D.B.
I would only take what the doctor or pharmacist recommends. It's not about starving - it's about having nothing in your intestines, and nothing with any food coloring or dark colors (think of all that Gatorade and Vitamin Water that people drink thinking it's "healthy" - it's not, and the red dye looks like blood on the testing). A lot of these drinks have additives too that can upset you. You need to fill up with only what will clean you out so the colonoscopy is successful. I know so many people who've gotten in there after a night of deprivation, only to find they didn't do a good enough clean-out and the test can't be completed.
Usually magnesium citrate is one of the top recommended solutions, and it requires you to drink a lot less of it to get the job done. There's something else some doctors recommend - I think it's something like Go Light or Go Lightly. It works but the pharmacist says a lot of people don't like it because the sheer volume you have to drink is difficult. Most doctors don't care which one you do so long as you do it right and it works.
A healthy person undergoing a colonoscopy doesn't really need "nutrition" for 24 hours - just liquids. These nutritional drinks are largely money-makers and they aren't going to make you feel satisfied anyway. I wouldn't spend my money on them and risk an inconclusive test. It's only once every ten years so I think I'd just tough it out, get some movies on DVD so you can pause them when you need to use the bathroom, and do something else constructive like start addressing Christmas card envelopes or paying bills. There's no way to make it fun but there are plenty of ways to make it less obsessive about food, which is your best bet.
Good luck!