Cats need animal protein. They have a higher requirement for protein in their diets than dogs and humans. They need protein so that they can route animo acids (protein is a chain of amino acids) in the process of gluconeogenesis (GNG) to supply the needs of the brain and other tissues requiring glucose. A significant proportion of protein is diverted into gluconeogenesis.
Gluconeogenesis means “the production of new glucose” in a literal translation.
In other words the cat’s body demands glucose from an an “obligatory amino acid based gluconeogenesis”.
The process of gluconeogenesis occurs even when there is protein malnutrition but at a reduced level. The amount of protein a cat needs depends on their age. For an adult cat, The Association of American Feed Control Officials (AAFCO) put the requirement at 26%.
P.S. As cats are unable to manufacture enough taurine from the amino acids methionine and cysteine they need it in their diets.
1. Hypercarnivory and the Brain: Protein Requirements of Cats Reconsidered
Regina Eisert and;
2. K Sturgess and K.J.Hurley in Nutrition and Welfare in The Welfare of Cats edited by Irene Rochlitz.