The NAD World is an interesting theory that may explain how fasting promotes healthy aging. Over the last few blogs I’ve discussed how factors in the blood play a big role in regulating how organs function.

Readers of this blog know that I take a non-conventional approach to gut health.  Focusing on what goes in the gut is important for gut health, but it’s not the only kid on the block. 

One of the biggest ways our blood promotes healthy organ function is by maintaining a large local stem cell pool.  As we age, stem cell pools decline and we begin to see a sharp decline in the function of many of our organs, the gut included.  Interestingly, when we infuse older mice with the blood of younger mice, they show improvements in organ function.  This process is now being studied in humans.

Part of this is from rejuvenation of stem cell pools by factors in young blood. And some of it is probably dilution of factors in old blood that do the opposite.  This has led researchers on an attempt to identify factors in blood to to reverse aging.

The NAD World identifies an enzyme called eNAMPT that partially explains some of the beneficial effects of fasting in animal models. It may be the the key to how fasting promotes healthy aging in mice.

The hypothalamus and aging

Recent work has identified the hypothalamus as a regulator of the aging process(1).  In this study, researchers observed that mice lose stem cells in the hypothalamus as they age. 

When young mice had these stem cells depleted, they aged at a faster rate.  Furthermore, injecting hypothalamic stem cells into older mice rejuvenated them, causing them to function as if they were younger.

The researchers identified that these stem cells were doing more than just replenishing cells in the hypothalamus.  In fact, they secrete messenger molecules called microRNAs that travel throughout the body and help regulate aging remotely.  Even in the absence of an increase in stem cells, infusing these microRNAs gave older mice more youthful function.

MicroRNAs regulate gene expression, usually by reducing it.  In the hypothalamus, we package these microRNAs into vessicles called exosomes.  Stem cells secrete these exosomes and they enter the circulation to regulate gene expression in remote tissues.

Taken from: https://benthamsciencepublishers.files.wordpress.com/2013/09/schematic-of-microrna.jpg

Given that the hypothalamus is the control center of the body, it makes sense that it plays a role in the aging process.  But it gets even more interesting than that.

Cellular senescence and the stem cell pool

There are many factors that regulate the number of stem cells you have in any given stem cell pool.  One big factor is cellular senescence, something I’ve mentioned previously.  When stem cells enter senescence, they no longer replicate and therefore can’t contribute to replenishing cells as they die.  They also secrete factors such as inflammatory cytokines that have detrimental effects on other stem cells.

Cellular senescence isn’t a one-trick pony, there are many factors that can lead to cellular senescence.  One factor that appears under our control is the level of the oxidized nicotinamide adenine dinucleotide, called NAD+.  NAD+ is a coenzyme in cells that generates energy for cellular processes.  As a coenzyme, it exists in 2 forms: an oxidized form(NAD+) and a reduced form(NADH).

Taken from:http://www.psoriasisdietplan.com/wp-content/uploads/2015/05/NAD_NADH.png

NAD+ and NADH levels go hand in hand. The total of both is consistent, but the ratio between the 2 depends on what’s going on in the cell. 

This ratio is very important with regard to cellular senescence.  When we process glucose in the cytoplasm of our cells, this reduces NAD+ to NADH.  The NADH then powers other processes and hopefully becomes NAD+ again.

Having a lower NAD+/NADH ratio can be very problematic. Two very important pathways to cell health utilize the oxidized from(NAD+), but not the reduced form(NADH).  These 2 pathways, the sirtuin and Poly(ADP-ribose) polymerase-1(PARP-1) pathways, are important for fixing DNA damage and repairing other cellular components. 

Cellular senescence occurs at a low NAD+/NADH ratio .  Increasing NAD+ rescues stem cells, including neural stem cells, from senescence(2, 3).

Through the sirtuin and/or PARP-1 pathway, NAD+ may play a big role in regulating the hypothalamic stem cell pool.  But it appears that part of this process may occur outside of the hypothalamus. And it utilizes something you probably spend most of your time trying to get rid of: your body fat.

The NAD World: Fat and fasting for brain health

The NAD World is a concept put forth by Shin-ichiro Imai in 2009 and recently updated in 2016 .  It illustrates the importance of inter-tissue communication for controlling the aging process.  The basics of the NAD+ world are pretty straightforward.

In our cells, sirtuin/NAD+/NAMPT axis measures nutrient status.  When we are in a fasted state, the NAD+/NADH ratio is high. This increases sirtuin activation and activates the circadian clock. 

The circadian clock then increases activity of the enzyme nicotinamide phosphoribosyltransferase (NAMPT), the rate-limiting enzyme for NAD+ synthesis. Sirtuins are NAD+-dependent enzymes, so this effectively allows the sirtuin train to keep pumping along by supplying NAD+. 

This pathway is the salvage pathway of NAD+ synthesis and is essential in humans.  The salvage pathway is important because when sirtuins utilize NAD+, they make nicotinamide(NAM), which inhibits sirtuin activity.

