Critical Alert: What You Must Know Before Supplementing NAD+—Is the NMN/NR We Take Consumed by Gut Microbiota?

Since scholars like Professor Sinclair brought NAD+ precursors such as NMN (nicotinamide mononucleotide) and NR (nicotinamide riboside) to prominence, these substances have become favorites among anti-aging enthusiasts. For instance, Jennifer Aniston, star of the classic American drama Friends, has publicly stated that she receives NAD+ injections weekly, calling it the "future trend"[1].

In the United States, the NAD+ market is booming—it was valued at $536 million in 2022 and is growing rapidly. However, whether the efficacy of NAD+ precursors lives up to their current commercial hype is questionable. Even in cutting-edge laboratories, many questions about NAD+ supplementation remain unresolved.

Recently, Professor Joseph A. Baur from the University of Pennsylvania published a landmark review that delves into little-known details about NAD+ and addresses long-standing controversies in the field[2].

As an indispensable substance for normal life activities, NAD+ (nicotinamide adenine dinucleotide) levels can decline under various stressors such as disease, pollution, and work pressure. This inevitable decline causes anxiety, but people have found that NAD+ levels can be quickly restored through supplements—even ordinary individuals with no background in scientific research have developed a keen interest in NAD+.

Yet, both in scientific theory and practical application, NAD+ supplementation is fraught with questions and controversies. For example, at the scientific level: How are NAD+ pools (reservoirs of NAD+ in mitochondria, nuclei, and endoplasmic reticulum) established and transported between different cellular compartments?

Some may ask: Isn’t it enough to know how NAD+ works? Is such detailed research necessary? For most anti-aging substances, it may not be—rapamycin, for instance, primarily acts in the cytoplasm because its target protein, mTORC1, is mostly located there[3].

But unlike most anti-aging substances, while NAD+ is known to mainly target mitochondria, large amounts of NAD+ have also been confirmed in the nucleus, cytoplasm, endoplasmic reticulum, and Golgi apparatus. Scientists estimate that for most cell types, NAD+ outside mitochondria accounts for more than 50% of the total cellular NAD+ pool.

We already understand some of NAD+’s functions:

However, research on how NAD+ crosses various biological membranes in cells to form NAD+ pools in different organelles is still in its infancy.

For mitochondria and nuclei: It is currently known that the mitochondrial membrane contains an NAD+ transporter called SLC25A51. Its deletion leads to almost no NAD+ in mitochondria, while NAD+ concentrations in the cytoplasm and nucleus increase.

For peroxisomes (another membrane-bound organelle), the membrane protein SLC25A17 is suspected to transport NAD+. As for the mechanisms underlying the formation and maintenance of NAD+ pools in the endoplasmic reticulum and Golgi apparatus, little is known.

This means that after we increase NAD+ levels by oral supplementation of NMN or NR (NAD+ precursors), critical questions remain largely unanswered: How much NAD+ is allocated to mitochondria for oxidative phosphorylation? How much is used to relieve endoplasmic reticulum "stress"? And how much is sent to the nucleus for DNA repair?

Another overlooked issue regarding the benefits of NAD+ supplementation is the role of gut microbiota. Before the NAD+ precursors we take orally are absorbed by our somatic cells, they must first pass through the intestines.

As mentioned earlier, all life activities rely on NAD+—and the microorganisms residing in our intestines are no exception. So, do the NAD+ precursors we consume get "hijacked" by these microbiota?

First, our daily diet contains various active forms of NAD+, including NADH, NADP+, and Nam (nicotinamide). Some are broken down into NR for direct absorption, while others are converted into NA (nicotinic acid) by gut microbiota in the form of Nam.

Unlike NAD+, which cannot be directly absorbed by cells, NA or NR can cross cell membranes and then be used for NAD+ synthesis[4].

Studies have found that when supplementing NAD+ precursors (e.g., Nam, NR, or NMN), the increased NAD+ levels observed in the liver mainly result from the microbiota-mediated NA pathway—not from the direct transfer of the administered molecules to hepatocytes. This means that part of the NAD+ precursors we ingest is intercepted by gut microbiota.

However, gut microbiota are not mere "robbers" in our NAD+ metabolism; they actually help maintain NA balance in the body. For example, after NAD+ in our cells exerts its positive effects through pathways like SIRT1, it is converted into Nam. Where does this Nam go?

As noted earlier, NR and other NAD+ precursors we ingest are converted into NA by gut microbiota in the form of Nam. Similarly, Nam produced when our body’s own NAD+ is consumed via pathways like SIRT1 is also transported to the intestines, where microorganisms convert it back into NA. NA is then recycled into somatic cells to synthesize NAD+.

