I'm not convinced that it is. I've done it. You have to concentrate and you have to execute constraint, but it doesn't have to be actually uncomfortable.
Well now, you're talking. This IS the hard bit.
It is. There have even been studies made where people have been put into an insulated box and food calories in compared with waste product calories out calculated. Lo and behold, the calories in and out all stack up. The calories extracted are close to what you would get from a bomb calorimeter. Yes, there are a few calories in undigested food that drop out as waste, and of course faecal material itself contains calories, albeit not in a form you would want to use, but the model as a whole works as well as it needs to. "Humans are not bomb calorimeters!" you may hear. No, they aren't, but they are close enough approximations to one.
As I keep saying, it's not about what goes on in your digestive tract. That's just going to do what it needs to do. The thing that makes or breaks it is your mind.