As it was explained to me by my medical team when I pushed (and pushed) for coffee:
* Caffeine is a diuretic, which can lead to dehydration after surgery, since many people have a hard time getting all the required water in. Caffeine can also affect the bowels, leading to more frequent and/or looser movements, which can also lead to dehydration and/or diarrhea.
* Most caffeinated drinks end up high in empty calories (think soda) and many people add heavy cream, sugar, high fructose laden flavorings, or chemically made sugar free sweeteners to their coffees.
* Many caffeinated drinks (soda, coffee, black tea) are also acidic, which can potentially cause ulcers in your pouch and/or your intestines.
That being said, I have totally ignored my medical team on this issue and have 1 coffee in the morning and usually a small iced coffee in the afternoon. I use ultra filtered high protein milk or a flavored protein shake in the place of creamers and have had no issues to date. However, I do make sure that the coffee is not taking the place of meals or making me unable to hit my water intake goals.