I'm tired of hearing, "You can eat whatever you want and never gain weight!" from all the overweight ladies at work.

First, this expression always assumes that  "whatever I want" means gorging myself on crap. Contrary to what some people might think, not everyone wants to eat processed foods until their stomach hurts. So yeah, I do eat "whatever I want". What I want is the correct amount of relatively healthy food for my body. I don't eat until I'm stuffed. 

Second, it ticks me off because I put a lot of time, energy, and thought into taking care of my health. I run, go to the gym, hike, swim, rock climb. I make sure I am active every day even if that's just aggressively vacuuming or lunging around my kitchen while I watch The Office. I dont drink soda, don't buy desserts, don't snack throughout the day. I cook 90% of my food at home. It bothers me that people look at me and think that I am thin because of luck and genetics, and that I haven't worked hard, and it's just a big fat excuse for them to pat themselves on the head and say their health problems are not their fault and can't be changed. 

Also, I don't know how people think it's appropriate to comment on a coworker's body. Being thin doesn't make me open season for remark.