However, your body’s natural levels of NAD+ decline with age. This decline is thought to be connected to the aging process, as well as declining energy levels. So that’s the bad news. The good news: If you support normal levels of NAD+ in the body by supplementing with a precursor like NR, you may be able to maintain mitochondrial functions—and thus promote healthy aging and energy levels. In fact, NR is clinically shown to increase levels of NAD+ in the body.* 

The problem is that finding NR in a naturally occurring source is challenging. There are trace levels in milk but not enough to see results. That’s why it’s suggested to find a supplement that contains NR, like mindbodygreen’s nr+.*

https://www.mindbodygreen.com/articles/feeling-sluggish-a-supplement-to-help-your-natural-energy-levels