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What is NAD+ effect on disease and aging

Dr. David Sinclair, Co-Director of the  Biological Mechanisms of Aging at Harvard Medical School was named to  the Times Magazine list of “Most Influential People in the World” after his research found a key cause of aging  and a potential weapon to reverse it.

He and his team found that as we age, our cells become less and less efficient due to the lack of an essential metabolite called Nicotinamide Adenine Dinucleotide (NAD+).

WHAT IS NAD+

NAD+ is a key co-enzyme that the mitochondria in every cell of our bodies depend on to fuel all basic functions. (3,4)

NAD+ play a key role in communicating between our cells nucleus and the Mitochondria that power all activity in our cells (5,6,7)

Scientists have now confirmed a direct link between falling NAD+ levels and aging in both animal and in human subjects.

Read more about NAD+  

NAD+ DECLINES WITH AGE

As we age, our bodies produce less NAD+ and the communication between the Mitochondria and cell nucleus is impaired. (5,8,10).

Over time,  decreasing NAD+ impairs the cell’s ability to make energy, which leads to aging and disease (8, 5) and perhaps even the key factor in why we age (5).

Read more about NAD+

NAD+ METABOLISM IN HUMANS

NAD+ can be synthesized in humans from several different molecules (precursors), thru  the De Novo  and Salvage Pathways.

The salvage pathway sustains 85% or more of our NAD+ (14), with approximately 3g of NAM metabolized to NMN and then to NAD 2-4 times per day (14).

Nampt is the rate-limiting step in the salvage process (97).

SUBLINGUAL NMN AND NAD+ BYPASS THE LIVER and NAMPT BOTTLENECKS

As we age, Nampt enzyme activity is lower, resulting in less NAM recycling, less NAD+, more disease and aging (97,101).

All NAD+ supplements can restore NAD+ in the Liver but does not solve NAD+ deficiency throughout the body. This research shows that NAD+ in the liver is metabolized by CD38 to NAM, and ONLY NAM is excreted to the rest of the body.

Sublingual administration of NMN or NAD+ supplements bypass the 2 bottlenecks – the liver and NAMPT processing of NAM.

Providing NAD+ or its immediate precursor, NMN, directly to the bloodstream is much more effective than dropping large quantities of NAM or NR into the liver and trying to force it to produce more NMN and NAD to the bloodstream.

 

CAN INCREASED NAD+ REVERSE AGING?

The 2013 study by Dr Sinclair at Harvard is explained in this video below:


There is reason to think that increased  NAD+ can at least slow the aging process.

When taken orally, NAD+  does not survive the digestive system long enough to enter your cells (14)

The ground-breaking paper published by Dr Sinclair found that supplementation with Nicotinamide MonoNucleotide (NMN), the immediate precursor to NAD+,  could boosts NAD+ levels in mice and resulted in the “equivalent of a human 60 year old becoming more like a 20 year old”(8).

More importantly, their research revealed that mitochondrial dysfunction is reversible with supplementation to boost NAD+

SUPPLEMENTATION TO  BOOST NAD+

In mammals, NAD+ can be created from simple elements present in the body such as Nicotinic Acid or Tryptophan thru the “de novo” pathway.

However the entire NAD+ pool is consumed 2-4 times a day and recycled thru the “salvage pathway”, which is far more important for maintaining NAD+ levels (14).

In the salvage pathway, Nicotinamide or Nicotinamide Riboside are first converted to NMN, which is then further converted to NAD+(14).

NMN is more correctly referred to as a NAD+ intermediate because NMN is the last step before conversion to NAD+

NAD+ Precursors:

  • Tryptophan
  • NA – Nicotinic Acid
  • NAM – Nicotinamide
  • NR – Nicotinamide Riboside
  • NMN – Nicotinamide Mononucleotide

The first 3 NAD+ precursors are well known and have been used for decades to treat various metabolic diseases such as dislipidemia and neurological conditions (10,11,14)

In 2006 Dr Charles Brenner discovered that NR can be phosphorylated to NMN in the body, to increase NAD+ without going thru NAM. This all owe it to bypass the “NAMPT bottleneck”.

Chromadex has licensed methods to produce NR and has been selling NR commercially since 2014. Dr Brenner publishes research on use NR in mice and humans for a wide range of disease and illness and is the chief scientific advisor in Chromadex.

