This is such an important question. Having done this prep a number of times, I can say that being able to walk in for your colonoscopy with your blood sugar at a reasonable level is a real accomplishment. I’ve managed it, just barely. The standard advice is to eliminate all insulin except for the basal dose and drink light-colored juice, like apple juice, as needed.
This works well for me but in the 2-3 hours before the procedure when they are saying no juice, it can get dicey. One time I had to get into a somewhat heated discussion with the gastroenterologist before leaving the house since my blood sugar was 80 and dropping. I made it clear that avoiding insulin shock came ahead of any test and he finally backed off his insistence on no juice, Everything went fine at that point and in all following colonoscopies.