Nicotinamide mononucleotide (NMN) is a molecule that's found in all living cells. More precisely, NMN is a ribo-nucleotide which is a foundational piece of RNA.
It is a nucleotide derivative of nicotinamide that is involved in various biological processes, including those related to cellular metabolism.*
NMN is being studied for its potential to influence cellular levels of nicotinamide adenine dinucleotide (NAD), a molecule involved in many biological functions.* NAD is an important coenzyme involved in cellular metabolism. NMN is often referred to as a direct NAD+ precursor.
NMN is commonly included in dietary supplement formulations that focus on topics such as aging, energy, and physical function.* Let's review some current scientific research around NMN and NAD+.

NAD+ levels decline with age
NAD+ is a critical molecule involved in numerous metabolic reactions, including energy production in nearly all cells. NMN serves as a biological precursor to NAD+, and some studies have explored its effects on NAD+ levels following oral administration.* NAD+ levels are thought to decline with age. This decline in NAD+ may also be associated with aging.

NMN may help support healthy NAD+ levels and healthy aging
In one recent study, the administration of NMN, the precursor to NAD+, was shown to increase NAD+ levels in healthy adults.* That increase in NAD+ levels is one of the key reasons why there is a growing interest in using NMN to help support healthy aging.*
NMN to help support exercise performance
A study showed that nicotinamide mononucleotide supplementation may enhance aerobic capacity in amateur runners. Results showed that the oxygen uptake (VO2), percentages of maximum oxygen uptake (VO2max), power at the first ventilatory threshold, and power at the second ventilatory threshold increased to a higher degree in subjects receiving NMN.*
Another study in Science, showed that NMN may increase muscle sensitivity insulin sensitivity in prediabetic women. That study also showed that NMN might have increased collagen production in the studied group.*

Other studies
Several studies have been conducted on different types of subjects, including mammals. These studies may indicate that NMN might also play a role in energy production, brain functions, including cognition, and blood sugars.
An example of such studies is how the NAD+ or sirtuin pathway might modulate longevity by activating mitochondrial UPR and FOXO signaling. Another research discusses the potential of NMN to help increase blood flow and endurance.
The list of studies around NMN and NAD+ in this article is certainly not exhaustive as other research are being conducted worldwide. You can also learn about niacin to get a better overview of vitamin B3 as well as NADH or NAC. You can also read about NMN 500mg.
More human studies are needed before we can make any additional conclusions on the additional benefits that NMN has on health.
As always, talk to your doctor before taking any supplements. Get professional advice if you think you might need any NMN supplement or are already taking one to ensure you're not exceeding the amounts you may need.