NMN Probably Won't Make You Live Forever

Longer maybe? A new Nature Medicine paper casts doubt.

Niacin (vitamin B3) is needed for a healthy gut, skin, and mind. Its deficiency leads to depression in early stages and schizophrenia in later stages, inflamed skin that gets much worse in the sun, and digestive problems resulting from poor absorption of nutrients. It is used to make NAD+, which declines in aging and disease. Longevity supplements like nicotinamide riboside (NR) and nicotinamide mononucleotide (NMN) are widely thought to help combat the diseases of aging and often hoped to make us live longer.

A new Nature Medicine paper from Stanley Hazen’s group at the Cleveland Clinic (of TMAO fame) puts this all into question.

It claims that the niacin metabolite known as 4PY promotes cardiovascular inflammation and contributes to early death.

This artful diagram summarizes the paper very well:

Who is right?

In this article we take a look at this paper and evaluate what it means in the context of what we already know about niacin and NAD+-boosting supplements specifically with respect to cardiovascular disease.

The Short Answer

While the diagram above is overly dramatic, 4-PY is likely to be relatively noxious and it is preferable to avoid needlessly generating it. This can be accomplished by hitting the RDA for iron and my custom nutrient targets for niacin, iron, riboflavin, and vitamin B6. Together this yields preformed niacin as well as the nutrients needed to convert the tryptophan in the protein to NAD+.

Supplements of niacin, nicotinamide, NR, or NMN should be capped at a maximum of 250 milligrams per day during long-term use (months or longer) and are best limited to the role of fixing a deficiency or aiming to improve a particular health condition that you verify responds well to the supplement. Higher doses should not be used for general health or out of a general concern for longevity. The cap should not be exceeded without substantial justification based on lab data and personal experience.

The need to place a limit on these supplements at the current time is a low-confidence judgment that could easily change with future research. It is substantially influenced by the present study.

Background on Niacin Metabolism

As covered in the niacin podcast (Part 1, Part 2), niacin is used to make NAD+ and NADPH, which are used ubiquitously in energy metabolism. NAD+ is also irreversibly broken down for a variety of purposes, including to serve as a neurotransmitter, to tag damaged DNA for repair, and to lengthen telomeres. It is most often broken down to nicotinamide. Since nicotinamide inhibits NAD+-degrading enzymes, whatever cannot quickly be salvaged back to NAD+ is methylated, and what is methylated tends to be excreted.

Niacin (nicotinic acid) supplements that exceed what can be turned into NAD+ are conjugated to glycine. Nicotinamide supplements that exceed what can be turned into NAD+ are methylated. NMN and NR generate marginally better nicotinamide levels in the liver than other niacin supplements. The liver then releases the nicotinamide to other tissues. Since these supplements are better at generating nicotinamide, they are more likely to be methylated.

Some of the methylated nicotinamide is broken down into N1-methyl-2-pyridone-5-carboxamide (2PY) and N1-methyl-4-pyridone-3-carboxamide (4PY). At higher doses there are other metabolites as well, such as nicotinamide N-oxide.

This conversion to 2PY and 4PY is catalyzed by aldehyde oxidase, which is an enzyme dependent on molybdenum, riboflavin, iron, and sulfur. It is also responsible for metabolizing nicotine to cotinine, and is one of the numerous enzymes capable of activating vitamin A by converting retinal to retinoic acid.

The ratio of 2PY to 4PY varies according to species. In humans, 2PY is the major metabolite, whereas in rats it is 4PY and in mice it is about equal. Across strains of rats, the ratio is constant. When the human gene is implanted into mice, however, 2PY becomes the major metabolite. This suggests that the ratio of 2PY to 4PY is a feature of the enzyme structure as dictated by the genetic code.

