I used to be one of those people that could eat anything and never gain a pound.
After they removed all my abdominal organs and then put them back again, things never quite worked right. I have something called gastroparesis, which basically means my digestive system no longer gives a fuck. It digests what it wants, when it wants. I now have the metabolism of a python.
Instead I'm not supposed to eat fresh fruit or vegetables. Also, I drink soda sometimes to make up the calories I miss from not eating. It's the stupidest doctor-approved diet ever, and makes me look like I have the pallet of a 6 year old when I'm trying to look like a responsible adult.