Commodore, on both the AOA and the AMA sites, it looks like you can just pop into a GI fellowship without doing IM. Of course, this is impossible. They both just throw all of the fellowships in with the residencies. But you are correct that in order to be accepted into a GI program, you must first complete three years of IM residency (MD or DO).
But as far as length, there is no difference between MD and DO programs. However, you may find some differences in the length of fellowships at different institutions independent of their MD or DO affiliation. In other words, the program length is dependent on the institution offering the fellowship and it has nothing to do with it being a MD or DO program. For example, take nephrology fellowships: you can find both two and three year fellowships out there. The difference is that some programs will require a third year dedicated almost solely to research, while other programs will just combine your research into the first two years.
Get it?
