NAD+ levels in our cells are indicators of how long we might live healthy lives. Increasing it, at least in mice, has significantly extended their lifespan, and caused a reversal to some of their aging. NAD+ (Nicotinamide adenine dinucleotide) is a coenzyme found in all living cells. As we age, levels of NAD+ decreases causing a resultant lack of energy and motivation. This is because one of the roles NAD+ serves is to move the energy from the foods we eat into basic cell functions. Even more important, another role is to turn off (down-regulate) genes that speed up the degenerative process. Part of this degenerative cycle entered into with decreased NAD+ involves the depletion of our mitochondrial DNA resulting in senescence and death. 
An enzyme called CD38 builds up in our cells as we age. This enzyme increases 2 to 3 times in aging cells. Future research will target this enzyme. Until then, longevity researchers have found a way to boost NAD+ levels using a form of Vitamin B3 called Nicotinamide Riboside (NR). Vitamin B3, as you may know, is niacin. You may remember niacin from that flushed feeling you got from eating B vitamin supplements. I personally can’t take niacin supplements because I get extremely flushed to the point of feeling sick. I have none of those problems when taking NR.
Scientists began focusing on NR after a 2013 study by Harvard researcher, David Sinclair and his team, was published that showed that NAD+ levels decline with age. Studies on yeast resulted in an amazing lengthening of the organism’s life span, nearly double. Moving up the evolutionary ladder, tests on round worms, c elegans caused them to live up to 16% longer. Studies on mice were even more amazing.
Mice were fed a high-fat diet to make them obese. After taking NR the mice had more energy and an improvement in factors associated with aging, such as improved insulin sensitivity. When put on a treadmill the supplemented mice were able to run more than 33% further than the mice that received no NR supplementation. Even more, in December of 2013 Price and his team reported that the muscle cell mitochondria in older mice were restored to their youthful state. This happened after only a few weeks of injections with another cellular NAD+ promoting compound, nicotinamide mononucleotide.
One of the pains of growing old is the difficulty in losing weight. You will be happy to know that NR helped the mice keep from gaining weight. In fact, they lost weight. This may be due to the fact that NR increases the metabolism in the mice which increased calorie-burning.
A study by researchers at the University of Iowa discovered that NR reduced the development of insulin resistance and sensory neuropathy or nerve damage in mice that were bred to have early type 2 diabetes. The results of the study published in Scientific Reports showed that NR was successful at reducing both fasting and and non-fasting blood glucose levels. Basically, mice getting NR just live longer. 
In February of 2016 a human clinical trial was started at the University of Copenhagen in collaboration with Chromadex, a company that makes a product called Niagen, focused on the ability of NR to overcome the effects of obesity and diabetes over an 8 week period.
Summing it all up
Supplementation with Nicotinamide Riboside increases NAD+ levels in cells where NAD+ levels traditionally decline with age. Increasing NAD+ levels has shown an increase in muscle mass, ability to fend off diseases associated with obesity and diabetes, and fights neurodegeneration. While most of the literature on NR are from studies in mice and other species, clinical trials are underway at the University of Copenhagen. NAD+ levels are also raised by extreme calorie restriction. Studies in primates have shown calorie restriction to have positive effects when NAD+ levels were raised.
As always, live longer,
- Declining NAD+ Induces a Pseudohypoxic State Disrupting Nuclear-Mitochondrial Communication during Aging, Ana P. Gomes, Nathan L. Price, Alvin J.Y. Ling, Javid J. Moslehi, Magdalene K. Montgomery, Luis Rajman, James P. White, João S. Teodoro, Christiane D. Wrann, Basil P. Hubbard, Evi M. Mercken, Carlos M. Palmeira, Rafael de Cabo, Anabela P. Rolo, Nigel Turner, Eric L. Bell, David A. Sinclair, Cell, Volume 155, Issue 7, p1624–1638, 19 December 2013
- Science. 2016 Apr 28. pii: aaf2693. ,NAD+ repletion improves mitochondrial and stem cell function and enhances life span in mice.,Zhang H1, Ryu D1, Wu Y2, Gariani K1, Wang X1, Luan P1, D'Amico D1, Ropelle ER3, Lutolf MP4, Aebersold R5, Schoonjans K6, Menzies KJ7, Auwerx J8.
- Cell Metabolism, 18 June 2016