uncooked peeled potatoes or other uncooked stachy foods like pasta will add in the weight. a good nutrient drink like boost or ensure will help with a vitimins boost. warmed oatmeal or baby foods are another favorite for an ailing rat that may be having trouble getting enough to eat or chewing. on that note check their teeth to make sure they aren't getting too long either as some older rats have problems keeping them worn down. carnberries are also good for helping prevent urinary tract infections and something else but i can't remember if this was the one that is suspected to help prevent mamorary tumors or respiratory flare-ups.... mind you either one has not yet been confirmed but there is a possbile link. i'm not sure if they've found any harder evidance with it yet or not.