Skinny Ratty!

Question
I have two rats, Olivia and Euphemia. Euphie is of questionable age; we got her from a pet store and they did not know how old she was at the time. I got her around November 2009.
Typically I put some lab blocks in the cage and the rats eat as they are hungry without overeating. I also give them occasional yogurt drops. However, lately I have noticed Euphie will eat ALL of the food, so I started feeding them throughout the day. This was working fine until about a week ago. Euphie has suddenly lost weight to the point if feeling very skinny. Normally she's a bit pugdier than the average rat. The weight loss and inability to put it back on is the only abnormal thing about her though. She's active, movement is fine, breathes normally. Her coat has some thin spots, but I suspect Livvy was barbering her. Something else to keep in mind is that I have been feedign the two rats the same amount. Livvy is a normal weight but Euphie loses weight.

Do you have any ideas what could be wrong?

Answer
The diet may be poor. Please check out my website, sandyscrittercity.com and refer to the page on diets.  Rats should have food at all times.  However, this doesnt rule out that she may not be sick.  Sometimes weight loss alone can be a sign of kidney disease or other problems.  However, since they were not given access to food all the time and are only getting lab blocks, it could be the diet. How much protein is in the lab blocks you use?