8/16/2023 0 Comments Protein turnoverIn the adult the protein turnover is associated with cell and organ protein maintenance since there is no net tissue growth under most circumstances. First, the higher rate of protein syn-thesis in the very young, compared with that in the adult, is related not only to the fact that a net deposi-tion of protein occurs during growth, which may account for about 30% of the total amount of protein synthesized in the 6 month old infant, but also to a high rate of protein turnover (synthesis and break-down) associated with tissue remodeling and repair, as well as to removal of abnormal proteins. Three points relevant to nutritional requirements may be drawn from these data. Nutr Res Rev 1999 12: 25–54, with permission of Cambridge University Press.)Ĥ–5 g protein/kg per day, respectively. Protein synthesis rates are high in the premature newborn, possibly about 11–14 g protein synthesized per kilogram of body weight per day, and these rates decline with growth and development so that in term babies and young adults these rates are about 7 g andįigure 4.6 The two endogenous nitrogen cycles that determine the status of body protein (nitrogen) balance. In the adult these two cycles operate so that they are effectively in balance (nitrogen intake = nitrogen excretion and protein synthesis = protein break-down), but the intensity of the two cycles differs, the flow of nitrogen (and amino acids) being about three times greater for the protein synthesis/breakdown component than for nitrogen intake/excretion cycle. the balance between protein synthesis and break-down (Figure 4.6).the balance between intake and excretion.In effect, there are two endogenous nitrogen cycles that determine the status of balance in body protein: Changes in the rates and efficiencies of one or more of these systems lead to an adjustment in whole body nitrogen (protein) balance and retention, with the net direc-tion and the extent of the balance depending upon the sum of the interactions occurring among the pre-vailing factor(s). This will be a topic for discussion in the following volume. Other factors, including hormones and immune system products, also regulate these systems. TNF, tumor necro-sis factor, IL, interleukin.ĭietary and nutritional factors determine, in part, the dynamic status of these systems such factors include the dietary intake levels relative to the host’s protein and amino acid requirements, the form and route of delivery of nutrients, i.e., parenteral (venous) and enteral (oral) nutritional support, and timing of intake during the day, especially in relation to the intake of the major energy-yielding substrates, which are the carbohydrates and fats in foods. amino acid synthesis, in the case of the nutritionally dispensable or conditionally indispensable amino acids.įigure 4.5 The major systems in amino acid uptake, utilization, and catabolism, with an indication of the processes involved and some factors that can affect them.amino acid interconversions, transformation, and eventually oxidation, with elimination of carbon dioxide and urea production.The principal metabolic systems responsible for the maintenance of body protein and amino acid homeo-stasis are shown in Figure 4.5. Protein synthesis, degradation, and turnover Turnover of proteins and amino acid metabolism
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