 Cats are obligate carnivores : their physiology has evolved to efficiently process meat , and they have difficulty digesting plant matter . In contrast to omnivores such as rats , which only require about 4 % protein in their diet , about 20 % of a cat 's diet must be protein . Cats are unusually dependent on a constant supply of the amino acid arginine , and a diet lacking arginine causes marked weight loss and can be rapidly fatal . Another unusual feature is that the cat cannot produce taurine , with taurine deficiency causing macular degeneration , wherein the cat 's retina slowly degenerates , causing irreversible blindness . 