Thus, gut microbiota act more like a "buffer" for NAD+ metabolism in the body: They prevent a sharp short-term surge in NAD+ levels after large-scale precursor supplementation, and through the NAD+-Nam-NA-NAD+ cycle, they ensure NAD+ levels do not plummet even when we take no nutrients.

Additionally, the "hijacking" of NAD+ precursors by gut microbiota may not be a bad thing. Scientists believe that increasing the host’s (our own) NAD+ levels may promote beneficial bacteria and inhibit harmful ones—for example, NAD+ precursor intake can restrict the survival of pathogenic bacteria in the intestines.

In summary, the role of gut microbiota in NAD+ supplementation is complex, and current research is limited to rodents. Thus, it remains unclear whether human gut microbiota "hijack" more or contribute more to NAD+ levels after we supplement with NAD+ precursors.

While the first two sections focus on questions of interest to scholars, the following topic is closely relevant to consumers: Is the money we spend on NAD+ supplements really worthwhile?

In Professor Joseph A. Baur’s view, people may have overhyped the anti-aging strategy of increasing NAD+ levels. Compared with clinical evidence, anecdotal claims about the benefits of NAD+ precursors are clearly exaggerated.

Studies show that high-dose NAD+ supplementation can negatively impact exercise capacity. However, there is ample evidence that supplementing NAD+ precursors can improve some aging indicators in cases of injury or disease. Therefore, the goal of NAD+ supplementation should be to restore NAD+ levels to normal when they are abnormally low.

The table below, summarized by the review authors, outlines clinical results of NAD+ precursor supplements. Overall, effective and ineffective cases are roughly evenly split: Effective cases are slightly more common in patients, while ineffective cases are more frequent in healthy individuals.

*Note: Symbols like "2" indicate the number of relevant clinical trials. For example, in the "Body Weight" row: 1 trial (using NMN) showed improvement, while 8 trials showed no improvement (1 using NA, 2 using Nam, 2 using NR, 3 using NMN).

Therefore, if you feel your physical condition improves after NAD+ supplementation, it is likely because your NAD+ levels had already dropped below normal before supplementation.

However, if you plan to test your NAD+ levels before such intervention, another problem arises: NAD+ level tests in muscle and blood do not reflect NAD+ turnover. It is possible that your NAD+ levels are not low, but its metabolic rate is slow—meaning supplementation may still be needed to promote its function.

For example, some parts of your body may be consuming NAD+ abnormally quickly, leading to insufficient supply in other areas. In this case, even if your blood NAD+ concentration shows no significant change, you may still benefit from NAD+ supplementation to restore health.

In conclusion, non-healthy individuals—such as those with chronic inflammation, kidney disease, various progeroid syndromes, and the elderly—can safely benefit from NAD+ precursor supplementation. For healthy individuals, more precise methods are needed to assess whether supplementation is truly necessary and what potential benefits it may bring, to avoid wasting money.

If you ultimately decide to supplement NAD+ after careful consideration, the next question is: Which method or NAD+ precursor should you choose? The review authors also discussed various strategies.

To understand the pros and cons of different NAD+ precursors, we first need to grasp the conversion relationships between key precursors:When NAD+ is consumed, it produces Nam. Nam can be converted into NMN via NAMPT (nicotinamide phosphoribosyltransferase). NR can also be catalyzed into NMN by NRK (nicotinamide riboside kinase). Finally, NMN is converted into NAD+ by NMNAT (nicotinamide mononucleotide adenylyltransferase).

Nam is the main precursor of NAD+ and is affordable. However, in the NAD+-Nam-NMN cycle, there is a "bottleneck"—NAMPT. Its activity may be insufficient to meet demand, leading to a backlog of Nam waiting for conversion and limited final NAD+ production.

Additionally, many beneficial biological processes (such as the SIRT1 pathway, which converts NAD+ to Nam) are inhibited by high concentrations of Nam.

NA operates independently of the NAD+-Nam-NMN cycle and is not limited by NAMPT like Nam. However, NA relies on NAPRT (nicotinic acid phosphoribosyltransferase). Studies have found that the NAD+ synthesis pathway dependent on NAPRT is associated with certain pre-cancerous and cancerous lesions.

Given that many studies show NA effectively reduces LDL cholesterol, total cholesterol, and triglycerides, individuals with dyslipidemia or cardiovascular disease risk may consider NA.

Tryptophan is an important raw material for NAD+ synthesis in the body. However, supplementing NAD+ via tryptophan is unpopular: The metabolic pathway from tryptophan to NAD+ involves many steps, and tryptophan is also converted into other substances. Excessive tryptophan metabolism may produce quinolinic acid, a neurotoxic compound.

