Starving rat

Question
I adopted 2 female rats in April of this year that were just weaned.  They had been purchased from a feed store I think and were described as "feeder rats" -- light brown.  (I got them from the original purchaser so don't really know about their beginnings).  They ate well and I had purchased a rat cage with 3 levels and  changed the bedding twice a week.  fresh water daily, etc.. They seemed to be growing normally, then one night one of them became extremely distressed -- laying on her side and writhing from side to side.  I thought she might be in labor or something as she had been getting fatter and fatter.  I went to bed expecting babies in the morning only to find her dead!  After about a week, I began to feel sorry for her "sister" and went to the local pet store to buy a companion for her.  They had 3 little hooded rat girls - newly weaned, and I took all three home so they wouldn't become snake food.  These little things grew like weeds and within 3 weeks had outgrown the original rat.  Her hair was thin from the beginning, but I didn't really notice it until I brought home these healthy babies.  The first rat's coat has become thinner and thinner and she has also become thinner and thinner even though she eats an amazing amount of food.  But she's literally starving before my eyes -- with ribs and hipbones jutting out.  Stools are formed and healthy looking -- she eats and drinks, but her energy level is extremely low.  Do you have any idea why she can't assimilate her food?  What ever she has doesn't seem to be contagious -- wondering if it's some type of congenital defect as I get the impression she came from breeders equivalent to "puppy mills" in dogdom.

Answer
From the sounds of it, her sister had either cancer or megacolon. I'd assume its something related to the two (so yes, congenital/hereditary), but honestly, I have no idea what to tell you. You're absolutely right.. feeder rats are bred in a mill-type situation (it's positively disgusting) and are far from the healthiest critters in the bunch. It's sad.

Other than a vet visit, I'm pretty much useless to you. Sorry!