Is this faster-than-light-like impossible daydream or is there any plausible path to it?
Does gas, painful cramping and diarrhea sound like a good approach?
Reminds me of Olestra, the zero calorie oil substitute that failed because of similar sounding side effects. Olestra wasn't consumed/absorbed in the gut so you'd just shit out oil and your half digested food would glide out with it since it didn't block stomach emptying like real food.
A vomitorium is simply the large exit pathway in stadia for rapid egress of many people. The idea that ancient Romans had facilities dedicated to vomiting has been debunked.
Those people that tell you they can eat anything and not put on weight have a very different idea of what "anything" is than fat people, with probably an actual exception here and there (along with teenagers).