NMN is also used in research for many of the same treatments as NR, and is now also being sold commercially, although there are no patents controlling the source of NMN. Dr Sinclair is the leading researchers in use of NMN.

We find the research using NMN provides far more dramatic results than those using NR (more below).

OTHER WAYS TO INCREASE NAD+

There are a number of lifestyle changes you can make  to increase NAD+ in you body.

It’s well known that Calorie Restriction  (CR) can extend longevity by 30–50% in many mammals (32)

CR has also been shown to increase NAD+ levels in the body , thru these pathways:

  • Lowering blood glucose levels minimizes inflammation, which consumes NAD+.
  • The ketone body BHB signals to increase AMPK to produce more NAD+
  • Burning Ketones for fuel instead of glucose requires 1/2 as much NAD+

– Ketone bodies mimic the life span extending properties of caloric restriction (veech,2017)

 KETOSIS BOOSTS NAD+

Ketosis is a metabolic state in which fat provides most of the fuel for the body. It occurs when there is limited access to glucose (blood sugar), which is the preferred fuel source for many cells in the body.

Ketosis can occur in many different diet plans whenever carb intake is low, but is most often associated the Ketogenic Diet, as that is a much easier method for restricting carbs (3, 4, 5, 6).

Recent research now shows  Ketosis  provides the benefit in life extension, lowering inflammation and boosting NAD+ (3,9).

Intermittent or Periodic Ketosis is also effective at extending lifespan and likely achieves much of the benefit (36,37).

Some research even shows more benefit from a cyclical rather than a full time Ketogenic Diet (71).

Ketogenic Diet Reduces Midlife Mortality and Improves Memory in Aging Mice (Newman, 2017)

The Red bars in the chart at left show the increased levels of the Ketone Body BHB produced from a cyclical Keto diet, resulting in Increased NAD+ and greatly improved neurological function and health.

Read more about nutrition for boosting NAD+

EXERCISE THAT BOOSTS NAD+

Researchers are finding that 2-3  short bouts of High Intensity Interval Training  (HIIT) per week is far more effective at lowering inflammation (and increasing NAD+), especially among older adults (55)

Exercise is very effective at boosting AMPK and NAD+, especially when performed at times of low blood glucose levels  ( more about HIIT ).

Short bouts of HIIT accomplishes the goal, while avoiding overtraining from endurance workouts  which increases inflammation and consumes NAD+ (55).

Read more about exercise for boosting NAD+

WHY WE BELIEVE NMN IS MORE EFFECTIVE THAN NR

DRAMATIC RESULTS WITH NMN IN ENDURANCE AND YOUTHFULLNESS

Below are the three studies that made the biggest splash’s about the potential for reversing aging by restoring NAD+ to youthful levels that have ALL been accomplished using NMN

After 6 days of NMN, 22 month old mice  had the muscle capacity, endurance and metabolism of 6 month old  mice (2013 Sinclair study)

NMN effectively mitigates age-associated physiological decline in mice (2016 Mills Long Term study)

“The old mice became as fit and strong as young mice” (Sinclair, 2018)

This third study recently published by Dr Sinclair is a  good example.

Mice that received NMN had nearly 100% increased endurance vs the control mice, and actually grew NEW blood vessels. This was after 60 days, in 20 month old mice (equivalent to 90 year old humans).

Along with the impressive increased endurance, the study shows  NAD+ increase is over 500% at 60 days

RESULTS NOT AS IMPRESSIVE WITH NR

In this 2016 study, 22-24 month old mice were given NR for 6 weeks.

Running distance and duration were improved approximately 20%.

The treatment duration was slightly shorter in this study than with NMN (6 weeks vs 8 weeks), there is a huge difference in the benefit with increased endurance from NMN nearly 100% vs the 20% with NR.

Treating Heart Disease

2 separate studies to treat a form of heart disease called Friedreich’s Ataxia with NR and NMN were published in 2017. Treatment with NMN was successful, while NR did not improve cardiac function.