Taken from: https://alivebynature.com/june/wp-content/uploads/nad-salvage3-3.png

In addition to local cellular NAD+ synthesis, there’s a remote pool of NAD+ regulated by fasting.  Sirtun 1 is activated in our fat cells as we enter a fasted state.  This causes them to release a special form of NAMPT called extracellular NAMPT, or eNAMPT. 

While we don’t know the particulars of how this raises NAD+ levels remotely, we know it does.  Knocking out NAMPT in the fat cells of mice causes a drop in systemic eNAMPT AND NAD+ in the hypothalamus but not the liver, muscle, or hippocampus(4).  That’s right, the level of NAD+ in the hypothalamus is dependent on circulating eNAMPT, which comes from fat.

The theory behind the NAD World is that there are frailty points in the body that don’t produce enough NAD+. These frailty points require adequate levels of systemic NAD+ due to low local NAMPT activity. 

NAD+ for these tissues supplied by eNAMPT, secreted by fat tissue during fasting.  Two of these frailty points are the hypothalamus and pancreas.

When NAD+ levels drop with age, sirtuin activity cannot proceed in these tissues and crucial repair processes go unperformed(5).  This may lead to organ dysfunction basically anywhere because the hypothalamus is the control center of the body.  It may also promote senescence.

Inflammation, NAD+, and the hypothalamic stem cell pool

Neural stem cells are dependent on adequate NAD+ for both self-renewal and differentiation(6).  Based on the evidence above, neural stem cells in the hypothalamus are dependent on systemic NAD+ to provide an adequate supply. 

But as we age, our NAD+ levels naturally decline.  This is through a variety of reasons. However, a recent study found a primary driver of this drop is an increase in CD38 activity(7).

Taken from: https://ars.els-cdn.com/content/image/1-s2.0-S1550413116302248-fx1.jpg

CD38 is a protein found on the cell surface of many cells of the immune system and increases with age.  CD38 breaks down NAD+ and the NAD+ precursor nicotinamide mononucleotide(NMN), which is made by NAMPT and eNAMPT. 

Since aging increases tissue CD38 expression, this reduces NAD+ availability for stem cells in the hypothalamus.  Furthermore, when CD38 utilizes NMN, it converts it to NAM. NAM actually inhibits sirtuin activity and requires NAMPT for conversion back to NMN.

We’ve discussed CD38 a number of times on the blog. Check out how CD38 disturb the circadian rhythm of autophagy here. You can also check out the basics of NAD+/NADH in energy metabolism in a podcast found here.

Increasing your declining NAD+ levels

There are a couple of states that can replete the NAD+ pool.  Both fasting and the ketogenic diet increase the NAD+/NADH ratio. 

In the hypothalamus, this has the added benefit of reducing inflammation, which attracts CD38 expressing cells to the area.  In fact, the ketogenic diet decreases inflammation in the hypothalamus via a mechanism that requires an increased NAD+/NADH ratio(8).

Supplementing nicotinamide riboside increases cellular NAD+ levels in humans(9).  But it’s not clear that it does this better than regular old niacin from food.

I don’t believe simply supplementing with nicotinamide riboside is an effective strategy for mimicking fasting or the ketogenic diet, though. This process seems timed, and NR is just a resource to help that clock keep ticking. 

Fasting does a lot more than change the NAD+/NADH ratio and also has an impact on other longevity pathways.  If you’re over 40, supplementing with NAD+ precursors such as nicotinamide riboside or nicotinamide mononucleotide may be a wise idea.

A final approach, and one that can be used in combination with the 2 approaches above, is to inhibit CD38.  There are a few substances that do this, including apigenin, quercetin, and fisetin(10).  These polyphenols have a long safety track record so the only concern would be adequate bioavailability.

Conclusion

Although still just a hypothesis, evidence is piling up in support for the NAD World.  The basic premise is that NAD+ availability in remote tissues is dependent on release of eNAMPT from fat tissue during fasting.

This may be how fasting promotes healthy aging, at least in animal models. This is of particular relevance to people in the developed world who don’t spend a lot of time in a fasted state.

Tissues such as the hypothalamus and pancreas act as frailty points that “break” once NAD+ availability drops.  This may be due to a multitude of factors including a decrease in the local stem cell pool. 

Given that recent evidence indicates that aging is regulated, at least to some extent, by microRNAs secreted by stem cells in the hypothalamus, the consequences of this could lead to an increased rate of aging and age-related decline.

A primary driver of the decline in NAD+ with age is a 2-3x increased expression of CD38 in tissues.  CD38 is highly expressed in cells of the immune system, and inflammation increases CD38 expression. 

This may be the primary mechanism by which chronic low grade inflammation drives the age-related decline in NAD+ levels.  Adding to the relevance of the NAD World, CD38 activity is increased 2.5 fold in human adipose tissue.

Overall, the NAD World provides a nice framework by which fasting promotes healthy aging.  Research is currently under way to better understand the NAD World and how it helps to regulate the aging process.  In my next blog I’ll uncover the relevance of the NAD World to the gut.