The amino acid tryptophan is converted to NAD+, but unlike supplements it will generally not risk creating an excess. Tryptophan beyond the need for the synthesis of proteins, serotonin, and melatonin flows down the kynurenine pathway, and with the help of iron, riboflavin, and B6, is either burned for energy or with the extra help of potassium converted into NAD+. Its degradation for energy is controlled by the enzyme aminocarboxymuconate semialdehyde decarboxylase (ACMSD). This is a zinc-dependent enzyme that is regulated by the NAD+ demand of glycolysis. If there is insufficient NAD+ to run glycolysis, dihydroxyacetone phosphate accumulates and shuts of ACMSD, preventing tryptophan from being broken down for energy and diverting it into synthesis of NAD+. If there is sufficient NAD+ to run glycolysis, the enzyme remains active and tryptophan is burned for energy.

The Findings of the New Nature Medicine Paper

The new Nature Medicine paper found that people in the highest quartile for circulating levels of 2PY and 4PY had more than double the risk of major adverse cardiovascular events.

Since failing to excrete these in the urine will make their blood levels rise, the researchers adjusted for kidney function. This made the association with 2PY lose statistical significance, and made the effect size for 4PY drop to a ~65% increase in risk.

The genetic variants rs10496731 and rs6430553 correlated strongly with one another and with the levels of 2PY and 4PY, and were found to be in the ACMSD enzyme. Knocking down this enzyme in mice elevated their levels of 2PY by 3.6-fold and their levels of 4PY by 49%.

They performed a Mendelian randomization study that did not support an association between these variants and heart disease risk. However, they attributed this to the fact that the variants were too weak to shift a person from one quartile of 2PY or 4PY to another.

The rs10496731 variant did correlate with soluble vascular cellular adhesion molecule-1 (sVCAM-1), and injecting mice with 4PY, but not 2PY, increased the expression of VCAM-1 by aortic endothelial cells. Similarly, injecting the mice with 4PY but not 2PY caused white blood cells to adhere to the blood vessel wall.

This provides evidence that 4PY promotes vascular inflammation, while 2PY does not. Further, this could explain the correlation between 4PY and major adverse cardiovascular events in humans.

Returning to the figure a moment:

… we can see some exaggeration. They did not assess protein intake, tryptophan intake, or niacin intake. They did find observational associations between 4PY and adverse cardiovascular events such as heart attacks and strokes, but they did not report any associations with death, and they did not provide any experimental evidence that 4PY causes heart attacks or strokes.

Nevertheless, they laid out mechanistic evidence that 4-PY causes vascular inflammation and charted a plausible hypothesis for how this could explain its observational association with heart attacks and strokes.

They argue that high-dose niacin is likely a net negative for heart disease now that statins are used to lower cholesterol, that we need more research on how NAD+-boosting strategies relate to cardiovascular risk, and that we should consider relaxing the mandate to fortify flour and cereal with niacin to allow non-fortified options.

This latter point is a strange position since that already prevails in the United States according to the FDA:

Let’s take a look at why 4-PY would be so noxious, then at the context of what we know about niacin and cardiovascular health.

Why Would 4-PY But Not 2-PY Be Toxic?

Take a look at the structure of 4-PY and 2-PY as illustrated in this figure of the paper:

4PY stands out as the only compound with two carbonyl groups (carbon double-bonded to oxygen, signified by =O) situated closely together. 2PY has a second carbonyl group, but it is interrupted by the bulky ring structure, pushing it far apart from the first carbonyl.

Carbonyl groups are very reactive, and they are the most reactive when they stand alone as aldehyde groups and do not have large bulky things getting in the way of their potential reactions.

If we take a look at methylglyoxal, the predominant precursor to advanced glycation endproducts (AGEs):

It also has two close-together carbonyls, but a lot less bulk around them. No nitrogen group on one side, no bulky ring on the other.

These two close-together carbonyls is what makes methylglyoxal so toxic. For example, they will react with the two nitrogens on the arginine residues of enzyme active sites and of receptors, forming a cap on top of the arginine, and neutralizing the positive charge that makes the enzyme or receptor active:

This forms methylglyoxal-derived hydroimidazolone-1 (MGH1), the quantitatively most important AGE in the human body.