“Remarkably, NMN administered to FXN-KO mice restores cardiac function to near-normal levels. “(Martin, 2017)

“In conclusion, NAD+ supplementation with NR in the FRDA model of mitochondrial heart disease does not alter SIRT3 activity or improve cardiac function.”(Stram, 2017)

COMBATTING ALZHEIMERS DISEASE

Alzheimer’s disease (AD) pathogenesis is widely believed to be driven by the production and deposition of the β-amyloid peptide (Aβ). Evidence now indicates that the solubility of Aβ, and the quantity of Aβ in different pools is related to disease state (r).Researchers believe that flaws in the processes governing production, accumulation or disposal of beta-amyloid are the primary cause of Alzheimer’s (r).

In studies published in 2017 and 2018 NMN decreased β-amyloid buildup, while NR did not.

“NR lessened pTau pathology in both 3xTgAD and 3xTgAD/Polβ+/− mice but had no impact on amyloid β peptide (Aβ) accumulation”(Hou, 2018)

“NMN decreased β-amyloid production, amyloid plaque burden, synaptic loss, and inflammatory responses in AD-Tg mice” (Yao, 2017)

NMN was able to mitigate most age-associated physiological declines in mice Treatment of old mice with NMN reversed all of these biochemical aspects of aging

Long-Term Administration of Nicotinamide Mononucleotide Mitigates Age-Associated Physiological Decline in Mice (mills, 2016)

Raising NAD+ levels in old mice restores mitochondrial function to that of a young mouse

Restore the mitochondrial homeostasis and key biochemical markers of muscle health in a 22-month-old mouse to levels similar to a 6-month-old mouse

Declining NAD+ Induces a Pseudohypoxic State Disrupting Nuclear-Mitochondrial Communication during Aging (Gomes, Sinclair,2013)

DNA Repair

This study showed supplementation with NMN was able to repair the DNA in cells damaged by radiation

The cells of old mice were indistinguishable from young mice after just one week of treatment.

A conserved NAD+ binding pocket that regulates protein-protein interactions during aging (Sinclair, 2017)

WEIGHT

NMN was immediately utilized and converted to NAD+ within 15 min, resulting in significant increases in NAD+ levels over 60 min

Administering NMN, a key NAD+ intermediate, can be an effective intervention to treat the pathophysiology of diet- and age-induced T2D

Surprisingly, just one dose of NMN normalized impaired glucose tolerance

Nicotinamide Mononucleotide, a Key NAD+ Intermediate, Treats the Pathophysiology of Diet- and Age-Induced Diabetes in Mice (Yoshino, 2011)

NAD(+) levels were increased significantly both in muscle and liver by NMN

NMN-supplementation can induce similar reversal of the glucose intolerance

NMN intervention is likely to be increased catabolism of fats NMN-supplementation does mimic exercise

Head to Head Comparison of Short-Term Treatment with the NAD(+) Precursor Nicotinamide Mononucleotide (NMN) and 6 Weeks of Exercise in Obese Female Mice (Uddin, 2016)

NMN significantly increased the level of NAD+ in the heart

NMN protected the heart from I/R injury

Nicotinamide mononucleotide, an intermediate of NAD+ synthesis, protects the heart from ischemia and repercussion (Yamamoto, 2014)

NMN reduces vascular oxidative stress

NMN treatment normalizes aortic stiffness in old mice

NMN represents a novel strategy for combating arterial aging

Nicotinamide mononucleotide supplementation reverses vascular dysfunction and oxidative stress with aging in mice (de Picciotto, 2016)

NMN can reduce myocardial inflammation NMN thus can cut off the initial inflammatory signal, leading to reduced myocardial inflammation

Short-term administration of Nicotinamide Mononucleotide preserves cardiac mitochondrial homeostasis and prevents heart failure (Zhang, 2017)

ENERGY

Remarkably, NMN administered to FXN-KO mice restores cardiac function to near-normal levels.

Restoration of cardiac function and energy metabolism upon NMN supplementation

Remarkable decrease in whole-body EE and cardiac energy wasting

Nicotinamide mononucleotide requires SIRT3 to improve cardiac function and bioenergetics in a Friedreich’s ataxia cardiomyopathy model

VISION

Exogenous NMN prevents photoreceptor degeneration and restores vision

NMN rescues retinal dysfunction in light-induced degeneration

 

NAMPT-mediated NAD+ biosynthesis is essential for vision in mice (lin, 2016)

Completed and pending publication

Beginning 2018

  • 2018 Sinclair Metrobio study – Phase 2

The Phase 1 study by Dr Sinclair has been completed, and they are ready to go forward with the Phase 2 study, so we can conclude there were positive results, and no negative side effects, else they would have to publish those immediately.

