Online, there can be found a lot of conflicting information about the absorbability and oral bioavailability of NMN.
It was previously thought that NMN could not be absorbed by cells (NMN would be too large to go through the cell membrane). Many online articles still claim this. However, in 2019 an NMN transporter was found, demonstrating that specific transporters exist that can shuttle NMN from outside into the cells:
https://www.nature.com/articles/s42255-018-0009-4
Also, many studies done in mice show beneficial effects of NMN on aging and in these studies NMN is given orally (by adding NMN in the drinking water or in the chow of the mice). This demonstrates that oral NMN can lead to beneficial effects.
The same is the case in various other species, including horses that are given NMN orally and that show rejuvenation of the oocytes and improved/restored fertility.
These are a few examples of studies in which NMN is given orally resulting in beneficial effects on health and longevity:
- Long-term administration of nicotinamide mononucleotide mitigates age-associated physiological decline in mice. Cell Metab, 2017
- Nicotinamide mononucleotide (NMN) supplementation rescues cerebromicrovascular endothelial function and neurovascular coupling responses and improves cognitive function in aged mice. Redox Biology, 2019
- Nicotinamide mononucleotide supplementation reverses vascular dysfunction and oxidative stress with aging in mice. Aging Cell, 2016
- Impairment of an endothelial NAD(+)-H2S signaling network is a reversible cause of vascular aging. Cell, 2018
- Nicotinamide mononucleotide (NMN) supplementation promotes anti-aging miRNA expression profile in the aorta of aged mice, predicting epigenetic rejuvenation and anti-atherogenic effects. Geroscience, 2019
- NAD+ Repletion Rescues Female Fertility during Reproductive Aging. Cell, 2020
- NAD+ supplementation rejuvenates aged gut adult stem cells. Aging Cell, 2019
- NAD+ controls neural stem cell fate in the aging brain. EB Journal, 2014
- Impairment of an endothelial NAD(+)-H2S signaling network is a reversible cause of vascular aging. Cell, 2018
- Nicotinamide mononucleotide promotes osteogenesis and reduces adipogenesis by regulating mesenchymal stromal cells via the SIRT1 pathway in aged bone marrow. Cell Death & Disease, 2019
Additionally, one of the most esteemed NMN experts in the world, Professor David Sinclair, takes NMN orally every morning. So apparently he trusts that taking NMN orally is able to exert beneficial effects inside cells.
This and other data suggest that NMN when taken orally enables various beneficial effects.
You can learn about NMN (and NR) below:
https://novoslabs.com/nmn-vs-nr-is-nmn-better-than-nr/ (In this article, we also quote studies about the oral bioavailability of NMN).