Technical Abstract: Because protein underlies all structural and dynamic functions in living organisms research into protein metabolism has contributed immensely to our present understanding of how metabolic and physiological homeostasis is maintained in good health and altered by diseases. Protein turnover (or kinetics), i.e. the rate at which protein is synthesized and broken down, is a fundamental biological process, and at the whole body level it is defined as the sum of turnovers of all the individual proteins of the body. The basis of most methods used to estimate protein turnover is to measure the whole body flux of an amino acid or of amino-nitrogen. Flux is the rate of flow of amino acids from protein breakdown and the diet through the body's metabolic amino acid pool into protein and other metabolic pathways such as oxidation. Once flux is determined, because dietary intake and oxidation can be measured, protein breakdown and synthesis can be estimated.