Hi Glen, thanks for your comments. The metformin/berberine idea is just a hypothesis (though something that could be easily tested), though it has been shown that metformin and berberine inhibit Complex I of the respiratory chain, which is important in replenishing the NAD+ pool by oxidizing NADH. While the experimental evidence is limited and still controversial, there are several studies that support this notion (a review can be found here https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6304344/)

I hope this helped a little.