No matter what we feed her, she keeps losing weight

I have a rat that seems to have chronic health issues. Whether she is simply very unfortunate, or has very poor genetics, I don't know.

What I do know is that no matter what we feed her, she will not gain any weight. She is maybe a year old, and completely thin. She is of a larger build compared to my first rat, but when it comes to weight I'd say the tiny one would win. Milly, the one who will not gain weight, is rail thin. She's not just skinny, she's frail. You can feel each of her ribs just by picking her up, and I can see the muscles in her stomach when I hold her upright. Even the tendons in her legs stick out and you can see them expand and contract when she walks.

She does not seem to have the interest in food that any of the others have. Nothing tempts her unless she is practically starving. Sweets, peanut butter, vegetables, you can put all of it right in front of her face and she won't even register it.

We've been giving her the same mix the others have, only we now give her whatever will make her eat. Cake, cookies, french fries, anything that will get her to just eat something.

Even when she does eat though, she doesn't gain weight. She's been eating cake and cookies and bits of our sweets for longer than a week now. We give her whatever she wants. She doesn't gain an ounce, she's gotten skinnier.

When I first bought her, she was a normal weight. She was fed on a bulk seed mix, mostly bird food, sunflower seeds and dried corn. Now she's gone away virtually to nothing, and I don't know what to do, or what's wrong with her.

I'm worried she's going to die very soon if nothing is done. I want to take her to the vet again, but I want answers before I do.