle appears to enter in the form of liberated NAM..." Indeed, the study found that NAD metabolism in muscle tissue was an "isolated system" untouched by circulating NMN.
Second, last month's University of Colorado study in Nature Communications ("the second ChromaDex study") says that orally ingested NR "effectively stimulates NAD metabolism," raising NAD in blood by 60% compared to a placebo. This result was consistent with the 2016 pilot study, also published in Nature Communications ("the first ChromaDex study"), which found that "human blood NAD+ can rise as much as 2.7-fold with a single oral dose of NR."
Moreover, Elysium Health's study from November 2017, published in Aging and Mechanisms of Disease ("the Elysium Study"), is in accord, finding that NAD+ in whole blood increased by approximately 40% or 90% compared to placebo and baseline, depending on whether the subjects took a single or double dose of Basis, which consists of 250mg of nicotinamide riboside and 50mg of pterostilbene.
Now, we would be tempted to harmonize these studies by supposing that the NR turned to NAM, as indicated in the Isotope and Mouse Muscle studies, and that the NAM then raised NAD levels, as per the ChromaDex and Elysium studies -- it could be so, since neither the ChromaDex nor the Elysium studies checked to see whether the NR got metabolized into NAM before working its magic to increase blood NAD.
The Plot Thickens But this is where the plot thickens. ANOTHER study in Cell Metabolism, ("the 23-author Nicotinamide study") published on March 6, 2018, and improbably signed by Baur, Sinclair, AND Brenner (+ 20 others), says that Nicotinamide supplementation by itself reduces NAD salvage and therefore "does not produce a net boost in tissue NAD levels."
So we are led to understand that NR gets reduced to NAM, NAM doesn't increase NAD, but NR supplementation nonetheless increased NAD in two placebo-controlled double-blind human studies.
What gives? It feels like they can't all be right.
The most obvious difference between the apparently conflicting results appears to be where the NAD+ was measured.
In the Elysium and ChromaDex studies, increased NAD+ was measured in blood, whereas in the Isotope and Mouse Muscle studies the NAD+ was measured in various tissues.
That would at least explain the apparent discrepancies in the data.
But it begs the question of whether measuring NAD increases in the blood makes sense if tissues don't get their NAD from NAD circulating in the blood, but from NAM, which can be supplemented directly without NR or NMN?
It also begs the question of how oral NR supplementation is generating positive effects for mice and men if the NR just gets turned into NAM, which cannot deliver "a net boost in tissue NAD levels" anyway because the NAM shuts down the NAD salvage pathway?
The second ChromaDex study does not clarify things. It found evidence of increased NAD activity, and no increase in NAM:
"NR also elevated levels of nicotinic acid adenine dinucleotide (NAAD) nearly fivefold above the placebo condition...confirming a previous report that NAAD is a highly sensitive and reliable biomarker of increased NAD+ metabolism and a product of NR utilization in humans. NR also elevated the mean concentration of nicotinamide (NaM), but this was not statistically significant..."
Great. So does oral ingestion of NR increase circulating NAM by 40-fold (the mouse muscle study) or by a statistically insignificant amount (the second ChromaDex study)?
UPDATE MARCH 2019: PROPOSED SOLUTION TO THE PARADOX
Chris Masterjohn, on the Peter Attia podcast, explains why NR is superior NAD replenishment method EVEN IF the NR is mostly converted to NAM in the liver on the first pass!
Masterjohn says (edited for clarity and length),
The 2018 Rabinowitz isotope tracer study shows that very little orally consumed NR makes it directly into muscle tissue, and the vast majority is converted to nicotinamide in the liver. Which would make you think that consuming oral nicotinamide directly would be a superior way of replenishing cellular NAD, but it is not.
In a prior study, Rabinowitz ALSO measured a much lower NAD response in muscle from oral NAM than from oral NR.
Here is how the findings can be reconciled.
Nicotinamide is taken up by the cells from the bloodstream, but nicotinamide cannot be stored in the cells, because nicotinamide inhibits sirtuin and PARP enzymes that are performing essential DNA repair functions.
So once the nicotinamide is taken in by the cell, it's a liability, and the cell can't have the nicotinamide hanging around. Instead, the cell makes an immediate decision to turn the nicotinamide into NAD, or to methylate the nicotinamide so that it can be excreted in the urine.
The same dynamic applies in the liver. When the liver encounters nicotinamide, it must immediately convert the nicotinamide to NAD, or methylate it for secretion.
But the liver can only make so much NAD at once, so if the amount of nicotinamide available exceeds what the liver can immediately turn into NAD, then the rest has to be excreted. You are going to see a lot more waste down the detoxification path if you flood the liver with nicotinamide.
By contrast, if you give the liver NR, instead of nicotinamide, NR poses no threat to sirtuins and PARPs. The NR is not a liability if the liver does not immediately convert it to NAD. The NR does not force the liver to make an immediate decision to convert or detoxify.
In fact, NR can ONLY be used to make NAD before it does anything else. So the liver HAS to turn the NR into NAD before the NR can ever be exposed to the methylation-detoxification pathway, and it can convert it now or later.
This matters because the liver is not just making NAD for itself. The liver also carries all of the NAD reserves for the rest of the body. So the liver doesn't only have NAD that is immediately being used in expiration, sirtuins and PARPs. The liver also has a reserve pool of NAD that it holds onto for the specific purpose of a slow release of nicotinamide to the rest of the tissues, which those tissues will take up. And then those tissues will face the immediate decision of whether to detoxify it or to make NAD.