My speculation is that these two close-together carbonyls explains why 4PY but not 2PY is toxic, and that, because of the extra bulk found around them, 4PY is far less toxic than methylglyoxal.

In support of this, let’s compare the ability of injected 4PY to generate leukocyte adhesion to mouse aorta in the Nature Medicine study to the ability of injected methylglyoxal to do the same thing to the microvasculature around the testicles of mice.

The Nature Medicine paper injected 30 milligrams per kilogram 4PY and achieved 12-16-fold greater than normal concentrations of 4PY to show leukocyte adhesion to the aorta.

The methylglyoxal paper injected 25 milligrams per kilogram and achieved a 3-fold elevation of methylglyoxal to show the same thing in the microvasculature around the testicles.

While the Nature Medicine paper did not investigate a dose-response, and while these two models are not exactly comparable, they are consistent with methylglyoxal being four to five times as toxic as 4PY relative to their respective normal circulating concentrations.

This would suggest a four- to five-fold elevation in 4PY would put 4PY on the level of normal, basal levels of methylglyoxal.

Obesity is associated with 35% higher methylglyoxal, while diabetics have 3.6-fold more methylglyoxal. Animal experiments show that raising methylglyoxal levels in animals to a similar extent as found in diabetic humans causes hyperglycemia acutely and causes severe diabetes chronically. Most likely the 35% elevation in obesity contributes a portion of the risk of future diabetes, and is part of a constellation of changes that together lead to diabetes.

So, we could then speculate that a 5.4-fold elevation of 4PY would raise concern as analogous to the rise of methylglyoxal found in obesity, and that a 14-fold elevation would be deeply concerning as analogous to the concentrations that are found in diabetes and induce diabetes in animals all on their own.

What Do Anti-Aging Supplements Do to 4PY?

This is what NR and NMN do to 4PY:

• In 12 men between the ages of 70 and 80, one gram of nicotinamide riboside per day for 21 days elevated circulating 4PY 8-fold from 0.48 to 3.82 micromoles per liter.

• In a 52-year-old male taking the same dose, these elevations were similar. 4PY took 8 hours after the first dose to reach its maximum, and then stayed elevated for as long as the man stayed on the supplement.

• In 13 prediabetic women with an average age of 62, 250 milligrams of nicotinamide mononucleotide for 10 weeks roughly tripled the sum of 2PY and 4PY, although the two metabolites were not reported separately.

Overall, we can surmise that 4PY triples on 250 milligrams of NAD+ precursors and increases 8-fold on 1000 milligrams.

Older studies on nicotinic acid and nicotinamide mostly looked at the urine rather than the blood, but there is little reason to think it works much differently, except that high doses of nicotinic acid are more likely to be conjugated to glycine than to generate methylated metabolites of nicotinamide.

The tripling that occurs on 250 milligrams fails to reach our 5.4-fold threshold of initial concern, whereas the 8-fold increase that occurs on 1000 milligrams is squarely between that threshold and the threshold for deep concern.

This suggests such supplements should be capped at 250 milligrams per day without substantial justification for the higher dose taken from lab data or personal experience.

Now let’s situate this new paper in the context of the existing literature on niacin and cardiovascular disease.

Food Niacin and Heart Disease Risk

Vascular endothelial dysfunction is an early sign of cardiovascular disease. It can be measured by testing the ability of the forearm blood vessel to dilate after being restricted. Endothelial function increases with increasing niacin intake up to about 50 milligrams per day with no tapering off, suggesting a protective effect of niacin against a very early step in the onset of cardiovascular disease.

In a cohort of individuals with fatty liver disease (quite representative of the modern general population, actually), niacin intake was inversely correlated with cardiovascular and all-cause mortality throughout the range of intake, which exceeded 75 milligrams per day.

