Umm...from an alternative health standpoint I'm going to caution using anything with refined carbs in it as a "fattening" agent. From a natural medicine standpoint, refined carbs like sugar and flours and starches will feed cancer. I'm not saying your rat has cancer but unexplained weight loss can sometimes signal the growth of some kind of tumour. Healthy oils can be added as a supplement that will help maintain weight. Fish is a nice food to feed as well and any of the high antioxident fruits and veggies...broccoli, blueberries (any berries), nuts and nut butters. I know this isn't the mainstream view but this may go beyond a matter of maintaining weight in an otherwise healthy rat and be more of a maintenance diet to prevent cancer growth. You just don't know at this point.