Having been "on a diet" for the past thirty five years and largely failed (cunning use of "largely" there) I think I have come up with a clever way of losing weight.
Cat poo, as you will certainly know, is considered a delicacy by dogs. This is because cats are quite inefficient when it comes to digesting food and what comes out is (yum!) still "food" in the broadest sense. Then there are tapeworms. These bad buggers grow right the way along our intestines using our partially digested food as their own. Let's combine the two ideas...
Insert into the intestine a tube running the whole length from stomach to bowel that takes a goodly proportion of the slop we pump out of our stomachs. Say 25%. Anything that goes down this "tube within a tube"* will not be absorbed into the bloodstream via the intestinal wall and - like cat poo - our faeces will have 25% undigested stuff. I'm not suggesting you then feed it to your dog, but you would certainly lose weight pretty quickly. In my egotistical style I'm calling this the Timworm®.
You know where you heard it first.
*It would need to be held clear of the intestinal wall by projections of some sort. I would think that PTFE might be a good material.