The dose they gave in the study was 400mg/kg.
What would the equivalent be in a human?
I found this study from a few years ago, that had this snippet.
"For example, whereas the effects of NMN on body weight gain, insulin sensitivity, tear production, and bone mineral density were dose-dependent, 100 mg/kg/day of NMN improved oxygen consumption, energy expenditure, and physical activity better than 300 mg/kg/day"
"Given that 100 mg/kg/day of NMN was able to mitigate most age-associated physiological declines in mice, an equivalent surface area dose for humans would be ~8 mg/kg/day (Freireich et al., 1966), providing hope to translate our findings to humans."
So it would seem that 100mg/kg would be in a human 8mg/kg
so for a 175lb human the 100mg equivalent would be 640mg a day.
this study was done with 4x that amount.
So it would seem to just show that more of a good thing is not better