Cats are obligate carnivores, which means they must eat meat to survive. Many people do not understand this, but cats get most of their essential nutrients from meat.
Cats have a higher protein requirement than non-obligate carnivores (mammals who can survive on diets consisting of foods other than meat but still may consume meat). If cats are fed diets that do not contain meat products, they can end up suffering from a variety of potentially serious, even life-threatening, metabolic problems.