If you note the shaded area representing the confidence interval, the sweet spot seems to be just under 50 milligrams a day, because this is where the shaded area dips below the dotted line, indicating a more statistically robust evidence of lower risk.

However, these studies don’t account for the synthesis of NAD+ from tryptophan, so without data on amino acid intake we cannot be certain in the need to reach just under 50 milligrams. The requirement will go down on a diet rich in protein, iron, riboflavin, B6, and potassium.

High-Dose Niacin and Heart Disease Risk

In meta-analysis, high-dose niacin (1-3 grams per day) by itself reduces stroke and acute coronary syndrome (heart attack, angina, and related conditions) by 26%, and this was derived primarily from two trials in the pre-statin era. When including modern trials where niacin was added to statin treatment, high-dose niacin was associated with with a 12% reduction in major adverse cardiac events that just barely failed to achieve statistical significance.

Another meta-analysis found that high-dose niacin was associated with a 10% increase in all-cause mortality that just barely failed to achieve statistical significance.

The first meta-analysis found that trials prior to 1990 showed most of the benefit, trials from 1990-2011 showed very little benefit, and trials since 2011 leaned toward niacin causing harm.

The fact that high-dose niacin really shines for cardiovascular disease when used without statins and that trials have become less favorable to niacin in the statin era is consistent with the idea that high-dose niacin primarily prevents heart disease by lowering cholesterol and is otherwise a mixed bag.

Since high-dose niacin trials use one to three grams of niacin per day, they should generate an awful lot of 4PY, in addition to depleting glycine and methyl groups. The fact that high-dose niacin doesn’t raise cardiovascular risk and that the worst case against it is that it might increase total mortality by 10% in the context of heart disease patients getting standard modern treatment is consistent with 4PY being relatively noxious, but is not consistent with it being a potent cardiovascular toxin.

However, the use of high-dose niacin to manage heart disease risk comes at the cost of a 34% increase in the risk of developing diabetes. Alex Leaf published a compelling hypothesis that this comes from a rebound increase in free fatty acids 3-6 hours after the dose, during which one would become temporarily glucose intolerant. He suggested that taking another dose of niacin or avoiding carbohydrates during this period could minimize the risk of diabetes.

While I find that logic compelling, I also now wonder if 4PY makes an independent contribution to the diabetes risk that is analogous to the effect of methylglyoxal.

From the several dozen clinical trials published of NMN and NR, few of them have any direct relation to cardiovascular disease. The most notable exception is a trial showing 250 milligrams of NMN per day for 12 weeks led to a 12% decrease in arterial stiffness that failed to reach statistical significance (P=0.097).

The Bottom Line

The new Nature Medicine study does not show a negative clinical effect of niacin supplementation on cardiovascular disease, but it does provide mechanistic evidence suggesting 4PY causes cardiovascular inflammation and puts forward a reasonable hypothesis for why this could explain the association between high circulating 4PY levels and major adverse cardiovascular events.

Because 4PY has two close-together carbonyl groups, it likely has analogous toxicity to dicarbonyls like methylglyoxal, but is probably much less toxic due to the bulky structures that could get in the way of reactions. If we use the relative ability of each compound to cause vascular inflammation in animal models and extrapolate to humans based on the known effect of NR and NMN supplementation on circulating 4PY, 250 milligrams per day is the upper limit of what does not raise concern.

It is not clearly established that doses even this high are necessary as long-term maintenance doses for any purpose, and it is best to focus on hitting the RDA for iron and my custom nutrient targets for niacin, iron, riboflavin, and vitamin B6. Together this yields preformed niacin as well as the nutrients needed to convert the tryptophan in the protein to NAD+.

It is best to limit niacin, nicotinamide, NR, and NMN supplements to 250 milligrams a day and to limit them to the role of fixing a deficiency over the short-term or aiming to improve a particular health condition that you verify responds well to the supplement.

Nevertheless this is a new avenue of research, clinical trials of NR and NMN are in their infancy, and this conclusion could easily be revised by the findings of future studies.