In the University of Washington study, participants are 50 healthy women between 55 and 70 years of age with slightly high blood glucose,BMI and triglyceride levels.

Using a dose of 2 capsules of 125mg NMN per day over a period of 8 weeks, researchers are testing for:

  • change in beta-cell function
  • works to control blood sugar
  • blood vessels dilate
  • effects of NMN on blood lipids
  • effects of NMN on body fat
  • markers of cardiovascular and metabolic health

The active supplementation portion of this study has ended, but testing of metabolic parameters will continue for 2 years after supplementation has ended.  So researchers know the immediate effects and  preliminary results are expected to be announced in 2018, with  final results expected in 2020.
 

Elevates NAD+ quickly throughout the body

In this 2016 study, mice were given a single dose of NMN in water.

NMN levels in blood showed it is quickly absorbed from the gut into blood circulation within 2’“3 min and then cleared from blood circulation into tissues within 15 min

Increases NAD+ and Sirt1 Dramatically in organs

The charts at left from 2017 study, NMN supplementation for 4 days significantly elevated NAD+ and SIRT1, which protected the mice from Kidney damage.

NAD+ and SIRT1 levels were HIGHER in OLD Mice than in YOUNG Mice that did not receive NMN.


 

Decline of NAD+ during Aging, Age-Related Diseases, and Cancer

Several evidences suggest a decline in NAD+ levels while we age, connecting NAD+ deficits to age-related diseases and cancer.

Inflammation increases during the aging process possibly due to the presence of senescent cells [1].

CD38 and bone marrow stromal cell antigen-1 (BST- 1) may provide explanations to NAD+ decline during aging.

CD38 is a membrane-bound hydrolase implicated in immune responses and metabolism. NAD+ can be degraded through its hydrolysis, deacetylation, or by NAD+ nucleosidases (also called NAD+ hydrolases or NADases) such as CD38.

Expression and activity of CD38 increase in older mice, promoting NMN degradation in vivo, responsible for NAD+ decline and mitochondrial dysfunctions [2].

Interestingly, loss of CD38 inhibits glioma progression and extends the survival of glioma- bearing mice.

Targeting CD38 in the tumor microenvironment may clearly serve as a novel therapeutic approach to treat glioma [3].

 

Daratumumab, a CD38 monoclonal antibody, rep- resents a first-in-class drug for the treatment of multiple myeloma. It promotes T cell expansion through inhibition of CD38+ immunosuppressive cells, improving patients’ responses [4].

These findings suggest that NAD+ boosters should be combined with CD38 inhibitors for a more efficient antiaging therapy.

 

NAD+ Biosynthesis Decreases during Aging, Age-Related Diseases, and Cancer 

NAD+ increases can also occur independently of the Preiss–Handler route. NAM and NR are important NAD+ precursors first converted to nicotinamide mononucleotide (NMN) by nicotinamide phosphoribosyltransferase (NAMPT) and NR kinase (NRK), respectively. NMN is then transformed into NAD+ by NMN adenylyltransferase [36].

As we age, our bodies undergo changes in metabolism, and a key part of these processes may affect de novo NAD+ synthesis, also called the L-tryptophan/kynurenine pathway (see Figure IB in Box 1). In mammals, the use of the de novo NAD+ biosynthetic pathway is limited to a few specific organs.

Finally, dysregulation of the kynurenine pathway is also linked to genetic disorders and age-related diseases such as obesity and cancer [14,15]. These age-associated changes in de novo NAD+ biosynthesis may have the potential to impact several biological processes, and thus contribute to age-related diseases and cancer in the elderly.

Animal models mimicking downregulation of NAD+ biosyn-thesis are needed to modulate its activity and understand its pathophysiological relevance in age-related pathologies and cancer.

