Does taking NMN/NR actually raise NAD+ inside cells, or just in the blood?

This is the most important nuance in the whole NAD+ conversation, and most marketing glosses over it.

What's well established: oral NMN and NR reliably raise NAD+ and its related metabolites in whole blood. One placebo-controlled trial found that 250 mg/day of NMN for 12 weeks nearly doubled whole blood NAD+ levels, with levels rising and then plateauing.6 A separate study reported the same significant rise in blood NAD+ metabolites after NMN supplementation in older men.4

What's far less settled: whether that blood increase translates into higher NAD+ inside the cells and tissues that actually matter — skeletal muscle, liver, brain. Here the human data is genuinely mixed. One direct commentary on the field noted plainly that several human studies have failed to show that NMN or NR supplementation increases NAD+ levels in skeletal muscle, even as whole blood levels rose, and called for future research to test tissue-specific effects directly.6 A twin study did find that NR altered gene expression tied to NAD+ metabolism and mitochondrial biogenesis in muscle tissue — changing the activity of enzymes involved in NAD+ synthesis7 — which is a promising downstream signal, but not direct proof of elevated intracellular NAD+ concentration.

Animal and cell-culture research does show NR raising intracellular and even mitochondrial NAD+ directly across multiple mammalian cell types,8 so the mechanism clearly exists. The open question is magnitude and consistency in living human tissue, not whether it's biologically possible.

Practical takeaway: blood NAD+ going up is real and reproducible; assuming your muscles, brain, or liver are getting a proportional boost is currently more of an inference than a proven fact.

NMN.com, "Clinical Study Indicates that NMN Efficaciously Increases Blood NAD+ in Humans" (summarizing Nakagawa et al.) — https://www.nmn.com/news/nmn-increases-blood-nad-humans

"Nicotinamide riboside improves muscle mitochondrial biogenesis, satellite cell differentiation, and gut microbiota in a twin study," Science Advances — https://www.science.org/doi/10.1126/sciadv.add5163

US Patent filing, "Methods of treating mitochondrial dysfunction" (NR intracellular/mitochondrial NAD+ data) — https://image-ppubs.uspto.gov/dirsearch-public/print/downloadPdf/10709724