Now, there are people sensitive to a certain kind of insoluble fibre, and if you are you might have some trouble on a vegan diet, but these people are in a minority. Apart from people wanting to avoid FODMAPs (the fibre group in question), the only problem I have heard of from friends becoming vegan is the transitory flatulence from eating beans.

Anecdotally the worst gastrointestinal problems I have ever had was on whatever version of the standard American diet I was served when visiting the states as a 20-something year old. I couldn't shit for 10 days...