Boosting NAD+ with Niacin in Age-Related Diseases and Cancer 

In humans, a lack of nicotinic acid (NA, also called niacin) in the diet causes the vitamin B3 deficiency disease pellagra, characterized by changes in the skin with very characteristic  pigmented sunburn-like rashes developing in areas that are exposed to sunlight. Likewise, people with chronic L-tryptophan-poor diets or malnutrition develop pellagra.

Furthermore, several epidemiologic studies in human reported an association between incidence of certain types of cancers and niacin deficiency [27].

In this regard, low dietary niacin has also been associated with an increased frequency of oral, gastric, and colon cancers, as well as esophageal dysplasia.

In some populations, it was shown that daily supplementation of niacin decreased esophageal cancer incidence and mortality. Although the molecular mechanisms of niacin deprivation and cancer incidence are not well understood, it has been recently reported that NAD+ depletion leads to DNA damage and increased tumorigenesis, and boosting NAD+ levels is shown to play a role in the prevention of liver and pancreatic cancers in mice [19,28,29].

Thus, malnutrition through inadequate amounts and/or diversity of food may affect the intra- cellular pools of nicotinamide and NAD+ thereby influencing cellular responses to genotoxic damage, which can lead to mutagenesis and cancer formation [19,27]. NAD+ boosters are therefore essential in patients at risk of exposure to genotoxic and mutagenic agents, including ionizing or UV radiations or, DNA damaging chemicals.

In addition, niacin deficiency in combination with carcinogenic agents was described to induce and increase tumorigenesis in rats and mice.

For instance, in rats, the lack of niacin together with carcinogen treatment increased tumorigenesis and death of rats [30,31]. Additionally, in mice, the incidence of skin tumours induced by UV was significantly reduced by local application of NAM or by niacin supplementation in the diet [32].

Boosting NAD+ with NAM in Age-Related Diseases and Cancer 

Recent research has focused on uncovering the consequences of a decrease in NAD+ during aging using age-related disease models. In PGC1a knockout mouse, a model of kidney failure, NAD+ levels are reportedly decreased, and boosting NAD+ by NAM improves kidney function [33].

NAM injections during four days re-establish local NAD+ levels via nicotinamide phos- phoribosyltransferase (NAmPRTase or NAMPT) activation and improve renal function in postischaemic PGC1a knockout mice [33].

Surgical resection of small renal tumors can induce kidney ischemia severely affecting the renal function. Therefore, NAD+ boosters can be beneficial to protect the organ from severe injury.

Moreover, in a model of muscular dystrophy in zebrafish, NAD+ increases, which functions as an agonist of muscle fiber–extracellular matrix adhesion, and corrects dystrophic phenotype recovering muscle architecture [34].

Boosting NAD+ with NR in Age-Related Diseases and Cancer 

Further research has extensively used NR to ameliorate the effects of NAD+ deficits in pleiotropic disorders. NR naturally occurs in milk [35,36]. NR is converted to NAD+ in two step reactions by nicotinamide riboside kinases (NRKs)-dependent phosphorylation and adenylylation by nicotinamide mononucleotide adenylyl transferases (NMNATs) [36].

It is considered to be a relevant NAD+ precursor in vivo. Evidences demonstrate the beneficial effect of NR in skeletal muscle aging [37,38] and mitochondrial-associated disorders, such as myopathies [39,40] or those characterized by impaired cytochrome c oxidase biogenesis affecting the respiratory chain [41].

In line of these findings, a mouse model of Duchenne muscular dystrophy present significant reductions in muscle NAD+ levels accompanied with increased poly-ADP-ribose polymerases (PARP) activity, and reduced expression of NAMPT [42].

Replenishing NAD+ stores with dietary NR supplementation improved muscle function in these mice through better mitochondrial function [42].

Additionally, enhanced NAD+ concen- trations by NR are apparently beneficial for some neurodegenerative diseases [43], as well as in noise-induced hearing loss [44].

NR-mediated NAD+ repletion is also protective, and even therapeutic, in certain metabolic disorders associated with cancer, such as fatty liver disease [28,45] and type 2 diabetes [28,46]. Metabolic disorders characterized by defective mitochon- drial function could also benefit from an increase in NAD+ levels.

Indeed, stimulation of the  oxidative metabolism in liver, muscle, and brown adipose tissue potentially protects against obesity [47]. Interestingly, NAMPT protein levels are not affected in chow- and high fat diet (HFD)-treated mice fed with NR, arguing that in models of obesity, NR directly increases NAD+ levels without affecting other salvage reactions [47].

