Cats lose weight when they are in heart failure due to:
increased energy requirements and
the production of inflammatory hormones.
Old-fashioned sources may suggest protein restriction for cats with heart failure to prevent "metabolic stress" on the liver and kidneys. There is
no evidence that protein restriction is needed for cats with heart failure. In fact, protein restriction can have a negative outcome because cats can lose valuable muscle mass, which can lead to decreased survival times. Unless severe kidney disease is present, a protein-restricted diet is not recommended for dietary management of heart failure. A high quality diet with at least 40-50% (cats) meat-based protein (on a dry matter basis) is recommended.