Hat tip to Ted Naiman on Twitter.
Basically, you poop out more glucose when you take metformin. Ted theorized this might be why there are diarrhea and the like with many who take metformin.
Edit: Also, to the extent berberine works similarly to metformin, perhaps this is why I did not like berberine: I already have stable blood sugar, and berberine made me feel worse, possibly due to lower blood glucose. Sadly, I did not have a CGM to test this.