Recently, diabetic mice with insulin resistance and sensory neuropathy treated with NR reportedly show a better glucose toler- ance, reduced weight gain and liver damage, and protection against hepatic steatosis and sensory and diabetic neuropathy [48].

 

Boosting NAD+ with NMN in Age-Related Diseases and Cancer 

NMN is also a key biosynthetic intermediate enhancing NAD+ synthesis and ameliorates various pathologies in mouse disease models [49,50].

Very recent research demonstrate that a 12- month-long NMN administration to regular chow-fed wild-type C57BL/6 mice during normal aging rapidly increases NAD+ levels in numerous tissues and blunts age-associated physio- logical decline in treated mice without any toxic effects [49]. NMN is also beneficial in treating age- and diet-induced diabetes, and vascular dysfunction associated with aging in mice [51,52].

Administration of NMN also protects the heart of mice from ischemia-reperfusion injury [53] and restores mitochondrial function in muscles of aged mice [37,54].

It has been speculated that NMN is a circulating NAD+ precursor, due to the extracellular activity of NAMPT [55]. However, the mechanisms by which extracellular NMN is converted to cellular NAD+ still remain elusive.

On the one hand, it is reported that NMN is directly trans- ported into hepatocytes [51]. On the other hand, NMN can be dephosphorylated to NR to support elevated NAD+ synthesis [56–59].

It is recently shown that NAM can be metabolized extracellularly into NMN by extracellular NAMPT. NMN is then converted into NR by CD73 [60]. Hence, NR is taken up by the cells and intracellularly phosphorylated firstly into NMN by NRKs and then, converted into NAD+ by NMNATs [60] (Figure 3).

Thus, mammalian cells require conversion of extracellular NMN to NR for cellular uptake and NAD+ synthesis. Consistent with these findings, in murine skeletal muscle specifically depleted for NAMPT, administration of NR rapidly restored muscle mass by entering the muscles and replenishing the pools of NAD+ through its conversion to NMN [38].

Interestingly, mice injected with NMN had increased NAM in their plasma that may come after initial conversion of NMN into NR [60]. However, degradation of NR into NAM could only be observed when cells were cultured in media supplementing with 10% FBS [60].

Finally, it is important to note that NR is stably associated with protein fractions in milk with a lifetime of weeks [35].

Notably, as reported above, NMN may be degraded by CD38 in older mice promoting NAD+ decline and mitochondrial dysfunctions [2], suggesting that NR may be more efficient than NMN in elderly.

Yet, the beneficial synergistic activation of sirtuins and metabolic pathways to replenish NAD+ pools cannot be excluded. However, given its efficient assimilation and high tolerance, NR represents still the most convenient and efficient NAD+ booster.

Overall, these findings suggest that NAD+ decrease in disease models and NAD+ precursors (NAM, NR or NMN) may circumvent NAD+ decline to generate adequate levels of NAD+ during aging and thus be used as preventive and therapeutic antiaging supplements.

NMN and NR  supplementations may be equivalent strategies to enhance NAD+ biosynthesis with their own limitations.

Side-Effects of Some NAD+ Boosters 

Clearly, several intermediates of the salvage pathway can be considered to boost NAD+ levels but some have contraindications. High doses of NA given to rats are needed to robustly increase NAD+ levels [61].

Additionally, relevant and unpleasant side effects through NA-induced prostaglandin- mediated cutaneous vasodilation (flushing) affecting patient compliance are due to the activation of the G-protein-coupled receptor GPR109A (HM74A) and represent a limitation in the pharma- cological use of NA [62].

NAM is much less efficient than NA as a lipid lowering agent and has also several side effects; in particular, it causes hepatic toxicity through NAM-mediated inhibition of sirtuins [63].

The metabolism of these conventional compounds to NAD+ is also different, as NA is converted via the three-step Preiss–Handler pathway, whereas NAM is metabolized into NMN via NAMPT and then to NAD+ by NMNATs [64]

Manipulating NAD+ by Manipulating Enzyme Activity of Salvage Reactions 

Enhancing the activity of enzymes that participate in salvage reactions can also be a strategic intervention to increase NAD+ concentrations. Different studies have addressed the importance of regulating the activity of NAMPT during disease, including metabolic disorders and cancer.

