I don't think so. My daughter's cat, who has an over-active thyroid, was losing weight to the point where it was really bad. The vet gave her a prescription diet aimed at building her up. She has gained a pound fairly quickly. She's returning to "regular" food (and thyroid medication) now that she's gotten through her crisis. When she was sick she wouldn't even eat regular cat food (only the special one made to be particularly appealing, apparently). She certainly wasn't gaining weight on regular cat food either. For sick and/or elderly pets with medical problems it's great.