Integration of amino acid metabolism during intense lactation

Purpose of review This review focuses on research from the author's laboratory and that of N.L. Trottier concerning protein and amino acid metabolism during intense lactation, a physiological state characterized by high rates of the net transfer of amino acids from physiological reserves in skeletal muscle to the mammary gland. These studies have broader interest for our understanding of the supply and distribution of amino acids towards key processes. Recent findings The synthesis of milk protein occurs at a distinct anatomical location, such that arterio-venous differences may be employed to identify all amino acid inputs into the mammary gland. These approaches can be used to determine the relative contributions to the mammary amino acid supply of systemic amino acid availability, transport system regulation and blood flow. Milk protein synthesis is partly reliant on the mobilization of endogenously stored protein in skeletal muscle, and our ability to modulate the size of physiological protein reserves and their rate of mobilization in an experimental context can be used to clarify the regulatory events that underlie the access to and eventual depletion of skeletal muscle protein. Summary Massive rates of milk production are achieved by increased mammary blood flow and amino acid extraction. Muscle protein mobilization is a key resource in the overall amino acid supply, and the absolute size of the protein reserve at parturition is a key factor in supporting lactation. The progressive depletion of muscle may ultimately provide insufficient substrate to support this process.