NAMPT is necessary in boosting NAD+ pools via the salvage pathway.

Consequently, NAMPT deletion provokes obesity-related insulin resistance, a phenotype rescued by boosting NAD+ levels in the white adipose tissue by giving NMN in drinking water [67].

Conversely, in a mouse model for atherosclerosis, NAMPT depletion promotes macro- phage reversal cholesterol transport, a key process for peripheral cholesterol efflux during atherosclerosis reversion [68].

Other recent reports suggest that NAMPT downregulation could be beneficial in treating pancreatic ductal adenocarcinoma [69,70] and colorectal cancer [71].

Recent findings show that Duchenne muscular dystrophy was accompanied by reduced levels of NAMPT in mice [42]. Moreover, NAMPT knockout mice exhibit a dramatic decline in intramuscular NAD+ content, accompanied by fiber degeneration and progressive loss of both muscle strength and treadmill endurance.

NR treatment induced a modest increase in intra- muscular NAD+ pools but sufficient to rapidly restore muscle mass. Importantly, overexpres- sion of NAMPT preserves muscle NAD+ levels and exercise capacity in aged mice [38].

Inhibitors against NAMPT are being used in several phase II clinical trials as anticancer therapy.

Given that NAMPT activation is important to boost NAD+ levels, therapy involving NAMPT inhibition should be considered with caution. Although levels of NAD+ remain to be determined in models with NAMPT depletion, further investigation on the effects of NAMPT modulation is clearly required.

The specific mechanisms and actual benefits of regulation of NAMPT activity remain elusive, evidencing the need of more specific disease models.

 

Can Dietary Restriction and Protein Catabolism Maintain NAD+ Levels?
Among the questions that still remain not well understood is why DR profoundly increases lifespan? Can DR affect NAD+ levels?

It is well established that overfeeding and obesity are important risk factors for cancer in humans [129] and obesity-induced liver and colorectal cancer, among others, can shorten lifespan.

Earlier research has also shown that both increased physical activity and reduction in caloric intake (without suffering malnourishment) can extend lifespan in yeasts, flies, worms, fish, rodents, and primates [3–8].

Furthermore, a recent study pointed to the importance of the ratio of macronutrients more than the caloric intake as the determinant factor in nutrition-mediated health status and lifespan extension [9].

Although in humans it is difficult to measure the beneficial effects of DR and currently there is no reliable data that describe the consequences of significantly limiting food intake, some studies have assessed how DR affects health status.

People practicing DR seem to be healthier, at least based on risk parameters such as LDL cholesterol, triglycerides, and blood pressure [130].

Activation of the salvage pathways during DR could be turned on and glucose restriction can stimulate SIRT1 through activation of the AMPK-NAMPT pathway resulting in inhibition of skeletal myoblast differentiation [131].

Interestingly, effects of NMN supplementation and exercise on glucose tolerance in HFD-treated mice are very similar [132].

Even though these effects are tissue-specific since exercise predominantly affects muscle, whereas NMN shows major effects in liver, and that mechanism of action can be different, exercise and NMN predominantly affect mitochondrial functions and may both contribute to the boost of NAD+.

It is thus tempting to speculate that L-tryptophan concentrations and thus the de novo NAD+ biosynthesis could fluctuate during DR ameliorating the aging process.

Recent studies in humans and mice suggest that moderate exercise can increase blood NAD+ levels and decrease L-tryptophan levels [137].

A possible explanation for this phenomenon is that DR,  and/or exercise, can induce autophagy and promote the release of several metabolites and essential amino acids [138].

 

Conclusion

 

Aging is proposed to be responsible for diverse pathologies, however, it should be considered as a disease among other diseases that appear in time while individuals age.

Although some questions still remain unclear, NAD+ precursors may present possible therapeutic solutions for the maintenance of NAD+ levels during aging and thus may provide prophylaxis to live longer and better.

Although more research is needed to understand the efficacy as well as potential adverse side effects of NAD+ Replacement Therapies in humans, recent studies already provided some pharmacological properties, showing low toxicity and high effectiveness.

 

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