In the past years of support of this project we developed a model to quantify in vivo for the first time in human subjects the rates of amino acid transmembrane transport of naturally-occurring amino acids into and out of muscle, and the rate of muscle protein breakdown, while at the same time also quantifying muscle protein synthesis. The general goal of this proposal is to extend our previous studies of the response to exercise to investigate the following hypotheses related to the effect of amino acid intake on muscle protein metabolism after exercise: Hypothesis 1. There is a curvilinear dose response of protein synthesis to the ingestion of a single bolus of AA which reaches a plateau, beyond which more AA intake causes no further stimulation of protein synthesis. Hypothesis 2. There is an interactive effect between sucrose and amino acids such that the addition of sucrose to any dose of AA will stimulate muscle protein synthesis to a greater extent than would be expected from the individual responses to that dose of AA or that dose of sucrose given separately. Hypothesis 3. An interactive effect between sucrose and AAs is due to the insulin response to sucrose, rather than to the energy (calories) provided by the sucrose. Hypothesis 4. Addition of sucrose to the AAs will increase the clearance of exogenous amino acids from the blood and intracellular compartment. Consequently, a second dose, taken one hour after the first dose, of sucrose plus AA will have the same stimulatory effect on net muscle protein synthesis as the first dose, whereas the response to a second dose of amino acids alone, taken 1 hour after the first dose, will have a diminished response as compared to the response to the initial dose. All studies will be performed using stable isotope methodology and arterial-venous sampling across the leg and muscle biopsies. The results of this study will also contribute to our understanding of the mechanism whereby nutritional intake affects muscle protein metabolism.
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