The concept that components of the body are continually being replaced has been around since the 6th century B.C., according to which the structure of the body was continuously broken down and being replaced by new structures and substances derived from food.1 We now understand that this applies appropriately to the proteins in the body, and to the concept of protein turnover. Protein turnover refers to the catabolism and synthesis of endogenous proteins.1 Within limits, this dynamic process is adaptable to the availability of amino acids for protein synthesis, hence it is influenced by sufficiency of dietary protein as well as the protein reservoirs provided by lean body mass (LBM).2 This paper will address what is sufficient protein for adult cats, and how this is best measured.