Tried lower carb prescription diet food as part of a study by the veterinary nutritionists we have at school- and he was worse. I'm honestly finding that, me personally, on days that I eat less than a hundred carbs, I feel hungrier and want to snack more, too. But, the cats are now on a commercial 'lower calorie' diet which seems to work. I put it out 2x a day and they nibble on it. He's a little sleeker than he used to be- and he never quite got 'morbidly obese'. He's about a 7/9, when he needs to be a 5 out of 9. Some pets come in with a body condition score of 12/9 ( or higher. think of a really fat sausage with toothpicks for legs, can barely walk down the hallway without nearly going into respiratory distress! )
I feel the biggest problem is me not being home as much as I was when I first got him. He seems very attached to me and gets pissy when I'm not here a lot, despite having another cat to annoy. I think when I graduate, this will help resolve some of the issues.
My cat is a carbaholic. It's terrible, but one of the funniest things ever because he is a large cat (ideal weight about 14 pounds) and he WILL go trash can diving. He also WILL steal doughnuts, bread, bagels, any sort of carby product out of the trash can and take off with it somewhere and nibble on it. I think this is a result of never being fed reliably by his first owner; he had to fend for himself, which is sad. So I never throw bread away until I'm ready to take the trash out, as a result! (The day he stole the doughnut, I was so worried he was going to wind up with some sort of pancreatitis from all the fat and sugar, because I could never find the leftovers. I finally found the leftovers and tossed them out, but was super thankful he had the good sense not to eat the whole thing!)
Some dogs exhibit pica- trying to eat anything and everything, and it's more of a behavioral disorder. Sounds like that is what your sister's dog had.. I can't remember if they try anti-depressants or not with them, or if it is more of a behavioral modification with them, but there are veterinary behaviorists that deal with things like that on a regular basis. As a slight aside, we call it counter surfing. Incidentally, my sister had issues with pica; she had her pituitary removed when she was 7 years old because of a tumor there (this was in the late 70s).. had to be on a lot of medications and such... just had no real control of her hunger, and would get into food all the time if she wasn't locked out of the kitchen. :(