But if the liver can safely hold on to the NR, and have a better ability to make NAD and then release nicotinamide on an as-needed basis, and on a continuous basis, then you have a superior way of ensuring the continuous delivery of nicotinamide optimized to reach the other tissues at a rate that the other tissues can take up and do something useful with.
I think what these studies are telling us is that we don't know yet what is happening. There are observed health benefits in mice and men that can't be explained if the NR is simply metabolized into NAM and offers no systemic advantages over NAM supplementation. NAM supplementation has not been shown to deliver these benefits, and indeed appears to suppress both NAD salvage and SIRT activity.
Instead, I think the following language from the first ChromaDex study captures the situation correctly:
"Because of the abundance of NAD+-dependent processes, the effects of NR and NMN may depend on multiple targets including sirtuins, PARP family members, cADPribose synthetases, NAD+-dependent oxidoreductases and NADPH-dependent ROS detoxification enzymes."
The NR story apparently is not as simple as NR in, NAD up.
It's frustrating to see so much evidence THAT nicotinamide riboside works without a good sense of HOW or WHY. But it looks like enduring that state of uncertainty may be our fate, for at least a few more years, because it's going to take a lot of studies to unravel this.
Coda: Help! I'm just a lawyer. If any biochemists or other experts would like to critique my analysis, I would welcome all the help I can get, and I will happily update this post with additional insights. Feel free to comment below or contact me directly via the web form.
UPDATE 1 (May 6, 2018):
One expert reminds me that there is a likely methodological error in the Isotope study, which is that they stopped measuring tissue NAD derived from NR after 135 minutes, even though there are indications that NAD metabolic activity derived from NR supplementation peaks after 6-8 hours, not 2 hours. Here is a chart from the isotope study (with my annotations):
You can see that NAD was still increasing when the measurement stopped. It would be interesting to see these charts at 2, 4, 6, and 8 hours, not just at 5, 15, 45, and 135 minutes.
Also, there is this curious quote from the Isotope study:
"Irrespective of the route of delivery, the main circulating product of the administered NR or NMN was NAM, which increased by about 20x within 5 min of i.v. NR or NMN; oral NR or NMN administration led to a more modest rise in circulating NAM."
It's hard to know what to make of that. How is it that intravenous NR is simultaneously getting converted to NAM more and faster than oral NR AND simultaneously being more effectively delivered to tissues?
UPDATE 2 (May 6, 2018):
I found additional studies in which orally ingested NR was effective when NAM was not, which challenges the Isotope study's suggestion that NR is no more effective than NAM:
1. NR in food may prevent heart failure (orally ingested NR rescued NAD synthesis when NAM metabolism was suppressed) (Circulation, December 2017)
2. NR replenished NAD in skeletal muscle (orally ingested NR rescued NAD synthesis in skeletal muscle when NAM salvage pathway was blocked) (This is the Mouse Muscle study above, but note that the study specifically finds that "a single week of NR supplementation was sufficient to dramatically restore exercise capacity" in the mice with impaired NAM salvage ability, that "NR is more effective than NAM," even though the study also found that "NR exerts only a subtle influence on the steady-state concentration of NAD in muscles." (Cell, August 2016)
3. NR-Kinase pathway is activated by tissue damage, which suggests that circulating NR must be available in addition to circulating NAM, or else why would the pathway activate (e.g., fish exposed to damaging noises
(Journal of Fish Biology, May 2014)?
So even if the liver is metabolizing some or a lot of NR into NAM, it isn't easy to accept that the liver is metabolizing all NR into NAM, given the direct and indirect evidence that NR is still having an effect different from and/or in addition to NAM, and given that we don't necessarily know where to look for NR, or how to detect the NR in all the places it might be,
UPDATE 3 (May 7, 2018): Another expert volunteers that cell adjust to their environment, so bioavailability of NR may be different with long-term supplementation than what is observed in a one-time test. The ChromaDex and Elysium studies looked at long-term supplementation, and the Isotope study did not. Here's how an expert would say it:
"Cells and tissues adapt to the metabolites they 'see'. Some adaptations are changes in the actions of existing (fixed number) protein molecules; these typically occur in minutes to hours. Others are the result of changes in amounts of those various proteins by new or discontinued synthesis or degradation. These adaptations can take days to weeks or months. The human studies span weeks. The cell-culture and mouse studies span hours to days."
UPDATE 4 (May 7, 2018):
Another industry expert chimes in, cautioning that we not conflate the mouse and human studies, pay attention to dosage differences, and recognize the significant unknowns involved in comparing NAD measurements from different types of cell:
1) Mice, as cute as they are, are not furry little people. They are significantly different from people in many important biological ways, so it is inappropriate to conflate results from the two species. Human studies trump rodent studies every time.
2) Dosage. Typical dosages for animal studies were in the 300 mg/kg range; in humans most often at 1000 mg. (The average human is assumed to way 70 kg.) The dosage in the mouse study that showed conversion of NR to Niacinamide (NAM) was only 50 mg/kg and was short term. Humans take NR for months and it takes a while, days, or weeks, to build up to peak blood levels of NAD+.
3) Measurement of NAD+ levels in liver are not the same as measures in blood.
Species, dosage regimen, tissue. All three of these things can make enormous differences in the measurement of any metabolite or drug concentration, over time. It is quite common that levels in a tissue, like liver, will be very different from those in blood. Even levels in whole blood vs. plasma (blood centrifuged to remove red blood cells) can be very different. I have seen no data on NAD+ levels in human liver, only mouse. Would you want to have a liver biopsy done on you? Heck no!!!