NR’s biggest advantage is that it bypasses the NAMPT bottleneck, avoiding Nam’s drawbacks. However, this theoretical advantage is mostly limited to cell experiments—most orally administered NR is first converted into Nam in the body. This may explain why NR has not shown a significant edge over Nam in clinical trials.

Studies suggest that NR may have greater advantages in antioxidant and anti-inflammatory effects compared to NA, Nam, and NMN[5]. Thus, individuals with systemic chronic inflammation may be more suitable for NR.

Like NR, NMN can bypass NAMPT, so their advantages are similar. However, a controversy surrounds NMN: Is it directly absorbed by cells, or is it first converted into NR before absorption? If the former is true, only one step (NMN → NAD+) is needed.

Some scholars have found that orally ingested NMN is first converted back to NR in the body[6–8]. If this is a universal phenomenon, it means three steps are required (NMN → NR → NMN → NAD+), which increases unnecessary loss.

Since this issue remains unresolved, it is difficult to determine whether NR or NMN is superior. Theoretically, there is no difference in benefits or risks between choosing NR and NMN (or it is impossible to judge). In this case, the quality of specific products has a greater impact than the substance itself.

Given the potential pitfalls of oral NAD+ precursor supplementation, direct intravenous NAD+ injection seems to theoretically avoid the shortcomings of the aforementioned precursors.

Even skeptical scientists only note: Scientific literature testing the efficacy of intravenous NAD+ is quite limited, and injection leads to a sustained increase in blood adenosine (a product of ATP breakdown), which may pose health risks.

Beyond this, the main disadvantages of intravenous NAD+ are poor convenience and high cost.

In fact, from the perspective of human clinical trials, it is currently difficult to conclude which NAD+ precursor is optimal. For example, when we say NR is effective for inflammation, it does not mean NMN or Nam cannot reduce inflammation—other NAD+ precursors simply lack clinical testing for anti-inflammatory effects.

However, when making a choice, consumers should combine existing clinical results with their own physical needs.

For instance, if you have poor sleep quality: Among NAD+ precursors tested for sleep improvement in clinical trials, NMN was effective. While this does not mean other precursors are ineffective, NMN may be the most suitable choice for you, as it has clinical trial support.

[1] Dougherty E. Meet NAD+, the Latest Celebrity Biohacking Trend. https://www.elle.com/beauty/health-fitness/a46598724/everything-you-need-to-know-nad/.[2] Migaud ME, Ziegler M, Baur JA. Regulation of and challenges in targeting NAD+ metabolism. Nature Reviews Molecular Cell Biology 2024.[3] Tee AR. The Target of Rapamycin and Mechanisms of Cell Growth. International Journal of Molecular Sciences 2018; 19.[4] Ahmad Z, Bashir K, Matsui A, Tanaka M, Sasaki R, Oikawa A, Hirai MY, Chaomurilege, Zu Y, Kawai-Yamada M, Rashid B, Husnain T, et al. Overexpression of nicotinamidase 3 (NIC3) gene and the exogenous application of nicotinic acid (NA) enhance drought tolerance and increase biomass in Arabidopsis. Plant Molecular Biology 2021; 107:63–84.[5] Dong Y, Wang X, Wei L, Liu Z, Chu X, Xiong W, Liu W, Li X. The Effectiveness of Four Nicotinamide Adenine Dinucleotide (NAD(+)) Precursors in Alleviating the High-Glucose-Induced Damage to Hepatocytes in Megalobrama amblycephala: Evidence in NAD(+) Homeostasis, Sirt1/3 Activation, Redox Defense, Inflammatory Response. Antioxidants (Basel, Switzerland) 2024; 13.[6] Grozio A, Mills KF, Yoshino J, Bruzzone S, Sociali G, Tokizane K, Lei HC, Cunningham R, Sasaki Y, Migaud ME, Imai S-I. Slc12a8 is a nicotinamide mononucleotide transporter. Nature Metabolism 2019; 1:47–57.[7] Schmidt MS, Brenner C. Absence of evidence that Slc12a8 encodes a nicotinamide mononucleotide transporter. Nature Metabolism 2019; 1:660–661.[8] Grozio A, Mills K, Yoshino J, Bruzzone S, Sociali G, Tokizane K, Lei HC, Sasaki Y, Migaud M, Imai S-I. Reply to: Absence of evidence that Slc12a8 encodes a nicotinamide mononucleotide transporter. Nature Metabolism 2019; 1:662–665.