NR is a compound found naturally in trace amounts in milk. But you would need to consume around one hundred thousand glasses of milk1 to achieve the human study quantities proven to safely and effectively boost NAD2.
NR has been studied in humans in the dose ranges from 100mg -1000mg per day. The current science suggests a single daily dose as low as 100mg provided a significant increase in NAD levels.2 NR has been reviewed by the FDA and granted NDI (New Dietary Ingredient) status permitting its use as a nutritional supplement.
1. Trammel, S. A. (2016). Nicotinamide riboside is a major NAD+ precursor vitamin in cow milk. Journal of Nutrition, 146(5), 957–63.
2. Trammell, S. A., Schmidt, M. S., Weidemann, B. J., Redpath, P., Jaksch, F., Dellinger, R. W., . . . Brenner, C. (2016). Nicotinamide riboside is uniquely and orally bioavailable in mice and humans. Nature Communications, 7, 12948. doi:10.1038/ncomms12948